Message ID | 20240203023645.31105-1-quic_wcheng@quicinc.com |
---|---|
Headers | show |
Series | Introduce QC USB SND audio offloading support | expand |
On 2/3/2024 3:36 AM, Wesley Cheng wrote: > Some platforms may have support for offloading USB audio devices to a > dedicated audio DSP. Introduce a set of APIs that allow for management of > USB sound card and PCM devices enumerated by the USB SND class driver. > This allows for the ASoC components to be aware of what USB devices are > available for offloading. > > Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> > --- ... > + > +/** > + * snd_soc_usb_add_port() - Add a USB backend port > + * @dev: USB backend device > + * @priv: private data > + * @connection_cb: connection status callback > + * > + * Register a USB backend device to the SND USB SOC framework. Memory is > + * allocated as part of the USB backend device. > + * > + */ > +int snd_soc_usb_add_port(struct snd_soc_usb *usb) > +{ > + > + Cosmetic, but why is there white space between start of function and body of function? > + mutex_lock(&ctx_mutex); > + list_add_tail(&usb->list, &usb_ctx_list); > + mutex_unlock(&ctx_mutex); > + > + return 0; > +} > +EXPORT_SYMBOL_GPL(snd_soc_usb_add_port); > +
On 2/3/2024 3:36 AM, Wesley Cheng wrote: > In case of notifying SND platform drivers of connection events, some of > these use cases, such as offloading, require an ASoC USB backend device to > be initialized before the events can be handled. If the USB backend device > has not yet been probed, this leads to missing initial USB audio device > connection events. > > Expose an API that traverses the usb_chip array for connected devices, and > to call the respective connection callback registered to the SND platform > driver. > > Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> > --- > sound/usb/card.c | 19 +++++++++++++++++++ > sound/usb/card.h | 2 ++ > sound/usb/qcom/qc_audio_offload.c | 2 ++ > 3 files changed, 23 insertions(+) > > diff --git a/sound/usb/card.c b/sound/usb/card.c > index 11b827b7a2a5..995b2df676ab 100644 > --- a/sound/usb/card.c > +++ b/sound/usb/card.c > @@ -202,6 +202,25 @@ struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx, > } > EXPORT_SYMBOL_GPL(snd_usb_find_suppported_substream); > > +/* > + * in case the platform driver was not ready at the time of USB SND > + * device connect, expose an API to discover all connected USB devices > + * so it can populate any dependent resources/structures. > + */ > +void snd_usb_rediscover_devices(void) > +{ > + int i; > + > + mutex_lock(®ister_mutex); > + for (i = 0; i < SNDRV_CARDS; i++) { > + if (usb_chip[i]) > + if (platform_ops && platform_ops->connect_cb) > + platform_ops->connect_cb(usb_chip[i]); if inside if, it can just be && or maybe move callback check before mutex lock and just return early if it is not present? > + } > + mutex_unlock(®ister_mutex); > +} > +EXPORT_SYMBOL_GPL(snd_usb_rediscover_devices); > + > /* > * disconnect streams > * called from usb_audio_disconnect() > diff --git a/sound/usb/card.h b/sound/usb/card.h > index 6d59995440c3..3a0d68f453a1 100644 > --- a/sound/usb/card.h > +++ b/sound/usb/card.h > @@ -222,11 +222,13 @@ int snd_usb_unregister_platform_ops(void); > #if IS_ENABLED(CONFIG_SND_USB_AUDIO) > struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx, > struct snd_pcm_hw_params *params, int direction); > +void snd_usb_rediscover_devices(void); > #else > static struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx, > struct snd_pcm_hw_params *params, int direction) > { > return NULL; > } > +static void snd_usb_rediscover_devices(void) { } > #endif /* IS_ENABLED(CONFIG_SND_USB_AUDIO) */ > #endif /* __USBAUDIO_CARD_H */ > diff --git a/sound/usb/qcom/qc_audio_offload.c b/sound/usb/qcom/qc_audio_offload.c > index 08af82ec22ad..9b0f98600e58 100644 > --- a/sound/usb/qcom/qc_audio_offload.c > +++ b/sound/usb/qcom/qc_audio_offload.c > @@ -1867,6 +1867,8 @@ static int __init qc_usb_audio_offload_init(void) > if (ret < 0) > goto release_qmi; > > + snd_usb_rediscover_devices(); > + > return 0; > > release_qmi: >
Hi Amadeusz, On 2/5/2024 12:20 AM, Amadeusz Sławiński wrote: > On 2/3/2024 3:36 AM, Wesley Cheng wrote: >> Some platforms may have support for offloading USB audio devices to a >> dedicated audio DSP. Introduce a set of APIs that allow for >> management of >> USB sound card and PCM devices enumerated by the USB SND class driver. >> This allows for the ASoC components to be aware of what USB devices are >> available for offloading. >> >> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> >> --- > > ... > >> + >> +/** >> + * snd_soc_usb_add_port() - Add a USB backend port >> + * @dev: USB backend device >> + * @priv: private data >> + * @connection_cb: connection status callback >> + * >> + * Register a USB backend device to the SND USB SOC framework. >> Memory is >> + * allocated as part of the USB backend device. >> + * >> + */ >> +int snd_soc_usb_add_port(struct snd_soc_usb *usb) >> +{ >> + >> + > > Cosmetic, but why is there white space between start of function and > body of function? > Thanks for catching this. Will fix it. Thanks Wesley Cheng
Hi Amadeusz, On 2/5/2024 1:01 AM, Amadeusz Sławiński wrote: > On 2/3/2024 3:36 AM, Wesley Cheng wrote: >> In case of notifying SND platform drivers of connection events, some of >> these use cases, such as offloading, require an ASoC USB backend >> device to >> be initialized before the events can be handled. If the USB backend >> device >> has not yet been probed, this leads to missing initial USB audio device >> connection events. >> >> Expose an API that traverses the usb_chip array for connected devices, >> and >> to call the respective connection callback registered to the SND platform >> driver. >> >> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> >> --- >> sound/usb/card.c | 19 +++++++++++++++++++ >> sound/usb/card.h | 2 ++ >> sound/usb/qcom/qc_audio_offload.c | 2 ++ >> 3 files changed, 23 insertions(+) >> >> diff --git a/sound/usb/card.c b/sound/usb/card.c >> index 11b827b7a2a5..995b2df676ab 100644 >> --- a/sound/usb/card.c >> +++ b/sound/usb/card.c >> @@ -202,6 +202,25 @@ struct snd_usb_stream >> *snd_usb_find_suppported_substream(int card_idx, >> } >> EXPORT_SYMBOL_GPL(snd_usb_find_suppported_substream); >> +/* >> + * in case the platform driver was not ready at the time of USB SND >> + * device connect, expose an API to discover all connected USB devices >> + * so it can populate any dependent resources/structures. >> + */ >> +void snd_usb_rediscover_devices(void) >> +{ >> + int i; >> + >> + mutex_lock(®ister_mutex); >> + for (i = 0; i < SNDRV_CARDS; i++) { >> + if (usb_chip[i]) >> + if (platform_ops && platform_ops->connect_cb) >> + platform_ops->connect_cb(usb_chip[i]); > > if inside if, it can just be && or maybe move callback check before > mutex lock and just return early if it is not present? > Thanks for pointing that out. Makes sense, and will update it. Thanks Wesley Cheng
