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Date: Sun, 2 Jun 2024 16:31:27 +0200
From: Andrew Lunn <andrew@lunn.ch>
To: Stefan Eichenberger <eichest@gmail.com>
Cc: o.rempel@pengutronix.de, kernel@pengutronix.de, andi.shyti@kernel.org,
	shawnguo@kernel.org, s.hauer@pengutronix.de, festevam@gmail.com,
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	andriy.shevchenko@linux.intel.com, u.kleine-koenig@pengutronix.de,
	marcelo.schmitt@analog.com, gnstark@salutedevices.com,
	francesco.dolcini@toradex.com, linux-i2c@vger.kernel.org,
	imx@lists.linux.dev, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-iio@vger.kernel.org,
	Stefan Eichenberger <stefan.eichenberger@toradex.com>
Subject: Re: [RFC PATCH] i2c: imx: avoid rescheduling when waiting for bus
 not busy
Message-ID: <f5d537e2-b102-415f-bc22-c949fd859344@lunn.ch>
References: <20240531142437.74831-1-eichest@gmail.com>
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On Fri, May 31, 2024 at 04:24:37PM +0200, Stefan Eichenberger wrote:
> From: Stefan Eichenberger <stefan.eichenberger@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).

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