[v4,1/4] i2c: imx: only poll for bus busy in multi master mode

From: Stefan Eichenberger <stefan.eichenberger@toradex.com>

According to the i.MX8M Mini reference manual chapter "16.1.4.2
Generation of Start" it is only necessary to poll for bus busy and
arbitration lost in multi master mode. This helps to avoid rescheduling
while the i2c bus is busy and avoids SMBus devices to timeout.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 drivers/i2c/busses/i2c-imx.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

Am Mittwoch, dem 02.10.2024 um 13:19 +0200 schrieb Stefan Eichenberger:
> From: Stefan Eichenberger <stefan.eichenberger@toradex.com>
> 
> According to the i.MX8M Mini reference manual chapter "16.1.4.2
> Generation of Start" it is only necessary to poll for bus busy and
> arbitration lost in multi master mode. This helps to avoid rescheduling
> while the i2c bus is busy and avoids SMBus devices to timeout.
> 
This is a backward incompatible change, as far as I can see. Until now
the driver would properly handle a multi-mastered bus, without any
specific configuration. Now it requires the new multi-master DT
property to be set, which isn't even documented in the binding to be
understood by this driver.

Are you sure that every single instance of a i.MX i2c bus is only
single mastered?

If this is a worthwhile performance improvement I guess you need to
flip the logic around by adding a new single-master DT property (or
something along those lines), which should go through proper DT binding
review. You can then use this property for boards/busses to opt into
skipping the arbitration lost check.

Regards,
Lucas

> Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
> Reviewed-by: Frank Li <Frank.Li@nxp.com>
> Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
> ---
>  drivers/i2c/busses/i2c-imx.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
> index 98539313cbc97..fbacdfaf6b28e 100644
> --- a/drivers/i2c/busses/i2c-imx.c
> +++ b/drivers/i2c/busses/i2c-imx.c
> @@ -216,6 +216,8 @@ struct imx_i2c_struct {
>  	struct i2c_client	*slave;
>  	enum i2c_slave_event last_slave_event;
>  
> +	bool			multi_master;
> +
>  	/* For checking slave events. */
>  	spinlock_t     slave_lock;
>  	struct hrtimer slave_timer;
> @@ -481,6 +483,9 @@ static int i2c_imx_bus_busy(struct imx_i2c_struct *i2c_imx, int for_busy, bool a
>  	unsigned long orig_jiffies = jiffies;
>  	unsigned int temp;
>  
> +	if (!i2c_imx->multi_master)
> +		return 0;
> +
>  	while (1) {
>  		temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2SR);
>  
> @@ -540,8 +545,8 @@ static int i2c_imx_trx_complete(struct imx_i2c_struct *i2c_imx, bool atomic)
>  		return -ETIMEDOUT;
>  	}
>  
> -	/* check for arbitration lost */
> -	if (i2c_imx->i2csr & I2SR_IAL) {
> +	/* In multi-master mode check for arbitration lost */
> +	if (i2c_imx->multi_master && (i2c_imx->i2csr & I2SR_IAL)) {
>  		dev_dbg(&i2c_imx->adapter.dev, "<%s> Arbitration lost\n", __func__);
>  		i2c_imx_clear_irq(i2c_imx, I2SR_IAL);
>  
> @@ -1468,6 +1473,8 @@ static int i2c_imx_probe(struct platform_device *pdev)
>  		goto rpm_disable;
>  	}
>  
> +	i2c_imx->multi_master = of_property_read_bool(pdev->dev.of_node, "multi-master");
> +
>  	/* Set up clock divider */
>  	i2c_imx->bitrate = I2C_MAX_STANDARD_MODE_FREQ;
>  	ret = of_property_read_u32(pdev->dev.of_node,