On Sat, 03 Feb 2024 03:36:40 +0100, Wesley Cheng wrote: > > In order to allow userspace/applications know about USB offloading status, > expose a sound kcontrol that fetches information about which sound card > index is associated with the ASoC platform card supporting offloading. In > the USB audio offloading framework, the ASoC BE DAI link is the entity > responsible for registering to the SOC USB layer. SOC USB will expose more > details about the current offloading status, which includes the USB sound > card and USB PCM device indexes currently being used. > > Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> The concept is understandable, but the control element name ("SNDUSB OFFLD playback available") looks non-intrusive and non-conformant. Use a bit more understandable name instead. This provides a card number where the offload driver is bound, and the name should indicate something about that. Also, about the implementation: > +static int > +snd_usb_offload_create_mixer(struct usb_mixer_interface *mixer, > + const struct snd_kcontrol_new *new_kctl) > +{ > + struct snd_kcontrol *kctl; > + struct usb_mixer_elem_info *elem; > + > + elem = kzalloc(sizeof(struct usb_mixer_elem_info), GFP_KERNEL); > + if (!elem) > + return -ENOMEM; > + > + elem->head.mixer = mixer; > + elem->val_type = USB_MIXER_S32; > + elem->control = 0; > + elem->head.id = 0; > + elem->channels = 1; > + > + kctl = snd_ctl_new1(new_kctl, elem); > + if (!kctl) { > + kfree(elem); > + return -ENOMEM; > + } > + kctl->private_free = snd_usb_mixer_elem_free; > + > + return snd_usb_mixer_add_control(&elem->head, kctl); This control has almost little to do with the standard USB interface, and it'll be much simpler if you create a raw control element. Pass the bus or the sysdev to private_data, and that's all you need in the get callback. Also, don't forget to set the proper access bits (it's read-only). thanks, Takashi
On Mon, 05 Feb 2024 10:01:03 +0100, Amadeusz Sławiński wrote: > > On 2/3/2024 3:36 AM, Wesley Cheng wrote: > > In case of notifying SND platform drivers of connection events, some of > > these use cases, such as offloading, require an ASoC USB backend device to > > be initialized before the events can be handled. If the USB backend device > > has not yet been probed, this leads to missing initial USB audio device > > connection events. > > > > Expose an API that traverses the usb_chip array for connected devices, and > > to call the respective connection callback registered to the SND platform > > driver. > > > > Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> > > --- > > sound/usb/card.c | 19 +++++++++++++++++++ > > sound/usb/card.h | 2 ++ > > sound/usb/qcom/qc_audio_offload.c | 2 ++ > > 3 files changed, 23 insertions(+) > > > > diff --git a/sound/usb/card.c b/sound/usb/card.c > > index 11b827b7a2a5..995b2df676ab 100644 > > --- a/sound/usb/card.c > > +++ b/sound/usb/card.c > > @@ -202,6 +202,25 @@ struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx, > > } > > EXPORT_SYMBOL_GPL(snd_usb_find_suppported_substream); > > +/* > > + * in case the platform driver was not ready at the time of USB SND > > + * device connect, expose an API to discover all connected USB devices > > + * so it can populate any dependent resources/structures. > > + */ > > +void snd_usb_rediscover_devices(void) > > +{ > > + int i; > > + > > + mutex_lock(®ister_mutex); > > + for (i = 0; i < SNDRV_CARDS; i++) { > > + if (usb_chip[i]) > > + if (platform_ops && platform_ops->connect_cb) > > + platform_ops->connect_cb(usb_chip[i]); > > if inside if, it can just be && or maybe move callback check before > mutex lock and just return early if it is not present? The callback check must be inside mutex; otherwise you'll get a race about the platform_ops registration. thanks, Takashi
On Sat, 03 Feb 2024 03:36:27 +0100, Wesley Cheng wrote: > > With USB audio offloading, an audio session is started from the ASoC > platform sound card and PCM devices. Likewise, the USB SND path is still > readily available for use, in case the non-offload path is desired. In > order to prevent the two entities from attempting to use the USB bus, > introduce a flag that determines when either paths are in use. > > If a PCM device is already in use, the check will return an error to > userspace notifying that the stream is currently busy. This ensures that > only one path is using the USB substream. > > Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> Hm, I'm not sure whether it's safe to hold chip->mutex there for the long code path. It even kicks off the auto-resume, which may call various functions at resuming, and some of them may re-hold chip->mutex. If it's only about the open flag, protect only the flag access with the mutex, not covering the all open function. At least the re-entry can be avoided by that. thanks, Takashi > --- > sound/usb/card.h | 1 + > sound/usb/pcm.c | 19 +++++++++++++++++-- > 2 files changed, 18 insertions(+), 2 deletions(-) > > diff --git a/sound/usb/card.h b/sound/usb/card.h > index ed4a664e24e5..6d59995440c3 100644 > --- a/sound/usb/card.h > +++ b/sound/usb/card.h > @@ -165,6 +165,7 @@ struct snd_usb_substream { > unsigned int pkt_offset_adj; /* Bytes to drop from beginning of packets (for non-compliant devices) */ > unsigned int stream_offset_adj; /* Bytes to drop