| Message ID | 20210626054113.246309-1-marcan@marcan.st |
|---|---|
| State | Needs Review / ACK |
| Headers | show |
| Series | [v3] i2c: i801: Safely share SMBus with BIOS/ACPI | expand |
On Sat, Jun 26, 2021 at 02:41:13PM +0900, Hector Martin wrote: > The i801 controller provides a locking mechanism that the OS is supposed > to use to safely share the SMBus with ACPI AML or other firmware. > > Previously, Linux attempted to get out of the way of ACPI AML entirely, > but left the bus locked if it used it before the first AML access. This > causes AML implementations that *do* attempt to safely share the bus > to time out if Linux uses it first; notably, this regressed ACPI video > backlight controls on 2015 iMacs after 01590f361e started instantiating > SPD EEPROMs on boot. > > Commit 065b6211a8 fixed the immediate problem of leaving the bus locked, > but we can do better. The controller does have a proper locking mechanism, > so let's use it as intended. Since we can't rely on the BIOS doing this > properly, we implement the following logic: > > - If ACPI AML uses the bus at all, we make a note and disable power > management. The latter matches already existing behavior. > - When we want to use the bus, we attempt to lock it first. If the > locking attempt times out, *and* ACPI hasn't tried to use the bus at > all yet, we cautiously go ahead and assume the BIOS forgot to unlock > the bus after boot. This preserves existing behavior. > - We always unlock the bus after a transfer. > - If ACPI AML tries to use the bus (except trying to lock it) while > we're in the middle of a transfer, or after we've determined > locking is broken, we know we cannot safely share the bus and give up. > > Upon first usage of SMBus by ACPI AML, if nothing has gone horribly > wrong so far, users will see: > > i801_smbus 0000:00:1f.4: SMBus controller is shared with ACPI AML. This seems safe so far. > > If locking the SMBus times out, users will see: > > i801_smbus 0000:00:1f.4: BIOS left SMBus locked > > And if ACPI AML tries to use the bus concurrently with Linux, or it > previously used the bus and we failed to subsequently lock it as > above, the driver will give up and users will get: > > i801_smbus 0000:00:1f.4: BIOS uses SMBus unsafely > i801_smbus 0000:00:1f.4: Driver SMBus register access inhibited > > This fixes the regression introduced by 01590f361e, and further allows > safely sharing the SMBus on 2015 iMacs. Tested by running `i2cdump` in a > loop while changing backlight levels via the ACPI video device. > > Fixes: 01590f361e ("i2c: i801: Instantiate SPD EEPROMs automatically") > Cc: <stable@vger.kernel.org> > Signed-off-by: Hector Martin <marcan@marcan.st> Jean, Heiner, what do we do with this topic? > --- > drivers/i2c/busses/i2c-i801.c | 96 ++++++++++++++++++++++++++++------- > 1 file changed, 79 insertions(+), 17 deletions(-) > > diff --git a/drivers/i2c/busses/i2c-i801.c b/drivers/i2c/busses/i2c-i801.c > index 04a1e38f2a6f..03be6310d6d7 100644 > --- a/drivers/i2c/busses/i2c-i801.c > +++ b/drivers/i2c/busses/i2c-i801.c > @@ -287,11 +287,18 @@ struct i801_priv { > #endif > struct platform_device *tco_pdev; > > + /* BIOS left the controller marked busy. */ > + bool inuse_stuck; > /* > - * If set to true the host controller registers are reserved for > - * ACPI AML use. Protected by acpi_lock. > + * If set to true, ACPI AML uses the host controller registers. > + * Protected by acpi_lock. > */ > - bool acpi_reserved; > + bool acpi_usage; > + /* > + * If set to true, ACPI AML uses the host controller registers in an > + * unsafe way. Protected by acpi_lock. > + */ > + bool acpi_unsafe; > struct mutex acpi_lock; > }; > > @@ -854,10 +861,37 @@ static s32 i801_access(struct i2c_adapter *adap, u16 addr, > int hwpec; > int block = 0; > int ret = 0, xact = 0; > + int timeout = 0; > struct i801_priv *priv = i2c_get_adapdata(adap); > > + /* > + * The controller provides a bit that implements a mutex mechanism > + * between users of the bus. First, try to lock the hardware mutex. > + * If this doesn't work, we give up trying to do this, but then > + * bail if ACPI uses SMBus at all. > + */ > + if (!priv->inuse_stuck) { > + while (inb_p(SMBHSTSTS(priv)) & SMBHSTSTS_INUSE_STS) { > + if (++timeout >= MAX_RETRIES) { > + dev_warn(&priv->pci_dev->dev, > + "BIOS left SMBus locked\n"); > + priv->inuse_stuck = true; > + break; > + } > + usleep_range(250, 500); > + } > + } > + > mutex_lock(&priv->acpi_lock); > - if (priv->acpi_reserved) { > + if (priv->acpi_usage && priv->inuse_stuck && !priv->acpi_unsafe) { > + priv->acpi_unsafe = true; > + > + dev_warn(&priv->pci_dev->dev, "BIOS uses SMBus unsafely\n"); > + dev_warn(&priv->pci_dev->dev, > + "Driver SMBus register access inhibited\n"); > + } > + > + if (priv->acpi_unsafe) { > mutex_unlock(&priv->acpi_lock); > return -EBUSY; > } > @@ -1639,6 +1673,16 @@ static bool i801_acpi_is_smbus_ioport(const struct i801_priv *priv, > address <= pci_resource_end(priv->pci_dev, SMBBAR); > } > > +static acpi_status > +i801_acpi_do_access(u32 function, acpi_physical_address address, > + u32 bits, u64 *value) > +{ > + if ((function & ACPI_IO_MASK) == ACPI_READ) > + return acpi_os_read_port(address, (u32 *)value, bits); > + else > + return