On Wed, Oct 02, 2024 at 04:40:32PM +0200, Lucas Stach wrote:
> Am Mittwoch, dem 02.10.2024 um 13:19 +0200 schrieb Stefan Eichenberger:
> > From: Stefan Eichenberger <stefan.eichenberger@toradex.com>
> > 
> > According to the i.MX8M Mini reference manual chapter "16.1.4.2
> > Generation of Start" it is only necessary to poll for bus busy and
> > arbitration lost in multi master mode. This helps to avoid rescheduling
> > while the i2c bus is busy and avoids SMBus devices to timeout.
> > 
> This is a backward incompatible change, as far as I can see. Until now
> the driver would properly handle a multi-mastered bus, without any
> specific configuration. Now it requires the new multi-master DT
> property to be set, which isn't even documented in the binding to be
> understood by this driver.
> 
> Are you sure that every single instance of a i.MX i2c bus is only
> single mastered?
> 
> If this is a worthwhile performance improvement I guess you need to
> flip the logic around by adding a new single-master DT property (or
> something along those lines), which should go through proper DT binding
> review. You can then use this property for boards/busses to opt into
> skipping the arbitration lost check.

According to the discussion here the property documentation should not
be added:
https://lore.kernel.org/linux-i2c/2bbddaxyjkxfmlgmq3yqcbzo7dsb2pq5bvdatk2y4ig4iintkt@35btqkdv7sy3/

However, the point regarding single-master and multi-master is correct.
We also discussed this internally and assumed the single-master use case
is more likely to be the default and that this patch series would fix
issues for other devices out there. However, your point is valid and if
preferred I can change it to single-master with the next version.

Francesco Dolcini <francesco.dolcini@toradex.com> I think you once had a
discussion regarding multi master mode for i2c on i.MX devices? Maybe
you can remember the details?

Regards,
Stefan

Am Mittwoch, dem 02.10.2024 um 16:56 +0200 schrieb Stefan Eichenberger:
> On Wed, Oct 02, 2024 at 04:40:32PM +0200, Lucas Stach wrote:
> > Am Mittwoch, dem 02.10.2024 um 13:19 +0200 schrieb Stefan Eichenberger:
> > > From: Stefan Eichenberger <stefan.eichenberger@toradex.com>
> > > 
> > > According to the i.MX8M Mini reference manual chapter "16.1.4.2
> > > Generation of Start" it is only necessary to poll for bus busy and
> > > arbitration lost in multi master mode. This helps to avoid rescheduling
> > > while the i2c bus is busy and avoids SMBus devices to timeout.
> > > 
> > This is a backward incompatible change, as far as I can see. Until now
> > the driver would properly handle a multi-mastered bus, without any
> > specific configuration. Now it requires the new multi-master DT
> > property to be set, which isn't even documented in the binding to be
> > understood by this driver.
> > 
> > Are you sure that every single instance of a i.MX i2c bus is only
> > single mastered?
> > 
> > If this is a worthwhile performance improvement I guess you need to
> > flip the logic around by adding a new single-master DT property (or
> > something along those lines), which should go through proper DT binding
> > review. You can then use this property for boards/busses to opt into
> > skipping the arbitration lost check.
> 
> According to the discussion here the property documentation should not
> be added:
> https://lore.kernel.org/linux-i2c/2bbddaxyjkxfmlgmq3yqcbzo7dsb2pq5bvdatk2y4ig4iintkt@35btqkdv7sy3/
> 
Ah, I wasn't aware of that.

> However, the point regarding single-master and multi-master is correct.
> We also discussed this internally and assumed the single-master use case
> is more likely to be the default and that this patch series would fix
> issues for other devices out there.
> 
I agree that the vast majority of busses is single master only and I
can see why you did it this way.

However, I still think it's a risky change, as some boards/DTs may rely
on the fact that the driver implicitly handled multi-master until now
and we can't retroactively change DTs in the wild.

I just looked up the dtschema for i2c-controller and there is a
"single-master" property defined already. I think it would be good if
you used this instead to have boards opt in to this optimization.

Regards,
Lucas

>  However, your point is valid and if
> preferred I can change it to single-master with the next version.
> 
> Francesco Dolcini <francesco.dolcini@toradex.com> I think you once had a
> discussion regarding multi master mode for i2c on i.MX devices? Maybe
> you can remember the details?
> 
> Regards,
> Stefan