linux-kernel - Re: [PATCH v3 3/6] i2c: at91: Send bus clear command if SDA is down

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Date:   Sat, 22 Feb 2020 12:44:33 +0100
From:   Wolfram Sang <wsa@...-dreams.de>
To:     Codrin Ciubotariu <codrin.ciubotariu@...rochip.com>
Cc:     linux-i2c@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kamel.bouhara@...tlin.com, Nicolas.Ferre@...rochip.com,
        alexandre.belloni@...tlin.com, Ludovic.Desroches@...rochip.com,
        robh@...nel.org, peda@...ntia.se, linux@...linux.org.uk
Subject: Re: [PATCH v3 3/6] i2c: at91: Send bus clear command if SDA is down

On Wed, Jan 15, 2020 at 01:54:19PM +0200, Codrin Ciubotariu wrote:
> After a transfer timeout, some faulty I2C slave devices might hold down
> the SDA pin. We can generate a bus clear command, hoping that the slave
> might release the pins.
> If the CLEAR command is not supported, we will use gpio recovery, if
> available, to reset the bus.
> 
> Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@...rochip.com>

One thing to improve:

> +	/*
> +	 * some faulty I2C slave devices might hold SDA down;
> +	 * we can send a bus clear command, hoping that the pins will be
> +	 * released
> +	 */
> +	if (has_clear_cmd) {
> +		if (!(dev->transfer_status & AT91_TWI_SDA)) {
> +			dev_dbg(dev->dev,
> +				"SDA is down; sending bus clear command\n");
> +			if (dev->use_alt_cmd) {
> +				unsigned int acr;
> +
> +				acr = at91_twi_read(dev, AT91_TWI_ACR);
> +				acr &= ~AT91_TWI_ACR_DATAL_MASK;
> +				at91_twi_write(dev, AT91_TWI_ACR, acr);
> +			}
> +			at91_twi_write(dev, AT91_TWI_CR, AT91_TWI_CLEAR);
> +		}

The inner if-block should be a seperate function, then you could do in
probe:

	if (has_clear_cmd)
		rinfo->recover_bus = <the above function>;
	else
		rinfo->recover_bus = i2c_generic_scl_recovery;

Then, i2c_recover_bus() will always do the right thing. More readable
and better maintainable IMO.

If this is not possible (maybe I overlooked some logic), then maybe this
will work:

	rinfo->recover_bus = <your custom function>;

and put the

	if (has_clear_cmd)

block there.


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