acpi_os_write_port(address, (u32)*value, bits); > +} > + > static acpi_status > i801_acpi_io_handler(u32 function, acpi_physical_address address, u32 bits, > u64 *value, void *handler_context, void *region_context) > @@ -1648,17 +1692,38 @@ i801_acpi_io_handler(u32 function, acpi_physical_address address, u32 bits, > acpi_status status; > > /* > - * Once BIOS AML code touches the OpRegion we warn and inhibit any > - * further access from the driver itself. This device is now owned > - * by the system firmware. > + * Non-i801 accesses pass through. > */ > - mutex_lock(&priv->acpi_lock); > + if (!i801_acpi_is_smbus_ioport(priv, address)) > + return i801_acpi_do_access(function, address, bits, value); > > - if (!priv->acpi_reserved && i801_acpi_is_smbus_ioport(priv, address)) { > - priv->acpi_reserved = true; > + if (!mutex_trylock(&priv->acpi_lock)) { > + mutex_lock(&priv->acpi_lock); > + /* > + * This better be a read of the status register to acquire > + * the lock... > + */ > + if (!priv->acpi_unsafe && > + !(address == SMBHSTSTS(priv) && > + (function & ACPI_IO_MASK) == ACPI_READ)) { > + /* > + * Uh-oh, ACPI AML is trying to do something with the > + * controller without locking it properly. > + */ > + priv->acpi_unsafe = true; > + > + dev_warn(&pdev->dev, "BIOS uses SMBus unsafely\n"); > + dev_warn(&pdev->dev, > + "Driver SMBus register access inhibited\n"); > + } > + } > > - dev_warn(&pdev->dev, "BIOS is accessing SMBus registers\n"); > - dev_warn(&pdev->dev, "Driver SMBus register access inhibited\n"); > + if (!priv->acpi_usage) { > + priv->acpi_usage = true; > + > + if (!priv->acpi_unsafe) > + dev_info(&pdev->dev, > + "SMBus controller is shared with ACPI AML. This seems safe so far.\n"); > > /* > * BIOS is accessing the host controller so prevent it from > @@ -1667,10 +1732,7 @@ i801_acpi_io_handler(u32 function, acpi_physical_address address, u32 bits, > pm_runtime_get_sync(&pdev->dev); > } > > - if ((function & ACPI_IO_MASK) == ACPI_READ) > - status = acpi_os_read_port(address, (u32 *)value, bits); > - else > - status = acpi_os_write_port(address, (u32)*value, bits); > + status = i801_acpi_do_access(function, address, bits, value); > > mutex_unlock(&priv->acpi_lock); > > @@ -1706,7 +1768,7 @@ static void i801_acpi_remove(struct i801_priv *priv) > ACPI_ADR_SPACE_SYSTEM_IO, i801_acpi_io_handler); > > mutex_lock(&priv->acpi_lock); > - if (priv->acpi_reserved) > + if (priv->acpi_usage) > pm_runtime_put(&priv->pci_dev->dev); > mutex_unlock(&priv->acpi_lock); > } > -- > 2.32.0 >
On Mon, 29 Nov 2021 10:00:55 +0100, Wolfram Sang wrote: > On Sat, Jun 26, 2021 at 02:41:13PM +0900, Hector Martin wrote: > > The i801 controller provides a locking mechanism that the OS is supposed > > to use to safely share the SMBus with ACPI AML or other firmware. > > > > Previously, Linux attempted to get out of the way of ACPI AML entirely, > > but left the bus locked if it used it before the first AML access. This > > causes AML implementations that *do* attempt to safely share the bus > > to time out if Linux uses it first; notably, this regressed ACPI video > > backlight controls on 2015 iMacs after 01590f361e started instantiating > > SPD EEPROMs on boot. > > > > Commit 065b6211a8 fixed the immediate problem of leaving the bus locked, > > but we can do better. The controller does have a proper locking mechanism, > > so let's use it as intended. Since we can't rely on the BIOS doing this > > properly, we implement the following logic: > > > > - If ACPI AML uses the bus at all, we make a note and disable power > > management. The latter matches already existing behavior. > > - When we want to use the bus, we attempt to lock it first. If the > > locking attempt times out, *and* ACPI hasn't tried to use the bus at > > all yet, we cautiously go ahead and assume the BIOS forgot to unlock > > the bus after boot. This preserves existing behavior. > > - We always unlock the bus after a transfer. > > - If ACPI AML tries to use the bus (except trying to lock it) while > > we're in the middle of a transfer, or after we've determined > > locking is broken, we know we cannot safely share the bus and give up. > > > > Upon first usage of SMBus by ACPI AML, if nothing has gone horribly > > wrong so far, users will see: > > > > i801_smbus 0000:00:1f.4: SMBus controller is shared with ACPI AML. This seems safe so far. > > > > If locking the SMBus times out, users will see: > > > > i801_smbus 0000:00:1f.4: BIOS left SMBus locked > > > > And if ACPI AML tries to use the bus concurrently with Linux, or it > > previously used the bus and we failed to subsequently lock it as > > above, the driver will give up and users will get: > > > > i801_smbus 0000:00:1f.4: BIOS uses SMBus unsafely > > i801_smbus 0000:00:1f.4: Driver SMBus register access inhibited > > > > This fixes the regression introduced by 01590f361e, and further allows > > safely sharing the SMBus on 2015 iMacs. Tested by running `i2cdump` in a > > loop while changing backlight levels via the ACPI video device. > > > > Fixes: 01590f361e ("i2c: i801: Instantiate SPD EEPROMs automatically") > > Cc: <stable@vger.kernel.org> > > Signed-off-by: Hector Martin <marcan@marcan.st> > > Jean, Heiner, what do we do with this topic? I like the idea, I need to give it a try and review the code.
