Subject: Re: [PATCH] i2c: at91: Send bus clear command if SCL or SDA is down

From: <[email-masked]>
Date: Thu, 19 Sep 2019 16:25:32 +0000

On 19.09.2019 18:06, kbouhara wrote:
> 
> 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 <[email-masked]>
>> ---
>>   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.
> 
> 

Indeed, they are not present on SAMA5D4 or earlier SoCs. It is supported 
on SAMA5D2 though. I will make a new version and implement the CLEAR 
command only for the SoCs that support it.

Thank you for your review.

Best regards,
Codrin
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