from beginning of stream (for non-compliant devices) */ > > + unsigned int opened:1; /* pcm device opened */ > unsigned int running: 1; /* running status */ > unsigned int period_elapsed_pending; /* delay period handling */ > > diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c > index 3adb09ce1702..c2cb52cd5d23 100644 > --- a/sound/usb/pcm.c > +++ b/sound/usb/pcm.c > @@ -1241,8 +1241,15 @@ static int snd_usb_pcm_open(struct snd_pcm_substream *substream) > struct snd_usb_stream *as = snd_pcm_substream_chip(substream); > struct snd_pcm_runtime *runtime = substream->runtime; > struct snd_usb_substream *subs = &as->substream[direction]; > + struct snd_usb_audio *chip = subs->stream->chip; > int ret; > > + mutex_lock(&chip->mutex); > + if (subs->opened) { > + mutex_unlock(&chip->mutex); > + return -EBUSY; > + } > + > runtime->hw = snd_usb_hardware; > /* need an explicit sync to catch applptr update in low-latency mode */ > if (direction == SNDRV_PCM_STREAM_PLAYBACK && > @@ -1259,13 +1266,17 @@ static int snd_usb_pcm_open(struct snd_pcm_substream *substream) > > ret = setup_hw_info(runtime, subs); > if (ret < 0) > - return ret; > + goto out; > ret = snd_usb_autoresume(subs->stream->chip); > if (ret < 0) > - return ret; > + goto out; > ret = snd_media_stream_init(subs, as->pcm, direction); > if (ret < 0) > snd_usb_autosuspend(subs->stream->chip); > + subs->opened = 1; > +out: > + mutex_unlock(&chip->mutex); > + > return ret; > } > > @@ -1274,6 +1285,7 @@ static int snd_usb_pcm_close(struct snd_pcm_substream *substream) > int direction = substream->stream; > struct snd_usb_stream *as = snd_pcm_substream_chip(substream); > struct snd_usb_substream *subs = &as->substream[direction]; > + struct snd_usb_audio *chip = subs->stream->chip; > int ret; > > snd_media_stop_pipeline(subs); > @@ -1287,6 +1299,9 @@ static int snd_usb_pcm_close(struct snd_pcm_substream *substream) > > subs->pcm_substream = NULL; > snd_usb_autosuspend(subs->stream->chip); > + mutex_lock(&chip->mutex); > + subs->opened = 0; > + mutex_unlock(&chip->mutex); > > return 0; > }
On Sat, 03 Feb 2024 03:36:24 +0100, Wesley Cheng wrote: > > Allow for checks on a specific USB audio device to see if a requested PCM > format is supported. This is needed for support when playback is > initiated by the ASoC USB backend path. > > Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> Just cosmetic: > +struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx, > + struct snd_pcm_hw_params *params, int direction) > +{ > + struct snd_usb_audio *chip; > + struct snd_usb_substream *subs; > + struct snd_usb_stream *as; > + const struct audioformat *fmt; > + > + /* > + * Register mutex is held when populating and clearing usb_chip > + * array. > + */ > + mutex_lock(®ister_mutex); > + chip = usb_chip[card_idx]; > + if (!chip) { > + mutex_unlock(®ister_mutex); > + return NULL; > + } > + > + if (enable[card_idx]) { > + list_for_each_entry(as, &chip->pcm_list, list) { > + subs = &as->substream[direction]; > + fmt = snd_usb_find_substream_format(subs, params); > + if (fmt) { > + mutex_unlock(®ister_mutex); > + return as; > + } > + } > + } > + mutex_unlock(®ister_mutex); I prefer having the single lock/unlock call pair, e.g. struct snd_usb_stream *as, *ret; ret = NULL; mutex_lock(®ister_mutex); chip = usb_chip[card_idx]; if (chip && enable[card_idx]) { list_for_each_entry(as, &chip->pcm_list, list) { subs = &as->substream[direction]; if (snd_usb_find_substream_format(subs, params)) { ret = as; break; } } } mutex_unlock(®ister_mutex); return ret; } In this case, we shouldn't reuse "as" for the return value since it can be non-NULL after the loop end. thanks, Takashi
On Tue, Feb 06, 2024 at 02:12:44PM +0100, Takashi Iwai wrote: > On Sat, 03 Feb 2024 03:36:24 +0100, > Wesley Cheng wrote: > > > > Allow for checks on a specific USB audio device to see if a requested PCM > > format is supported. This is needed for support when playback is > > initiated by the ASoC USB backend path. > > > > Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> > > Just cosmetic: > > > +struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx, > > + struct snd_pcm_hw_params *params, int direction) > > +{ > > + struct snd_usb_audio *chip; > > + struct snd_usb_substream *subs; > > + struct snd_usb_stream *as; > > + const struct audioformat *fmt; > > + > > + /* > > + * Register mutex is held when populating and clearing usb_chip > > + * array. > > + */ > > + mutex_lock(®ister_mutex); > > + chip = usb_chip[card_idx]; > > + if (!chip) { > > + mutex_unlock(®ister_mutex); > > + return NULL; > > + } > > + > > + if (enable[card_idx]) { > > + list_for_each_entry(as, &chip->pcm_list, list) { > > + subs = &as->substream[direction]; > > + fmt = snd_usb_find_substream_format(subs, params); > > + if (fmt) { > > + mutex_unlock(®ister_mutex); > > + return as; > > + } > > + } > > + } > > + mutex_unlock(®ister_mutex); > > I prefer having the single lock/unlock call pair, e.g. > > struct snd_usb_stream *as, *ret; > > ret = NULL; > mutex_lock(®ister_mutex); > chip = usb_chip[card_idx]; > if (chip && enable[card_idx]) { > list_for_each_entry(as, &chip->pcm_list, list) { > subs = &as->substream[direction]; > if (snd_usb_find_substream_format(subs, params)) { > ret = as; > break; > } > } > } > mutex_unlock(®ister_mutex); > return ret; > } > > In this case, we shouldn't reuse "as" for the return value since it > can be non-NULL after the loop end. Why not just use guard(mutex) for this, making it all not an issue? thanks, greg k-h