Tested-by: Alex Henrie <alexh@vpitech.com>
On Sat, 26 Jun 2021 14:41:13 +0900 Hector Martin <marcan@marcan.st> wrote: > + /* > + * The controller provides a bit that implements a mutex mechanism > + * between users of the bus. First, try to lock the hardware mutex. > + * If this doesn't work, we give up trying to do this, but then > + * bail if ACPI uses SMBus at all. > + */ > + if (!priv->inuse_stuck) { > + while (inb_p(SMBHSTSTS(priv)) & SMBHSTSTS_INUSE_STS) { > + if (++timeout >= MAX_RETRIES) { > + dev_warn(&priv->pci_dev->dev, > + "BIOS left SMBus locked\n"); > + priv->inuse_stuck = true; > + break; > + } > + usleep_range(250, 500); > + } > + } > + > mutex_lock(&priv->acpi_lock); Hi Hector, I do have a question about the order of mutex locking here: Wouldn't it be better to lock priv->acpi_lock before trying to lock the hardware mutex? As it is, if a large number of threads are all trying to read from the SMBus concurrently, couldn't the loop time out and print the message "BIOS left SMBus locked" when it's really not the BIOS's fault? -Alex
Tested on a SECO SBC-B68 and a UDOO X86.
The BIOS AML code queries the Embedded Controller over SMBus,
respecting the hardware semaphore implementation.
I get this line on kernel log and everything works as expected.
[ 7.270172] i801_smbus 0000:00:1f.3: SMBus controller is shared with ACPI AML. This seems safe so far.
Tested with continous use of i2c-tools (i2cdump) with temperature reads
in thermal_zone (that triggers AML code).
Tested-by: Ettore Chimenti <ek5.chimenti@gmail.com>
Hi all, I know it's been a while, and I have to apologize for that. The original plan was to merge the patch series from Heiner Kallweit first, but as it turns out, it was never resubmitted after my (admittedly slow) review and things got stuck. Anyway, I finally took some time to review this patch. I have a good headache now, and some comments below. On Sat, 26 Jun 2021 14:41:13 +0900, Hector Martin wrote: > The i801 controller provides a locking mechanism that the OS is supposed > to use to safely share the SMBus with ACPI AML or other firmware. > > Previously, Linux attempted to get out of the way of ACPI AML entirely, > but left the bus locked if it used it before the first AML access. This > causes AML implementations that *do* attempt to safely share the bus > to time out if Linux uses it first; notably, this regressed ACPI video > backlight controls on 2015 iMacs after 01590f361e started instantiating > SPD EEPROMs on boot. > > Commit 065b6211a8 fixed the immediate problem of leaving the bus locked, > but we can do better. The controller does have a proper locking mechanism, > so let's use it as intended. Since we can't rely on the BIOS doing this > properly, we implement the following logic: > > - If ACPI AML uses the bus at all, we make a note and disable power > management. The latter matches already existing behavior. > - When we want to use the bus, we attempt to lock it first. If the > locking attempt times out, *and* ACPI hasn't tried to use the bus at > all yet, we cautiously go ahead and assume the BIOS forgot to unlock > the bus after boot. This preserves existing behavior. > - We always unlock the bus after a transfer. If I'm not mistaken, this also matches already existing behavior (since commit 065b6211a8). > - If ACPI AML tries to use the bus (except trying to lock it) while > we're in the middle of a transfer, or after we've determined > locking is broken, we know we cannot safely share the bus and give up. > > Upon first usage of SMBus by ACPI AML, if nothing has gone horribly > wrong so far, users will see: > > i801_smbus 0000:00:1f.4: SMBus controller is shared with ACPI AML. This seems safe so far. > > If locking the SMBus times out, users will see: > > i801_smbus 0000:00:1f.4: BIOS left SMBus locked > > And if ACPI AML tries to use the bus concurrently with Linux, or it > previously used the bus and we failed to subsequently lock it as > above, the driver will give up and users will get: > > i801_smbus 0000:00:1f.4: BIOS uses SMBus unsafely > i801_smbus 0000:00:1f.4: Driver SMBus register access inhibited > > This fixes the regression introduced by 01590f361e, and further allows > safely sharing the SMBus on 2015 iMacs. Tested by running `i2cdump` in a > loop while changing backlight levels via the ACPI video device. > > Fixes: 01590f361e ("i2c: i801: Instantiate SPD EEPROMs automatically") > Cc: <stable@vger.kernel.org> I object to this patch going to stable, as the original regression has already been addressed by an earlier patch. The "Fixes" tag doesn't seem appropriate, for the same reason. This patch is way too intrusive and risky to be considered for stable kernels, it clearly fails to meet the first two acceptance criteria. It should live for some time upstream before distributions may consider backporting it. > Signed-off-by: Hector Martin <marcan@marcan.st> > --- > drivers/i2c/busses/i2c-i801.c | 96 ++++++++++++++++++++++++++++------- > 1 file changed, 79 insertions(+), 17 deletions(-) > > diff --git a/drivers/i2c/busses/i2c-i801.c b/drivers/i2c/busses/i2c-i801.c > index 04a1e38f2a6f..03be6310d6d7 100644 > --- a/drivers/i2c/busses/i2c-i801.c > +++ b/drivers/i2c/busses/i2c-i801.c > @@ -287,11 +287,18 @@ struct i801_priv { > #endif > struct platform_device *tco_pdev; > > + /* BIOS left the controller marked busy. */ > + bool inuse_stuck; > /* > - * If set to true the host controller registers are reserved for > - * ACPI AML use. Protected by acpi_lock. > + * If set to true, ACPI AML uses the host controller registers. > + * Protected by acpi_lock. > */ > - bool acpi_reserved; > + bool acpi_usage; > + /* > + * If set to true, ACPI AML uses the host controller registers in an > + * unsafe way. Protected by acpi_lock. > + */ > + bool acpi_unsafe; > struct mutex acpi_lock; > }; > > @@ -854,10 +861,37 @@ static s32 i801_access(struct i2c_adapter *adap, u16 addr, > int hwpec; > int block = 0; > int ret = 0, xact = 0; > + int timeout = 0; > struct i801_priv *priv = i2c_get_adapdata(adap); > > + /* > + * The controller provides a bit that implements a mutex mechanism > + * between users of the bus. First, try to lock the hardware mutex. > + * If this doesn't work, we give up trying to do this, but then > + * bail if ACPI uses SMBus at all. > + */ > + if (!priv->inuse_stuck) { > + while (inb_p(SMBHSTSTS(priv)) & SMBHSTSTS_INUSE_STS) { Do I understand correctly that there are 3 cases where we would be looping here: 1* The SMBus was used by the BIOS at boot time and they forgot to clear SMBHSTSTS_INUSE_STS. The expected outcome is a timeout. 