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Subject: Re: [PATCH] i2c: at91: Send bus clear command if SCL or SDA is down
To:     Codrin Ciubotariu <codrin.ciubotariu@microchip.com>,
        linux-i2c@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
Cc:     ludovic.desroches@microchip.com, nicolas.ferre@microchip.com,
        alexandre.belloni@bootlin.com, wsa@the-dreams.de
References: <20190911095854.5141-1-codrin.ciubotariu@microchip.com>
From:   kbouhara <kamel.bouhara@bootlin.com>
Message-ID: <1ed845e5-3835-f1aa-099a-b67c3bc16076@bootlin.com>
Date:   Thu, 19 Sep 2019 17:06:18 +0200
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On 9/11/19 11:58 AM, Codrin Ciubotariu wrote:
> After a transfer timeout, some faulty I2C slave devices might hold down
> the SCL or the SDA pins. We can generate a bus clear command, hoping that
> the slave might release the pins.
>
> Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
> ---
>   drivers/i2c/busses/i2c-at91-master.c | 20 ++++++++++++++++++++
>   drivers/i2c/busses/i2c-at91.h        |  6 +++++-
>   2 files changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/i2c/busses/i2c-at91-master.c b/drivers/i2c/busses/i2c-at91-master.c
> index a3fcc35ffd3b..5f544a16db96 100644
> --- a/drivers/i2c/busses/i2c-at91-master.c
> +++ b/drivers/i2c/busses/i2c-at91-master.c
> @@ -599,6 +599,26 @@ static int at91_do_twi_transfer(struct at91_twi_dev *dev)
>   		at91_twi_write(dev, AT91_TWI_CR,
>   			       AT91_TWI_THRCLR | AT91_TWI_LOCKCLR);
>   	}
> +
> +	/*
> +	 * After timeout, some faulty I2C slave devices might hold SCL/SDA down;
> +	 * we can send a bus clear command, hoping that the pins will be
> +	 * released
> +	 */
> +	if (!(dev->transfer_status & AT91_TWI_SDA) ||
> +	    !(dev->transfer_status & AT91_TWI_SCL)) {
> +		dev_dbg(dev->dev,
> +			"SDA/SCL are 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);

This bit is not documented on SoCs before SAMA5D2/D4, this write 
shouldn't be done unconditionally.


-- 
Kamel Bouhara, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com