On Tue, 06 Feb 2024 15:50:21 +0100, Greg KH wrote: > > On Tue, Feb 06, 2024 at 02:12:44PM +0100, Takashi Iwai wrote: > > On Sat, 03 Feb 2024 03:36:24 +0100, > > Wesley Cheng wrote: > > > > > > Allow for checks on a specific USB audio device to see if a requested PCM > > > format is supported. This is needed for support when playback is > > > initiated by the ASoC USB backend path. > > > > > > Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> > > > > Just cosmetic: > > > > > +struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx, > > > + struct snd_pcm_hw_params *params, int direction) > > > +{ > > > + struct snd_usb_audio *chip; > > > + struct snd_usb_substream *subs; > > > + struct snd_usb_stream *as; > > > + const struct audioformat *fmt; > > > + > > > + /* > > > + * Register mutex is held when populating and clearing usb_chip > > > + * array. > > > + */ > > > + mutex_lock(®ister_mutex); > > > + chip = usb_chip[card_idx]; > > > + if (!chip) { > > > + mutex_unlock(®ister_mutex); > > > + return NULL; > > > + } > > > + > > > + if (enable[card_idx]) { > > > + list_for_each_entry(as, &chip->pcm_list, list) { > > > + subs = &as->substream[direction]; > > > + fmt = snd_usb_find_substream_format(subs, params); > > > + if (fmt) { > > > + mutex_unlock(®ister_mutex); > > > + return as; > > > + } > > > + } > > > + } > > > + mutex_unlock(®ister_mutex); > > > > I prefer having the single lock/unlock call pair, e.g. > > > > struct snd_usb_stream *as, *ret; > > > > ret = NULL; > > mutex_lock(®ister_mutex); > > chip = usb_chip[card_idx]; > > if (chip && enable[card_idx]) { > > list_for_each_entry(as, &chip->pcm_list, list) { > > subs = &as->substream[direction]; > > if (snd_usb_find_substream_format(subs, params)) { > > ret = as; > > break; > > } > > } > > } > > mutex_unlock(®ister_mutex); > > return ret; > > } > > > > In this case, we shouldn't reuse "as" for the return value since it > > can be non-NULL after the loop end. > > Why not just use guard(mutex) for this, making it all not an issue? Heh, it's too new ;) That should work gracefully, yes. Takashi
Hi Takashi, On 2/6/2024 5:07 AM, Takashi Iwai wrote: > On Sat, 03 Feb 2024 03:36:27 +0100, > Wesley Cheng wrote: >> >> With USB audio offloading, an audio session is started from the ASoC >> platform sound card and PCM devices. Likewise, the USB SND path is still >> readily available for use, in case the non-offload path is desired. In >> order to prevent the two entities from attempting to use the USB bus, >> introduce a flag that determines when either paths are in use. >> >> If a PCM device is already in use, the check will return an error to >> userspace notifying that the stream is currently busy. This ensures that >> only one path is using the USB substream. >> >> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> > > Hm, I'm not sure whether it's safe to hold chip->mutex there for the > long code path. It even kicks off the auto-resume, which may call > various functions at resuming, and some of them may re-hold > chip->mutex. > That's a good point. > If it's only about the open flag, protect only the flag access with > the mutex, not covering the all open function. At least the re-entry > can be avoided by that. > Sure, let me re-order the check/assignment and the mutex locking. Since this is now checked here in USB PCM and the QC offload driver, we want to make sure that if there was some application attempting to open both at the same time, we prevent any possible races. I think the best way to address this would be something like: static int snd_usb_pcm_open(struct snd_pcm_substream *substream) { ... mutex_lock(&chip->mutex); if (subs->opened) { mutex_unlock(&chip->mutex); return -EBUSY; } subs->opened = 1; mutex_unlock(&chip->mutex); //Execute bulk of PCM open routine ... return 0; // If any errors are seen, unwind err_resume: snd_usb_autosuspend(subs->stream->chip); err_open: mutex_lock(&chip->mutex); subs->opened = 0; mutex_unlock(&chip->mutex); return ret; } Set the opened flag first, so that if QC offload checks it, it can exit early and vice versa. Otherwise, if we set the opened flag at the same position as the previous patch, we may be calling the other routines in parallel to the QC offload enable stream routine. The only thing with this patch is that we'd need some error handling unwinding. Thanks Wesley Cheng
Hi Takashi, On 2/6/2024 5:12 AM, Takashi Iwai wrote: > On Sat, 03 Feb 2024 03:36:24 +0100, > Wesley Cheng wrote: >> >> Allow for checks on a specific USB audio device to see if a requested PCM >> format is supported. This is needed for support when playback is >> initiated by the ASoC USB backend path. >> >> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> > > Just cosmetic: > >> +struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx, >> + struct snd_pcm_hw_params *params, int direction) >> +{ >> + struct snd_usb_audio *chip; >> + struct snd_usb_substream *subs; >> + struct snd_usb_stream *as; >> + const struct audioformat *fmt; >> + >> + /* >> + * Register mutex is held when populating and clearing usb_chip >> + * array. >> + */ >> + mutex_lock(®ister_mutex); >> + chip = usb_chip[card_idx]; >> + if (!chip) { >> + mutex_unlock(®ister_mutex); >> + return NULL; >> + } >> + >> + if (enable[card_idx]) { >> + list_for_each_entry(as, &chip->pcm_list, list) { >> + subs = &as->substream[direction]; >> + fmt = snd_usb_find_substream_format(subs, params); >> + if (fmt) { >> + mutex_unlock(®ister_mutex); >> + return as; >> + } >> + } >> + } >> + mutex_unlock(®ister_mutex); > > I prefer having the single lock/unlock call pair, e.g. > > struct snd_usb_stream *as, *ret; > > ret = NULL; > mutex_lock(®ister_mutex); > chip = usb_chip[card_idx]; > if (chip && enable[card_idx]) { > list_for_each_entry(as, &chip->pcm_list, list) { > subs = &as->substream[direction]; > if (snd_usb_find_substream_format(subs, params)) { > ret = as; > break; > } > } > } > mutex_unlock(®ister_mutex); > return ret; > } > > In this case, we shouldn't reuse "as" for the return value since it > can be non-NULL after the loop end. > > Sure will do, thanks for taking a look. Thanks Wesley Cheng