2* ACPI is using the SMBus, and the implementation is a good citizen (acquires the hardware lock before starting a transfer, releases it when done), and we are in the middle of an ACPI-side SMBus transfer. The expectation is that the ACPI-side transfer will complete and the hardware lock be released before we reach the timeout. 3* ACPI is using the SMBus, the implementation is a bad citizen and did not release the hardware lock after a transfer. The expected outcome is a timeout. Case 1 wouldn't be a problem, but we can't differentiate it from case 3, which is a problem, therefore we always consider a timeout to be the problematic case 3 and inhibit driver access when that happens. Right? > + if (++timeout >= MAX_RETRIES) { Note: I had to change this to use time_is_before_jiffies(), as MAX_RETRIES has been removed from the i2c-i801 driver by commit 44c54c4ec391 ("i2c: i801: Improve status polling") meanwhile. > + dev_warn(&priv->pci_dev->dev, > + "BIOS left SMBus locked\n"); > + priv->inuse_stuck = true; > + break; > + } > + usleep_range(250, 500); > + } > + } > + > mutex_lock(&priv->acpi_lock); > - if (priv->acpi_reserved) { > + if (priv->acpi_usage && priv->inuse_stuck && !priv->acpi_unsafe) { > + priv->acpi_unsafe = true; > + > + dev_warn(&priv->pci_dev->dev, "BIOS uses SMBus unsafely\n"); > + dev_warn(&priv->pci_dev->dev, > + "Driver SMBus register access inhibited\n"); > + } > + > + if (priv->acpi_unsafe) { > mutex_unlock(&priv->acpi_lock); > return -EBUSY; > } > @@ -1639,6 +1673,16 @@ static bool i801_acpi_is_smbus_ioport(const struct i801_priv *priv, > address <= pci_resource_end(priv->pci_dev, SMBBAR); > } > > +static acpi_status > +i801_acpi_do_access(u32 function, acpi_physical_address address, > + u32 bits, u64 *value) > +{ > + if ((function & ACPI_IO_MASK) == ACPI_READ) > + return acpi_os_read_port(address, (u32 *)value, bits); > + else > + return acpi_os_write_port(address, (u32)*value, bits); > +} > + > static acpi_status > i801_acpi_io_handler(u32 function, acpi_physical_address address, u32 bits, > u64 *value, void *handler_context, void *region_context) > @@ -1648,17 +1692,38 @@ i801_acpi_io_handler(u32 function, acpi_physical_address address, u32 bits, > acpi_status status; > > /* > - * Once BIOS AML code touches the OpRegion we warn and inhibit any > - * further access from the driver itself. This device is now owned > - * by the system firmware. > + * Non-i801 accesses pass through. Technically, this is "Non-SMBus access". It could still be an access to other functions of the i82801 chipset, but we don't care about these. > */ > - mutex_lock(&priv->acpi_lock); > + if (!i801_acpi_is_smbus_ioport(priv, address)) > + return i801_acpi_do_access(function, address, bits, value); > > - if (!priv->acpi_reserved && i801_acpi_is_smbus_ioport(priv, address)) { > - priv->acpi_reserved = true; > + if (!mutex_trylock(&priv->acpi_lock)) { > + mutex_lock(&priv->acpi_lock); I was pretty confused by this construct at first, I think it deserves a comment. My understanding of it is: 1* We first try to acquire the software lock without waiting. If it works, it means that no transfer is in progress on the Linux side, and we can proceed with the ACPI I/O request, as we hold the lock. 2* If immediate acquisition of the software lock failed, we wait for the lock to be released by i801_access(). Once we have the lock, there are two options. Either the first thing done by the ACPI side is reading SMBHSTSTS_INUSE_STS, it is a good citizen and we can continue (we hold both the software lock and the hardware lock). Or the ACPI side accesses another register directly, which means it does not check SMBHSTSTS_INUSE_STS, it is a bad citizen and there is no safe way to access the SMBus on the Linux side. We prevent further access from the Linux side, as we did before this patch. Please correct me if I'm wrong. > + /* > + * This better be a read of the status register to acquire > + * the lock... > + */ > + if (!priv->acpi_unsafe && > + !(address == SMBHSTSTS(priv) && > + (function & ACPI_IO_MASK) == ACPI_READ)) { > + /* > + * Uh-oh, ACPI AML is trying to do something with the > + * controller without locking it properly. > + */ > + priv->acpi_unsafe = true; > + > + dev_warn(&pdev->dev, "BIOS uses SMBus unsafely\n"); > + dev_warn(&pdev->dev, > + "Driver SMBus register access inhibited\n"); > + } > + } > > - dev_warn(&pdev->dev, "BIOS is accessing SMBus registers\n"); > - dev_warn(&pdev->dev, "Driver SMBus register access inhibited\n"); > + if (!priv->acpi_usage) { > + priv->acpi_usage = true; > + > + if (!priv->acpi_unsafe) > + dev_info(&pdev->dev, > + "SMBus controller is shared with ACPI AML. This seems safe so far.