[RFC PATCH] i2c: imx: avoid rescheduling when waiting for bus not busy

[Andy Shevchenko]

On Fri, May 31, 2024 at 04:24:37PM +0200, Stefan Eichenberger wrote:
> From: Stefan Eichenberger <stefan.eichenberger at toradex.com>
> 
> On our i.MX8M Mini based module we have an ADS1015 I2C ADC connected to
> the I2C bus. The ADS1015 I2C ADC will timeout after 25ms when the I2C
> bus is idle. The imx i2c driver will call schedule when waiting for the
> bus to become idle after switching to master mode. When the i2c
> controller switches to master mode it pulls SCL and SDA low, if the
> ADS1015 I2C ADC sees this for more than 25 ms without seeing SCL
> clocking, it will timeout and ignore all signals until the next start
> condition occurs (SCL and SDA low). This can occur when the system load
> is high and schedule returns after more than 25 ms.
> 
> This rfc tries to solve the problem by using a udelay for the first 10
> ms before calling schedule. This reduces the chance that we will
> reschedule. However, it is still theoretically possible for the problem
> to occur. To properly solve the problem, we would also need to disable
> interrupts during the transfer.
> 
> After some internal discussion, we see three possible solutions:
> 1. Use udelay as shown in this rfc and also disable the interrupts
>    during the transfer. This would solve the problem but disable the
>    interrupts. Also, we would have to re-enable the interrupts if the
>    timeout is longer than 1ms (TBD).
> 2. We use a retry mechanism in the ti-ads1015 driver. When we see a
>    timeout, we try again.
> 3. We use the suggested solution and accept that there is an edge case
>    where the timeout can happen.
> 
> There may be a better way to do this, which is why this is an RFC.

...

> +			/*
> +			 * Avoid rescheduling in the first 10 ms to avoid
> +			 * timeouts for SMBus like devices
> +			 */
> +			if (time_before(jiffies, orig_jiffies + msecs_to_jiffies(10)))
> +				udelay(10);
> +			else
> +				schedule();

Isn't there cond_resched() or so for such things?
More info here: 494e46d08d35 ("airo: Replace in_atomic() usage.")

-- 
With Best Regards,
Andy Shevchenko