Hi Takashi, On 2/6/2024 4:57 AM, Takashi Iwai wrote: > On Sat, 03 Feb 2024 03:36:40 +0100, > Wesley Cheng wrote: >> >> In order to allow userspace/applications know about USB offloading status, >> expose a sound kcontrol that fetches information about which sound card >> index is associated with the ASoC platform card supporting offloading. In >> the USB audio offloading framework, the ASoC BE DAI link is the entity >> responsible for registering to the SOC USB layer. SOC USB will expose more >> details about the current offloading status, which includes the USB sound >> card and USB PCM device indexes currently being used. >> >> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> > > The concept is understandable, but the control element name ("SNDUSB > OFFLD playback available") looks non-intrusive and non-conformant. > Use a bit more understandable name instead. > > This provides a card number where the offload driver is bound, and the > name should indicate something about that. > Hmmm, does USB sound have a naming convention that it usually follows for mixer/control interfaces? For something that is more closely related, how about: "USB offload capable card" > Also, about the implementation: > >> +static int >> +snd_usb_offload_create_mixer(struct usb_mixer_interface *mixer, >> + const struct snd_kcontrol_new *new_kctl) >> +{ >> + struct snd_kcontrol *kctl; >> + struct usb_mixer_elem_info *elem; >> + >> + elem = kzalloc(sizeof(struct usb_mixer_elem_info), GFP_KERNEL); >> + if (!elem) >> + return -ENOMEM; >> + >> + elem->head.mixer = mixer; >> + elem->val_type = USB_MIXER_S32; >> + elem->control = 0; >> + elem->head.id = 0; >> + elem->channels = 1; >> + >> + kctl = snd_ctl_new1(new_kctl, elem); >> + if (!kctl) { >> + kfree(elem); >> + return -ENOMEM; >> + } >> + kctl->private_free = snd_usb_mixer_elem_free; >> + >> + return snd_usb_mixer_add_control(&elem->head, kctl); > > This control has almost little to do with the standard USB interface, > and it'll be much simpler if you create a raw control element. > Pass the bus or the sysdev to private_data, and that's all you need in > the get callback. > Sure, I'll remove the need to register over the SND USB mixer API, and just use the core SND control APIs. > Also, don't forget to set the proper access bits (it's read-only). > Thanks for pointing this out, will fix. Thanks Wesley Cheng > > thanks, > > Takashi
On Wed, 07 Feb 2024 02:24:46 +0100, Wesley Cheng wrote: > > Hi Takashi, > > On 2/6/2024 4:57 AM, Takashi Iwai wrote: > > On Sat, 03 Feb 2024 03:36:40 +0100, > > Wesley Cheng wrote: > >> > >> In order to allow userspace/applications know about USB offloading status, > >> expose a sound kcontrol that fetches information about which sound card > >> index is associated with the ASoC platform card supporting offloading. In > >> the USB audio offloading framework, the ASoC BE DAI link is the entity > >> responsible for registering to the SOC USB layer. SOC USB will expose more > >> details about the current offloading status, which includes the USB sound > >> card and USB PCM device indexes currently being used. > >> > >> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> > > > > The concept is understandable, but the control element name ("SNDUSB > > OFFLD playback available") looks non-intrusive and non-conformant. > > Use a bit more understandable name instead. > > > > This provides a card number where the offload driver is bound, and the > > name should indicate something about that. > > > > Hmmm, does USB sound have a naming convention that it usually follows > for mixer/control interfaces? The old rule is found in Documentation/sound/designs/control-names.rst (although the prefix and the suffix are often dropped for non-standard controls). > For something that is more closely related, how about: > "USB offload capable card" Yes, it looks better. But usually each word begins with an upper letter. Takashi
On Wed, 07 Feb 2024 01:08:00 +0100, Wesley Cheng wrote: > > Hi Takashi, > > On 2/6/2024 5:07 AM, Takashi Iwai wrote: > > On Sat, 03 Feb 2024 03:36:27 +0100, > > Wesley Cheng wrote: > >> > >> With USB audio offloading, an audio session is started from the ASoC > >> platform sound card and PCM devices. Likewise, the USB SND path is still > >> readily available for use, in case the non-offload path is desired. In > >> order to prevent the two entities from attempting to use the USB bus, > >> introduce a flag that determines when either paths are in use. > >> > >> If a PCM device is already in use, the check will return an error to > >> userspace notifying that the stream is currently busy. This ensures that > >> only one path is using the USB substream. > >> > >> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> > > > > Hm, I'm not sure whether it's safe to hold chip->mutex there for the > > long code path. It even kicks off the auto-resume, which may call > > various functions at resuming, and some of them may re-hold > > chip->mutex. > > > > That's a good point. > > > If it's only about the open flag, protect only the flag access with > > the mutex, not covering the all open function. At least the re-entry > > can be avoided by that. > > > > Sure, let me re-order the check/assignment and the mutex locking. > Since this is now checked here in USB PCM and the QC offload driver, > we want to make sure that if there was some application attempting to > open both at the same time, we prevent any possible races. > > I think the best way to address this would be something like: > > static int snd_usb_pcm_open(struct snd_pcm_substream *substream) > { > ... > mutex_lock(&chip->mutex); > if (subs->opened) { > mutex_unlock(&chip->mutex); > return -EBUSY; > } > subs->opened = 1; > mutex_unlock(&chip->mutex); > > //Execute bulk of PCM open routine > ... > return 0; > > // If any errors are seen, unwind > err_resume: > snd_usb_autosuspend(subs->stream->chip); > err_open: > mutex_lock(&chip->mutex); > subs->opened = 0; > mutex_unlock(&chip->mutex); > > return ret; > } > > Set the opened flag first, so that if QC offload checks it, it can > exit early and vice versa. Otherwise, if we set the opened flag at > the same position as the previous patch, we may be calling the other > routines in parallel to the QC offload enable stream routine. The > only thing with this patch is that we'd need some error handling > unwinding. The above is what I had in mind. But, thinking on this again, you might be able to get the same result by using the ALSA PCM core substream open_mutex and hw_opened flag. This is already held and set at snd_pcm_core() (the hw_opened flag is set after open callback, though). The offload driver can use those instead of the own lock and flag, too, although it's not really well-mannered behavior (hence you need proper comments). thanks, Takashi