\n"); > > /* > * BIOS is accessing the host controller so prevent it from I see a problem with this implementation. It is possible that i801_access() gets called in the middle of an ACPI-side SMBus transfer, because the software lock is not held for the whole duration of an ACPI-side transfer (contrary to Linux-side transfers). Before this patch, the first step of the ACPI-side transfer would have blocked any future access from the Linux side, so we were safe. After this patch, acpi_usage has been set to true at this point, but that alone won't prevent access from the Linux side. Access will only be prevented if SMBHSTSTS_INUSE_STS is considered stuck. If the ACPI implementation is a bad citizen, I'm afraid there is no guarantee of that happening. SMBHSTSTS will eventually be read during the ACPI-side transfer, and that will acquire the hardware lock as a side effect, but not as the first step. Specifically, the scenario that would cause problem is: ACPI side | Linux side ---------------------------------------+---------------------------------- [first step of a transfer] | mutex_trylock(&priv->acpi_lock) | priv->acpi_usage = true | i801_acpi_do_access(random register) | mutex_unlock(&priv->acpi_lock) | | take hardware lock | mutex_lock(&priv->acpi_lock) | priv->acpi_unsafe is false -> OK | perform transfer | release hardware lock | mutex_unlock(&priv->acpi_lock) [second step of a transfer] | mutex_trylock(&priv->acpi_lock) | i801_acpi_do_access(random register) | mutex_unlock(&priv->acpi_lock) | (...) | Obviously the ACPI-side transfer will be screwed, as whatever register value was set during the first step will inevitably have been overwritten by the Linux-side transfer. I admit that the probability of this scenario happening is extremely low, because it will take only one completed ACPI-side transfer (bad citizen case) to take the hardware lock and not release it, which in turn will prevent any further access from the Linux side. The above scenario can therefore only happen if the very first SMBus transfer from the bad citizen ACPI side happens at exactly the same time as a transfer from the Linux side. Question: why do you check that the first action of ACPI AML is to acquire the hardware lock *only* if that action was attempted while the Linux side was performing a transfer? That event is quite unlikely to happen. Can't we perform that test unconditionally the very first time i801_acpi_io_handler() is called? Unless the i2c-i801 driver gets initialized in the middle of an ACPI-side SMBus transfer, the first time i801_acpi_io_handler() is called, and i801_acpi_is_smbus_ioport() returns true, should correspond to the first step of an ACPI-side SMBus transfer. If that's not a SMBHSTSTS read then we know we can't trust the ACPI AML implementation, and Linux-side access should be prohibited. No need to wait for a transfer collision to make that decision. What do you think?
On 15.12.2022 15:26, Jean Delvare wrote: > Hi all, > > I know it's been a while, and I have to apologize for that. The > original plan was to merge the patch series from Heiner Kallweit first, > but as it turns out, it was never resubmitted after my (admittedly > slow) review and things got stuck. > Indeed, this has been stuck. I have to set finishing this series on my task list again. > Anyway, I finally took some time to review this patch. I have a good > headache now, and some comments below. > [...]
Hi, I'm going to be honest - it's been so long I completely lost all mental state of this patch, so at this point I'm mostly at the same place you are here. On 15/12/2022 23.26, Jean Delvare wrote: > I object to this patch going to stable, as the original regression has > already been addressed by an earlier patch. The "Fixes" tag doesn't > seem appropriate, for the same reason. > > This patch is way too intrusive and risky to be considered for stable > kernels, it clearly fails to meet the first two acceptance criteria. It > should live for some time upstream before distributions may consider > backporting it. That's fair. The original intent was to fix the regression and implement it properly in one go. >> + /* >> + * The controller provides a bit that implements a mutex mechanism >> + * between users of the bus. First, try to lock the hardware mutex. >> + * If this doesn't work, we give up trying to do this, but then >> + * bail if ACPI uses SMBus at all. >> + */ >> + if (!priv->inuse_stuck) { >> + while (inb_p(SMBHSTSTS(priv)) & SMBHSTSTS_INUSE_STS) { > > Do I understand correctly that there are 3 cases where we would be > looping here: > > 1* The SMBus was used by the BIOS at boot time and they forgot to clear > SMBHSTSTS_INUSE_STS. The expected outcome is a timeout. > > 2* ACPI is using the SMBus, and the implementation is a good citizen > (acquires the hardware lock before starting a transfer, releases it > when done), and we are in the middle of an ACPI-side SMBus transfer. > The expectation is that the ACPI-side transfer will complete and the > hardware lock be released before we reach the timeout. > > 3* ACPI is using the SMBus, the implementation is a bad citizen and did > not release the hardware lock after a transfer. The expected outcome > is a timeout. > > Case 1 wouldn't be a problem, but we can't differentiate it from case > 3, which is a problem, therefore we always consider a timeout to be the > problematic case 3 and inhibit driver access when that happens. Right? Not quite. inuse_stuck does not inhibit driver access by itself until we see *some* access from ACPI (acpi_usage). It is impossible for BIOS to forget to clear SMBHSTSTS_INUSE_STS *and* for ACPI to be a good citizen with the bus, because that would immediately deadlock on the ACPI access regardless of what the OS does (unless there is some weird logic/workaround to clear the lock on first access in the ACPI AML, but let's hope nobody does that). So the expected cases are: 1. SMBus was left locked by the BIOS but is *not* used by ACPI (inuse_stuck == true, acpi_usage == false). In this case we expect a timeout once, then we give up on checking the hardware lock altogether in the future. 2. SMBus is/was busy on a well-behaved ACPI transfer (inuse_stuck == false, acpi_usage == true). In this case we make note of the ACPI usage but nothing bad happens (no timeout). 