linux-kernel - Re: [RFC PATCH] i2c: imx: avoid rescheduling when waiting for bus not busy

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Date: Sun, 2 Jun 2024 16:31:27 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Stefan Eichenberger <eichest@...il.com>
Cc: o.rempel@...gutronix.de, kernel@...gutronix.de, andi.shyti@...nel.org,
	shawnguo@...nel.org, s.hauer@...gutronix.de, festevam@...il.com,
	jic23@...nel.org, lars@...afoo.de, nuno.sa@...log.com,
	andriy.shevchenko@...ux.intel.com, u.kleine-koenig@...gutronix.de,
	marcelo.schmitt@...log.com, gnstark@...utedevices.com,
	francesco.dolcini@...adex.com, linux-i2c@...r.kernel.org,
	imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
	Stefan Eichenberger <stefan.eichenberger@...adex.com>
Subject: Re: [RFC PATCH] i2c: imx: avoid rescheduling when waiting for bus
 not busy

On Fri, May 31, 2024 at 04:24:37PM +0200, Stefan Eichenberger wrote:
> From: Stefan Eichenberger <stefan.eichenberger@...adex.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).

Does the I2C specification say anything about this behaviour, or is it
specific to this device?

> 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.

2. has the advantage you fix it for any system with this device, not
just those using an IMX. Once question would be, is such a retry safe
in all conditions. Does the timeout happen before any non idempotent
operation is performed?

If the I2C specification allows this behaviour, maybe a more generic
solution is needed, since it could affect more devices?

	Andrew