Hi Takashi, On 2/6/2024 11:05 PM, Takashi Iwai wrote: > On Wed, 07 Feb 2024 01:08:00 +0100, > Wesley Cheng wrote: >> >> Hi Takashi, >> >> On 2/6/2024 5:07 AM, Takashi Iwai wrote: >>> On Sat, 03 Feb 2024 03:36:27 +0100, >>> Wesley Cheng wrote: >>>> >>>> With USB audio offloading, an audio session is started from the ASoC >>>> platform sound card and PCM devices. Likewise, the USB SND path is still >>>> readily available for use, in case the non-offload path is desired. In >>>> order to prevent the two entities from attempting to use the USB bus, >>>> introduce a flag that determines when either paths are in use. >>>> >>>> If a PCM device is already in use, the check will return an error to >>>> userspace notifying that the stream is currently busy. This ensures that >>>> only one path is using the USB substream. >>>> >>>> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> >>> >>> Hm, I'm not sure whether it's safe to hold chip->mutex there for the >>> long code path. It even kicks off the auto-resume, which may call >>> various functions at resuming, and some of them may re-hold >>> chip->mutex. >>> >> >> That's a good point. >> >>> If it's only about the open flag, protect only the flag access with >>> the mutex, not covering the all open function. At least the re-entry >>> can be avoided by that. >>> >> >> Sure, let me re-order the check/assignment and the mutex locking. >> Since this is now checked here in USB PCM and the QC offload driver, >> we want to make sure that if there was some application attempting to >> open both at the same time, we prevent any possible races. >> >> I think the best way to address this would be something like: >> >> static int snd_usb_pcm_open(struct snd_pcm_substream *substream) >> { >> ... >> mutex_lock(&chip->mutex); >> if (subs->opened) { >> mutex_unlock(&chip->mutex); >> return -EBUSY; >> } >> subs->opened = 1; >> mutex_unlock(&chip->mutex); >> >> //Execute bulk of PCM open routine >> ... >> return 0; >> >> // If any errors are seen, unwind >> err_resume: >> snd_usb_autosuspend(subs->stream->chip); >> err_open: >> mutex_lock(&chip->mutex); >> subs->opened = 0; >> mutex_unlock(&chip->mutex); >> >> return ret; >> } >> >> Set the opened flag first, so that if QC offload checks it, it can >> exit early and vice versa. Otherwise, if we set the opened flag at >> the same position as the previous patch, we may be calling the other >> routines in parallel to the QC offload enable stream routine. The >> only thing with this patch is that we'd need some error handling >> unwinding. > > The above is what I had in mind. > > But, thinking on this again, you might be able to get the same result > by using the ALSA PCM core substream open_mutex and hw_opened flag. > This is already held and set at snd_pcm_core() (the hw_opened flag is > set after open callback, though). The offload driver can use those > instead of the own lock and flag, too, although it's not really > well-mannered behavior (hence you need proper comments). > I think I had looked into this as well previously, and it was difficult to achieve, because from the USB offloading perspective, we don't ever call: snd_usb_pcm_open() This is actually where we populate the pcm_substream parameter within struct snd_usb_substream based on when userspace opens the USB SND PCM device (which is not the case for offloading). So the offload driver doesn't have a way to fetch the struct snd_pcm that is allocated to the PCM device created by the USB SND card. Thanks Wesley Cheng
On 2/6/2024 6:53 AM, Takashi Iwai wrote: > On Tue, 06 Feb 2024 15:50:21 +0100, > Greg KH wrote: >> >> On Tue, Feb 06, 2024 at 02:12:44PM +0100, Takashi Iwai wrote: >>> On Sat, 03 Feb 2024 03:36:24 +0100, >>> Wesley Cheng wrote: >>>> >>>> Allow for checks on a specific USB audio device to see if a requested PCM >>>> format is supported. This is needed for support when playback is >>>> initiated by the ASoC USB backend path. >>>> >>>> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> >>> >>> Just cosmetic: >>> >>>> +struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx, >>>> + struct snd_pcm_hw_params *params, int direction) >>>> +{ >>>> + struct snd_usb_audio *chip; >>>> + struct snd_usb_substream *subs; >>>> + struct snd_usb_stream *as; >>>> + const struct audioformat *fmt; >>>> + >>>> + /* >>>> + * Register mutex is held when populating and clearing usb_chip >>>> + * array. >>>> + */ >>>> + mutex_lock(®ister_mutex); >>>> + chip = usb_chip[card_idx]; >>>> + if (!chip) { >>>> + mutex_unlock(®ister_mutex); >>>> + return NULL; >>>> + } >>>> + >>>> + if (enable[card_idx]) { >>>> + list_for_each_entry(as, &chip->pcm_list, list) { >>>> + subs = &as->substream[direction]; >>>> + fmt = snd_usb_find_substream_format(subs, params); >>>> + if (fmt) { >>>> + mutex_unlock(®ister_mutex); >>>> + return as; >>>> + } >>>> + } >>>> + } >>>> + mutex_unlock(®ister_mutex); >>> >>> I prefer having the single lock/unlock call pair, e.g. >>> >>> struct snd_usb_stream *as, *ret; >>> >>> ret = NULL; >>> mutex_lock(®ister_mutex); >>> chip = usb_chip[card_idx]; >>> if (chip && enable[card_idx]) { >>> list_for_each_entry(as, &chip->pcm_list, list) { >>> subs = &as->substream[direction]; >>> if (snd_usb_find_substream_format(subs, params)) { >>> ret = as; >>> break; >>> } >>> } >>> } >>> mutex_unlock(®ister_mutex); >>> return ret; >>> } >>> >>> In this case, we shouldn't reuse "as" for the return value since it >>> can be non-NULL after the loop end. >> >> Why not just use guard(mutex) for this, making it all not an issue? > > Heh, it's too new ;) > That should work gracefully, yes. > Thanks Greg/Takashi. That is nifty. Thanks Wesley Cheng