3. ACPI is/has used SMBus and left it locked (inuse_stuck == true, acpi_usage == true). In this case we give up on future driver accesses entirely. Only case 3 leads to the driver bailing out on further accesses. > Note: I had to change this to use time_is_before_jiffies(), as > MAX_RETRIES has been removed from the i2c-i801 driver by commit > 44c54c4ec391 ("i2c: i801: Improve status polling") meanwhile. Ack. > Technically, this is "Non-SMBus access". It could still be an access to > other functions of the i82801 chipset, but we don't care about these. Right, makese sense. > >> */ >> - mutex_lock(&priv->acpi_lock); >> + if (!i801_acpi_is_smbus_ioport(priv, address)) >> + return i801_acpi_do_access(function, address, bits, value); >> >> - if (!priv->acpi_reserved && i801_acpi_is_smbus_ioport(priv, address)) { >> - priv->acpi_reserved = true; >> + if (!mutex_trylock(&priv->acpi_lock)) { >> + mutex_lock(&priv->acpi_lock); > > I was pretty confused by this construct at first, I think it deserves a > comment. My understanding of it is: > > 1* We first try to acquire the software lock without waiting. If it > works, it means that no transfer is in progress on the Linux side, > and we can proceed with the ACPI I/O request, as we hold the lock. > > 2* If immediate acquisition of the software lock failed, we wait for > the lock to be released by i801_access(). Once we have the lock, > there are two options. Either the first thing done by the ACPI side > is reading SMBHSTSTS_INUSE_STS, it is a good citizen and we can > continue (we hold both the software lock and the hardware lock). Or > the ACPI side accesses another register directly, which means it > does not check SMBHSTSTS_INUSE_STS, it is a bad citizen and there is > no safe way to access the SMBus on the Linux side. We prevent > further access from the Linux side, as we did before this patch. > > Please correct me if I'm wrong. Yes, I believe that was the intent. > > I see a problem with this implementation. It is possible that > i801_access() gets called in the middle of an ACPI-side SMBus transfer, > because the software lock is not held for the whole duration of an > ACPI-side transfer (contrary to Linux-side transfers). Before this > patch, the first step of the ACPI-side transfer would have blocked any > future access from the Linux side, so we were safe. After this patch, > acpi_usage has been set to true at this point, but that alone won't > prevent access from the Linux side. Access will only be prevented if > SMBHSTSTS_INUSE_STS is considered stuck. If the ACPI implementation is > a bad citizen, I'm afraid there is no guarantee of that happening. > SMBHSTSTS will eventually be read during the ACPI-side transfer, and > that will acquire the hardware lock as a side effect, but not as the > first step. > > Specifically, the scenario that would cause problem is: > > ACPI side | Linux side > ---------------------------------------+---------------------------------- > [first step of a transfer] | > mutex_trylock(&priv->acpi_lock) | > priv->acpi_usage = true | > i801_acpi_do_access(random register) | > mutex_unlock(&priv->acpi_lock) | > | take hardware lock > | mutex_lock(&priv->acpi_lock) > | priv->acpi_unsafe is false -> OK > | perform transfer > | release hardware lock > | mutex_unlock(&priv->acpi_lock) > [second step of a transfer] | > mutex_trylock(&priv->acpi_lock) | > i801_acpi_do_access(random register) | > mutex_unlock(&priv->acpi_lock) | > (...) | > > Obviously the ACPI-side transfer will be screwed, as whatever register > value was set during the first step will inevitably have been > overwritten by the Linux-side transfer. > > I admit that the probability of this scenario happening is extremely > low, because it will take only one completed ACPI-side transfer (bad > citizen case) to take the hardware lock and not release it, which in > turn will prevent any further access from the Linux side. The above > scenario can therefore only happen if the very first SMBus transfer > from the bad citizen ACPI side happens at exactly the same time as a > transfer from the Linux side. Agreed, this is a problem. > Question: why do you check that the first action of ACPI AML is to > acquire the hardware lock *only* if that action was attempted while the > Linux side was performing a transfer? That event is quite unlikely to > happen. Can't we perform that test unconditionally the very first time > i801_acpi_io_handler() is called? Unless the i2c-i801 driver gets> initialized in the middle of an ACPI-side SMBus transfer [...] I wonder if there's some way to close this race? This doesn't sound all that unlikely (consider: ACPI backlight is also a module, device init happens in parallel, so we could well end up probing i2c-i801 in the middle of an ACPI SMBus transfer more often than you'd expect at boot time). How about this: instead of checking for the lock access on the *first* call to i801_acpi_io_handler, we add an acpi_must_lock flag. This is initially false, but it is set on completion of every Linux-side i2c transfer, and cleared once we see ACPI take the lock properly. The ACPI handler then checks it and expects a lock access if it is true. So as soon as Linux does an i2c transfer, we expect ACPI to be taking the lock next time it touches the hardware, and we know to bail on the Linux side in the future if it does not. * If the i2c driver probes in the middle of a well-behaved ACPI SMBus transfer, nothing bad happens, since if we try to do a Linux-side transfer it will block on the lock until ACPI is done. Further ACPI SMBus transfers after a Linux transfer will pass the locking check. * If the driver probes in the middle of a misbehaved ACPI transfer but is otherwise idle, nothing happens until the first Linux transfer, then the next ACPI access after that will have us bail and disable driver access. * If we probe *and* try to make a transfer all in the middle of a misbehaved ACPI transfer, then we are going to step on its toes and break it, but at least we will notice as soon as the Linux side is done (or possibly failed due to the collision) and ACPI tries to touch the controller again, so we will get out of its way in the future and there's at least a chance it will recover for future accesses. Further closing that last edge case to avoid ever conflicting with broken ACPI implementations would require some sort of mechanism to know whether ACPI AML started running an i2c transfer method before the i2c-i801 driver loaded, which might be too intrusive a change to be wrth it for such a corner case. Though maybe there's an easy way? If there's something like a global AML lock we could just have the probe sequence be: 1. Register the ACPI IO handler 2. Take the AML lock 3. Set acpi_must_lock = true 4. Release the AML lock 5. Finally register the i2c controller That makes sure we serialize on any ongoing ACPI shenanigans at probe time and allows us to truly check that the first access from ACPI after that is a lock, before Linux has a chance to do anything itself. But I don't know off the top of my head whether there is such a lock. - Hector
