linux-kernel - Re: [PATCH] i2c: imx: increase retries on arbitration loss

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Date:   Fri, 30 Dec 2022 18:09:57 +0100
From:   Francesco Dolcini <francesco@...cini.it>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     Primoz Fiser <primoz.fiser@...ik.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, linux-kernel@...r.kernel.org,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        upstream@...ts.phytec.de, Marco Felsch <m.felsch@...gutronix.de>,
        Oleksij Rempel <linux@...pel-privat.de>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-i2c@...r.kernel.org,
        francesco.dolcini@...adex.com, wsa@...nel.org,
        Francesco Dolcini <francesco@...cini.it>
Subject: Re: [PATCH] i2c: imx: increase retries on arbitration loss

On Fri, Dec 30, 2022 at 05:47:42PM +0100, Francesco Dolcini wrote:
> On Fri, Dec 30, 2022 at 05:12:09PM +0100, Oleksij Rempel wrote:
> > On Fri, Dec 30, 2022 at 03:40:58PM +0100, Francesco Dolcini wrote:
> > > +Wolfram
> > > 
> > > On Wed, Dec 28, 2022 at 09:01:46AM +0100, Primoz Fiser wrote:
> > > > On 16. 12. 22 13:51, Francesco Dolcini wrote:
> > > > > On Fri, Dec 16, 2022 at 01:23:29PM +0100, Primoz Fiser wrote:
> > > > > > The only solid point in the thread seems to be that in that case we are not
> > > > > > covering up the potential i2c hardware issues?
> > > > > 
> > > > > I believe that in this case we should just have a warning in the kernel.
> > > > > The retry potentially work-around a transient issue and we do not hide any hardware
> > > > > issue at the same time. It seems an easy win-win solution.
> > > > 
> > > > I would agree about throwing a warning message in retry case.
> > > > 
> > > > Not sure how would it affect other i2c bus drivers using retries > 0.
> > > > Retries might be pretty rare with i2c-imx but some other drivers set this to
> > > > 5 for example. At least using _ratelimited printk is a must using this
> > > > approach.
> > > 
> > > Wolfram, Uwe, Oleksij
> > > 
> > > Would it be acceptable to have a warning when we have I2C retries, and
> > > with that in place enabling retries on the imx driver?
> > > 
> > > It exists hardware that requires this to work correctly,
> > 
> > Well, this is persistent confusion in this monolog. It will not make it
> > correctly.
> > 
> > > and at a
> > > minimum setting the retry count from user space is not going to solve
> > > potential issues during initial driver probe.
> > 
> > I assume it is not clear from programmer point of view. Lets try other way:
> > 
> > - The I2C slave could not correctly interpret the data on SDA because the SDA
> >   high or low-level voltages do not reach its appropriate input
> >   thresholds.
> > 
> > This means:
> > 
> > You have this:
> > 
> >     /-\    /-\ ----- 2.5Vcc
> > ___/   \__/   \___
> > 
> > Instead of this:
> > 
> >      /-\     /-\ ----- 3.3Vcc
> >     /  \    /   \
> > ___/    \__/     \___
> > 
> > This is bad, because master or slave will not be able to interpret the pick level
> > correctly. It may see some times 0 instead of 1. This means, what ever we are
> > writing we are to the slave or reading from the slave is potentially corrupt
> > and only __sometimes__ the master was able to detect it. 
> > 
> > - The I2C slave missed an SCL cycle because the SCL high or low-level voltages
> >   do not reach its appropriate input thresholds.
> > 
> > This means, the bus frequency is too high for current configured or physical PCB
> > designed. So, you will have different kind of corruptions and some times they
> > will be detected. 
> > 
> > - The I2C slave accidently interpreted a spike etc. as an SCL cycle.
> > 
> > This means the noise level is to high. The driver strange should be increased
> > or PCB redesign should be made. May be there are more options. If not done,
> > data corruption can be expected.
> > 
> > None of this issue can be "fixed" by retries or made more "robust".
> > Doing more retries means: we do what ever we do until the system was not able to
> > detect the error.
> 
> Hello Oleksij,
> thanks for the detailed explanation, appreciated.
> 
> Given that is it correct that the i2c imx driver return EAGAIN in such a
> case (arbitration error)? You made it crystal clear that there is no
> such thing as try again for this error, I would be inclined to prepare a
> patch to fix this.
> 
> diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
> index cf5bacf3a488..a2a581c8ae07 100644
> --- a/drivers/i2c/busses/i2c-imx.c
> +++ b/drivers/i2c/busses/i2c-imx.c
> @@ -492,7 +492,7 @@ static int i2c_imx_bus_busy(struct imx_i2c_struct *i2c_imx, int for_busy, bool a
>                 /* check for arbitration lost */
>                 if (temp & I2SR_IAL) {
>                         i2c_imx_clear_irq(i2c_imx, I2SR_IAL);
> -                       return -EAGAIN;
> +                       return -EIO;
>                 }
> 
>                 if (for_busy && (temp & I2SR_IBB)) {
> 

Just a small addition, the tegra i2c driver is interesting.
It returns EAGAIN only when an arbitration error is detected on multi master
node, otherwise it tries the bus recovery procedure.

	/* start recovery upon arbitration loss in single master mode */
	if (i2c_dev->msg_err == I2C_ERR_ARBITRATION_LOST) {
		if (!i2c_dev->multimaster_mode)
			return i2c_recover_bus(&i2c_dev->adapter);

		return -EAGAIN;
	}

Francesco