Hi Takashi, On 2/7/2024 4:02 PM, Wesley Cheng wrote: > Hi Takashi, > > On 2/6/2024 11:05 PM, Takashi Iwai wrote: >> On Wed, 07 Feb 2024 01:08:00 +0100, >> Wesley Cheng wrote: >>> >>> Hi Takashi, >>> >>> On 2/6/2024 5:07 AM, Takashi Iwai wrote: >>>> On Sat, 03 Feb 2024 03:36:27 +0100, >>>> Wesley Cheng wrote: >>>>> >>>>> With USB audio offloading, an audio session is started from the ASoC >>>>> platform sound card and PCM devices. Likewise, the USB SND path is >>>>> still >>>>> readily available for use, in case the non-offload path is >>>>> desired. In >>>>> order to prevent the two entities from attempting to use the USB bus, >>>>> introduce a flag that determines when either paths are in use. >>>>> >>>>> If a PCM device is already in use, the check will return an error to >>>>> userspace notifying that the stream is currently busy. This >>>>> ensures that >>>>> only one path is using the USB substream. >>>>> >>>>> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> >>>> >>>> Hm, I'm not sure whether it's safe to hold chip->mutex there for the >>>> long code path. It even kicks off the auto-resume, which may call >>>> various functions at resuming, and some of them may re-hold >>>> chip->mutex. >>>> >>> >>> That's a good point. >>> >>>> If it's only about the open flag, protect only the flag access with >>>> the mutex, not covering the all open function. At least the re-entry >>>> can be avoided by that. >>>> >>> >>> Sure, let me re-order the check/assignment and the mutex locking. >>> Since this is now checked here in USB PCM and the QC offload driver, >>> we want to make sure that if there was some application attempting to >>> open both at the same time, we prevent any possible races. >>> >>> I think the best way to address this would be something like: >>> >>> static int snd_usb_pcm_open(struct snd_pcm_substream *substream) >>> { >>> ... >>> mutex_lock(&chip->mutex); >>> if (subs->opened) { >>> mutex_unlock(&chip->mutex); >>> return -EBUSY; >>> } >>> subs->opened = 1; >>> mutex_unlock(&chip->mutex); >>> >>> //Execute bulk of PCM open routine >>> ... >>> return 0; >>> >>> // If any errors are seen, unwind >>> err_resume: >>> snd_usb_autosuspend(subs->stream->chip); >>> err_open: >>> mutex_lock(&chip->mutex); >>> subs->opened = 0; >>> mutex_unlock(&chip->mutex); >>> >>> return ret; >>> } >>> >>> Set the opened flag first, so that if QC offload checks it, it can >>> exit early and vice versa. Otherwise, if we set the opened flag at >>> the same position as the previous patch, we may be calling the other >>> routines in parallel to the QC offload enable stream routine. The >>> only thing with this patch is that we'd need some error handling >>> unwinding. >> >> The above is what I had in mind. >> >> But, thinking on this again, you might be able to get the same result >> by using the ALSA PCM core substream open_mutex and hw_opened flag. >> This is already held and set at snd_pcm_core() (the hw_opened flag is >> set after open callback, though). The offload driver can use those >> instead of the own lock and flag, too, although it's not really >> well-mannered behavior (hence you need proper comments). >> > > I think I had looked into this as well previously, and it was difficult > to achieve, because from the USB offloading perspective, we don't ever > call: snd_usb_pcm_open() > > This is actually where we populate the pcm_substream parameter within > struct snd_usb_substream based on when userspace opens the USB SND PCM > device (which is not the case for offloading). So the offload driver > doesn't have a way to fetch the struct snd_pcm that is allocated to the > PCM device created by the USB SND card. > Sorry, took a look at it again, and found a way. Although not pretty, we can access it using: subs->stream->pcm->streams[direction].substream->hw_opened Thanks Wesley Cheng
On Thu, 08 Feb 2024 02:12:00 +0100, Wesley Cheng wrote: > > Hi Takashi, > > On 2/7/2024 4:02 PM, Wesley Cheng wrote: > > Hi Takashi, > > > > On 2/6/2024 11:05 PM, Takashi Iwai wrote: > >> On Wed, 07 Feb 2024 01:08:00 +0100, > >> Wesley Cheng wrote: > >>> > >>> Hi Takashi, > >>> > >>> On 2/6/2024 5:07 AM, Takashi Iwai wrote: > >>>> On Sat, 03 Feb 2024 03:36:27 +0100, > >>>> Wesley Cheng wrote: > >>>>> > >>>>> With USB audio offloading, an audio session is started from the ASoC > >>>>> platform sound card and PCM devices. Likewise, the USB SND path > >>>>> is still > >>>>> readily available for use, in case the non-offload path is > >>>>> desired. In > >>>>> order to prevent the two entities from attempting to use the USB bus, > >>>>> introduce a flag that determines when either paths are in use. > >>>>> > >>>>> If a PCM device is already in use, the check will return an error to > >>>>> userspace notifying that the stream is currently busy. This > >>>>> ensures that > >>>>> only one path is using the USB substream. > >>>>> > >>>>> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> > >>>> > >>>> Hm, I'm not sure whether it's safe to hold chip->mutex there for the > >>>> long code path. It even kicks off the auto-resume, which may call > >>>> various functions at resuming, and some of them may re-hold > >>>> chip->mutex. > >>>> > >>> > >>> That's a good point. > >>> > >>>> If it's only about the open flag, protect only the flag access with > >>>> the mutex, not covering the all open function. At least the re-entry > >>>> can be avoided by that. > >>>> > >>> > >>> Sure, let me re-order the check/assignment and the mutex locking. > >>> Since this is now checked here in USB PCM and the QC offload driver, > >>> we want to make sure that if there was some application attempting to > >>> open both at the same time, we prevent any possible races. > >>> > >>> I think the best way to address this would be something like: > >>> > >>> static int snd_usb_pcm_open(struct snd_pcm_substream *substream) > >>> { > >>> ... > >>> mutex_lock(&chip->mutex); > >>> if (subs->opened) { > >>> mutex_unlock(&chip->mutex); > >>> return -EBUSY; > >>> } > >>> subs->opened = 1; > >>> mutex_unlock(&chip->mutex); > >>> > >>> //Execute bulk of PCM open routine > >>> ... > >>> return 0; > >>> > >>> // If any errors are seen, unwind > >>> err_resume: > >>> snd_usb_autosuspend(subs->stream->chip); > >>> err_open: > >>> mutex_lock(&chip->mutex); > >>> subs->opened = 0; > >>> mutex_unlock(&chip->mutex); > >>> > >>> return ret; > >>> } > >>> > >>> Set the opened flag first, so that if QC offload checks it, it can > >>> exit early and vice versa. Otherwise, if we set the opened flag at > >>> the same position as the previous patch, we may be calling the other > >>> routines