On Sat, 17 Dec 2022 22:28:25 +0900, Hector Martin wrote: > On 15/12/2022 23.26, Jean Delvare wrote: > > Question: why do you check that the first action of ACPI AML is to > > acquire the hardware lock *only* if that action was attempted while the > > Linux side was performing a transfer? That event is quite unlikely to > > happen. Can't we perform that test unconditionally the very first time > > i801_acpi_io_handler() is called? Unless the i2c-i801 driver gets> initialized in the middle of an ACPI-side SMBus transfer [...] > > I wonder if there's some way to close this race? This doesn't sound all > that unlikely (consider: ACPI backlight is also a module, device init > happens in parallel, so we could well end up probing i2c-i801 in the > middle of an ACPI SMBus transfer more often than you'd expect at boot time). > > How about this: instead of checking for the lock access on the *first* > call to i801_acpi_io_handler, we add an acpi_must_lock flag. This is > initially false, but it is set on completion of every Linux-side i2c > transfer, and cleared once we see ACPI take the lock properly. The ACPI > handler then checks it and expects a lock access if it is true. > > So as soon as Linux does an i2c transfer, we expect ACPI to be taking > the lock next time it touches the hardware, and we know to bail on the > Linux side in the future if it does not. > > * If the i2c driver probes in the middle of a well-behaved ACPI SMBus > transfer, nothing bad happens, since if we try to do a Linux-side > transfer it will block on the lock until ACPI is done. Further ACPI > SMBus transfers after a Linux transfer will pass the locking check. > * If the driver probes in the middle of a misbehaved ACPI transfer but > is otherwise idle, nothing happens until the first Linux transfer, then > the next ACPI access after that will have us bail and disable driver access. > * If we probe *and* try to make a transfer all in the middle of a > misbehaved ACPI transfer, then we are going to step on its toes and > break it, but at least we will notice as soon as the Linux side is done > (or possibly failed due to the collision) and ACPI tries to touch the > controller again, so we will get out of its way in the future and > there's at least a chance it will recover for future accesses. I see what you want to do and I think it should work. > Further closing that last edge case to avoid ever conflicting with > broken ACPI implementations would require some sort of mechanism to know > whether ACPI AML started running an i2c transfer method before the > i2c-i801 driver loaded, which might be too intrusive a change to be wrth > it for such a corner case. Though maybe there's an easy way? If there's > something like a global AML lock we could just have the probe sequence be: > > 1. Register the ACPI IO handler > 2. Take the AML lock > 3. Set acpi_must_lock = true > 4. Release the AML lock > 5. Finally register the i2c controller > > That makes sure we serialize on any ongoing ACPI shenanigans at probe > time and allows us to truly check that the first access from ACPI after > that is a lock, before Linux has a chance to do anything itself. But I > don't know off the top of my head whether there is such a lock. I don't think there is, but that would be a question for the ACPI list. Anyway, for it to be useful, it would have a to be a high-level lock (taken before starting an ACPI-side SMBus transfer, released after the ACPI-side SMBus transfer has been fully processes). If it is taken for every I/O, or even if it isn't held while waiting for the transfer to complete, it won't solve the problem. And I suspect that if such a high-level lock existed, we would have been using it in the first place to guarantee exclusive access to the SMBus controller. The most important thing is to get exclusive access to work properly for well-behaved ACPI implementations. I know that the Linux driver hasn't been a good citizen with the hardware lock until you partially fixed that 1.5 year ago, but I hope that other operating systems did that earlier, which would have encouraged well-behaved ACPI implementations. So hopefully misbehaving ACPI implementations aren't many on recent systems.
diff --git a/drivers/i2c/busses/i2c-i801.c b/drivers/i2c/busses/i2c-i801.c index 04a1e38f2a6f..03be6310d6d7 100644 --- a/drivers/i2c/busses/i2c-i801.c +++ b/drivers/i2c/busses/i2c-i801.c @@ -287,11 +287,18 @@ struct i801_priv { #endif struct platform_device *tco_pdev; + /* BIOS left the controller marked busy. */ + bool inuse_stuck; /* - * If set to true the host controller registers are reserved for - * ACPI AML use. Protected by acpi_lock. + * If set to true, ACPI AML uses the host controller registers. + * Protected by acpi_lock. */ - bool acpi_reserved; + bool acpi_usage; + /* + * If set to true, ACPI AML uses the host controller registers in an + * unsafe way. Protected by acpi_lock. + */ + bool acpi_unsafe; struct mutex acpi_lock; }; @@ -854,10 +861,37 @@ static s32 i801_access(struct i2c_adapter *adap, u16 addr, int hwpec; int block = 0; int ret = 0, xact = 0; + int timeout = 0; struct i801_priv *priv = i2c_get_adapdata(adap); + /* + * The controller provides a bit that implements a mutex mechanism + * between users of the