linux-kernel - Re: [PATCH V3] i2c: tegra: Add Bus Clear Master Support

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Message-ID: <783993ec-6d88-9a84-b1bd-cd44d45e2584@gmail.com>
Date:   Wed, 23 Jan 2019 16:17:19 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Sowjanya Komatineni <skomatineni@...dia.com>
Cc:     "thierry.reding@...il.com" <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Mantravadi Karthik <mkarthik@...dia.com>,
        Shardar Mohammed <smohammed@...dia.com>,
        Timo Alho <talho@...dia.com>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH V3] i2c: tegra: Add Bus Clear Master Support

23.01.2019 2:26, Sowjanya Komatineni пишет:
> 
> 
>>>>> +	if (i2c_dev->msg_err == I2C_ERR_ARBITRATION_LOST) {
>>>>> +		if (!i2c_dev->is_multimaster_mode)
>>>>> +			return tegra_i2c_issue_bus_clear(i2c_dev);
>>>>> +		return -EAGAIN;
>>>>
>>>> This changes the returned errno from -EIO to -EAGAIN for the 
>>>> supports_bus_clear=false case, is it okay and intentional?
>>>>  
>>>
>>> Yes EAGAIN is intentional to allow for transfer retry.
>>> During single master mode, ARBITRATION LOST notification happens when 
>>> 1. I2C Master sees the bus is occupied by some other device when a 
>>> transfer is initiated 2. I2C Master lost the bus during arbitration 
>>> incase if slave device pulls SDA line low continuously for some 
>>> unknown reason If arbitration lost is due to cause 1, retry helps to 
>>> continue with transfer once bus is released by the slave and it just 
>>> added delay in communication due to bus release delay by slave. In 
>>> case of 2nd cause, retry never succeeds in cases where bus clear is 
>>> not supported.
>>
>> It's unclear whether the "never succeeds retry" may fail with the EAGAIN, causing an endless retry-loop. Could you please clarify this moment?
> 
> during master transmit mode, on arbitration lost and if master doesn’t support bus clear to recover then transfer will return EAGAIN.
> I2c core base driver performs retries if return code from i2c_transfer is EAGAIN up to specified retries in i2c adapter and returns the ret code from the last retry.
> Retry is not endless as i2c core base performs retry only up to specified adapter retries.
> Following return code from documentation
> https://www.kernel.org/doc/Documentation/i2c/fault-codes
> 
> 

Could you please point at the code that sets the number of retries for the Tegra's I2C? Looks it is always 0 and hence EAGAIN won't do anything useful.