in parallel to the QC offload enable stream routine. The > >>> only thing with this patch is that we'd need some error handling > >>> unwinding. > >> > >> The above is what I had in mind. > >> > >> But, thinking on this again, you might be able to get the same result > >> by using the ALSA PCM core substream open_mutex and hw_opened flag. > >> This is already held and set at snd_pcm_core() (the hw_opened flag is > >> set after open callback, though). The offload driver can use those > >> instead of the own lock and flag, too, although it's not really > >> well-mannered behavior (hence you need proper comments). > >> > > > > I think I had looked into this as well previously, and it was > > difficult to achieve, because from the USB offloading perspective, > > we don't ever call: snd_usb_pcm_open() > > > > This is actually where we populate the pcm_substream parameter > > within struct snd_usb_substream based on when userspace opens the > > USB SND PCM device (which is not the case for offloading). So the > > offload driver doesn't have a way to fetch the struct snd_pcm that > > is allocated to the PCM device created by the USB SND card. > > > > Sorry, took a look at it again, and found a way. Although not pretty, > we can access it using: > subs->stream->pcm->streams[direction].substream->hw_opened Yes, it's not easy to follow it. So if we want to this path, worth for a detailed comment. That said, I don't mind to introduce the new local mutex and flag as you did if the above became too messy in the end. thanks, Takashi
Hi Takashi, On 2/8/2024 12:33 AM, Takashi Iwai wrote: > On Thu, 08 Feb 2024 02:12:00 +0100, > Wesley Cheng wrote: >> >> Hi Takashi, >> >> On 2/7/2024 4:02 PM, Wesley Cheng wrote: >>> Hi Takashi, >>> >>> On 2/6/2024 11:05 PM, Takashi Iwai wrote: >>>> On Wed, 07 Feb 2024 01:08:00 +0100, >>>> Wesley Cheng wrote: >>>>> >>>>> Hi Takashi, >>>>> >>>>> On 2/6/2024 5:07 AM, Takashi Iwai wrote: >>>>>> On Sat, 03 Feb 2024 03:36:27 +0100, >>>>>> Wesley Cheng wrote: >>>>>>> >>>>>>> With USB audio offloading, an audio session is started from the ASoC >>>>>>> platform sound card and PCM devices. Likewise, the USB SND path >>>>>>> is still >>>>>>> readily available for use, in case the non-offload path is >>>>>>> desired. In >>>>>>> order to prevent the two entities from attempting to use the USB bus, >>>>>>> introduce a flag that determines when either paths are in use. >>>>>>> >>>>>>> If a PCM device is already in use, the check will return an error to >>>>>>> userspace notifying that the stream is currently busy. This >>>>>>> ensures that >>>>>>> only one path is using the USB substream. >>>>>>> >>>>>>> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com> >>>>>> >>>>>> Hm, I'm not sure whether it's safe to hold chip->mutex there for the >>>>>> long code path. It even kicks off the auto-resume, which may call >>>>>> various functions at resuming, and some of them may re-hold >>>>>> chip->mutex. >>>>>> >>>>> >>>>> That's a good point. >>>>> >>>>>> If it's only about the open flag, protect only the flag access with >>>>>> the mutex, not covering the all open function. At least the re-entry >>>>>> can be avoided by that. >>>>>> >>>>> >>>>> Sure, let me re-order the check/assignment and the mutex locking. >>>>> Since this is now checked here in USB PCM and the QC offload driver, >>>>> we want to make sure that if there was some application attempting to >>>>> open both at the same time, we prevent any possible races. >>>>> >>>>> I think the best way to address this would be something like: >>>>> >>>>> static int snd_usb_pcm_open(struct snd_pcm_substream *substream) >>>>> { >>>>> ... >>>>> mutex_lock(&chip->mutex); >>>>> if (subs->opened) { >>>>> mutex_unlock(&chip->mutex); >>>>> return -EBUSY; >>>>> } >>>>> subs->opened = 1; >>>>> mutex_unlock(&chip->mutex); >>>>> >>>>> //Execute bulk of PCM open routine >>>>> ... >>>>> return 0; >>>>> >>>>> // If any errors are seen, unwind >>>>> err_resume: >>>>> snd_usb_autosuspend(subs->stream->chip); >>>>> err_open: >>>>> mutex_lock(&chip->mutex); >>>>> subs->opened = 0; >>>>> mutex_unlock(&chip->mutex); >>>>> >>>>> return ret; >>>>> } >>>>> >>>>> Set the opened flag first, so that if QC offload checks it, it can >>>>> exit early and vice versa. Otherwise, if we set the opened flag at >>>>> the same position as the previous patch, we may be calling the other >>>>> routines in parallel to the QC offload enable stream routine. The >>>>> only thing with this patch is that we'd need some error handling >>>>> unwinding. >>>> >>>> The above is what I had in mind. >>>> >>>> But, thinking on this again, you might be able to get the same result >>>> by using the ALSA PCM core substream open_mutex and hw_opened flag. >>>> This is already held and set at snd_pcm_core() (the hw_opened flag is >>>> set after open callback, though). The offload driver can use those >>>> instead of the own lock and flag, too, although it's not really >>>> well-mannered behavior (hence you need proper comments). >>>> >>> >>> I think I had looked into this as well previously, and it was >>> difficult to achieve, because from the USB offloading perspective, >>> we don't ever call: snd_usb_pcm_open() >>> >>> This is actually where we populate the pcm_substream parameter >>> within struct snd_usb_substream based on when userspace opens the >>> USB SND PCM device (which is not the case for offloading). So the >>> offload driver doesn't have a way to fetch the struct snd_pcm that >>> is allocated to the PCM device created by the USB SND card. >>> >> >> Sorry, took a look at it again, and found a way. Although not pretty, >> we can access it using: >> subs->stream->pcm->streams[direction].substream->hw_opened > > Yes, it's not easy to follow it. So if we want to this path, worth > for a detailed comment. That said, I don't mind to introduce the new > local mutex and flag as you did if the above became too messy in the > end. > If you don't mind, I prefer to keep it the way it was with the local mutex and flag. It makes it a lot easier to follow, and for other users to adopt as well compared to the long equation above :). Thanks Wesley Cheng