bus. First, try to lock the hardware mutex. + * If this doesn't work, we give up trying to do this, but then + * bail if ACPI uses SMBus at all. + */ + if (!priv->inuse_stuck) { + while (inb_p(SMBHSTSTS(priv)) & SMBHSTSTS_INUSE_STS) { + if (++timeout >= MAX_RETRIES) { + dev_warn(&priv->pci_dev->dev, + "BIOS left SMBus locked\n"); + priv->inuse_stuck = true; + break; + } + usleep_range(250, 500); + } + } + mutex_lock(&priv->acpi_lock); - if (priv->acpi_reserved) { + if (priv->acpi_usage && priv->inuse_stuck && !priv->acpi_unsafe) { + priv->acpi_unsafe = true; + + dev_warn(&priv->pci_dev->dev, "BIOS uses SMBus unsafely\n"); + dev_warn(&priv->pci_dev->dev, + "Driver SMBus register access inhibited\n"); + } + + if (priv->acpi_unsafe) { mutex_unlock(&priv->acpi_lock); return -EBUSY; } @@ -1639,6 +1673,16 @@ static bool i801_acpi_is_smbus_ioport(const struct i801_priv *priv, address <= pci_resource_end(priv->pci_dev, SMBBAR); } +static acpi_status +i801_acpi_do_access(u32 function, acpi_physical_address address, + u32 bits, u64 *value) +{ + if ((function & ACPI_IO_MASK) == ACPI_READ) + return acpi_os_read_port(address, (u32 *)value, bits); + else + return acpi_os_write_port(address, (u32)*value, bits); +} + static acpi_status i801_acpi_io_handler(u32 function, acpi_physical_address address, u32 bits, u64 *value, void *handler_context, void *region_context) @@ -1648,17 +1692,38 @@ i801_acpi_io_handler(u32 function, acpi_physical_address address, u32 bits, acpi_status status; /* - * Once BIOS AML code touches the OpRegion we warn and inhibit any - * further access from the driver itself. This device is now owned - * by the system firmware. + * Non-i801 accesses pass through. */ - mutex_lock(&priv->acpi_lock); + if (!i801_acpi_is_smbus_ioport(priv, address)) + return i801_acpi_do_access(function, address, bits, value); - if (!priv->acpi_reserved && i801_acpi_is_smbus_ioport(priv, address)) { - priv->acpi_reserved = true; + if (!mutex_trylock(&priv->acpi_lock)) { + mutex_lock(&priv->acpi_lock); + /* + * This better be a read of the status register to acquire + * the lock... + */ + if (!priv->acpi_unsafe && + !(address == SMBHSTSTS(priv) && + (function & ACPI_IO_MASK) == ACPI_READ)) { + /* + * Uh-oh, ACPI AML is trying to do something with the + * controller without locking it properly. + */ + priv->acpi_unsafe = true; + + dev_warn(&pdev->dev, "BIOS uses SMBus unsafely\n"); + dev_warn(&pdev->dev, + "Driver SMBus register access inhibited\n"); + } + } - dev_warn(&pdev->dev, "BIOS is accessing SMBus registers\n"); - dev_warn(&pdev->dev, "Driver SMBus register access inhibited\n"); + if (!priv->acpi_usage) { + priv->acpi_usage = true; + + if (!priv->acpi_unsafe) + dev_info(&pdev->dev, + "SMBus controller is shared with ACPI AML. This seems safe so far.\n"); /* * BIOS is accessing the host controller so prevent it from @@ -1667,10 +1732,7 @@ i801_acpi_io_handler(u32 function, acpi_physical_address address, u32 bits, pm_runtime_get_sync(&pdev->dev); } - if ((function & ACPI_IO_MASK) == ACPI_READ) - status = acpi_os_read_port(address, (u32 *)value, bits); - else - status = acpi_os_write_port(address, (u32)*value, bits); + status = i801_acpi_do_access(function, address, bits, value); mutex_unlock(&priv->acpi_lock); @@ -1706,7 +1768,7 @@ static void i801_acpi_remove(struct i801_priv *priv) ACPI_ADR_SPACE_SYSTEM_IO, i801_acpi_io_handler); mutex_lock(&priv->acpi_lock); - if (priv->acpi_reserved) + if (priv->acpi_usage) pm_runtime_put(&priv->pci_dev->dev); mutex_unlock(&priv->acpi_lock); }
The i801 controller provides a locking mechanism that the OS is supposed to use to safely share the SMBus with ACPI AML or other firmware. Previously, Linux attempted to get out of the way of ACPI AML entirely, but left the bus locked if it used it before the first AML access. This causes AML implementations that *do* attempt to safely share the bus to time out if Linux uses it first; notably, this regressed ACPI video backlight controls on 2015 iMacs after 01590f361e started instantiating SPD EEPROMs on boot. Commit 065b6211a8 fixed the immediate problem of leaving the bus locked, but we can do better. The controller does have a proper locking mechanism, so let's use it as intended. Since we can't rely on the BIOS doing this properly, we implement the following logic: - If ACPI AML uses the bus at all, we make a note and disable power management. The latter matches already existing behavior. - When we want to use the bus, we attempt to lock it first. If the locking attempt times out, *and* ACPI hasn't tried to use the bus at all yet, we cautiously go ahead and assume the BIOS forgot to unlock the bus after boot. This preserves existing behavior. - We always unlock the bus after a transfer. - If ACPI AML tries to use the bus (except trying to lock it) while we're in the middle of a transfer, or after we've determined locking is broken, we know we cannot safely share the bus and give up. Upon first usage of SMBus by ACPI AML, if nothing has gone horribly wrong so far, users will see: i801_smbus 0000:00:1f.4: SMBus controller is shared with ACPI AML. This seems safe so far. If locking the SMBus times out, users will see: i801_smbus 0000:00:1f.4: BIOS left SMBus locked And if ACPI AML tries to use the bus concurrently with Linux, or it previously used the bus and we failed to subsequently lock it as above, the driver will give up and users will get: i801_smbus 0000:00:1f.4: BIOS uses SMBus unsafely i801_smbus 0000:00:1f.4: Driver SMBus register access inhibited This fixes the regression introduced by 01590f361e, and further allows safely sharing the SMBus on 2015 iMacs. Tested by running `i2cdump` in a loop while changing backlight levels via the ACPI video device. Fixes: 01590f361e ("i2c: i801: Instantiate SPD EEPROMs automatically") Cc: <stable@vger.kernel.org> Signed-off-by: Hector Martin <marcan@marcan.st> --- drivers/i2c/busses/i2c-i801.c | 96 ++++++++++++++++++++++++++++------- 1 file changed, 79 insertions(+), 17 deletions(-)