linux-kernel - Re: [PATCHv6 1/2] i2c: designware: introduce a custom scl recovery for SoCFPGA platforms

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Date:   Tue, 12 Jul 2022 07:52:12 -0500
From:   Dinh Nguyen <dinguyen@...nel.org>
To:     Wolfram Sang <wsa@...nel.org>, jarkko.nikula@...ux.intel.com,
        andriy.shevchenko@...ux.intel.com, mika.westerberg@...ux.intel.com,
        robh+dt@...nel.org, krzk+dt@...nel.org, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        christopher.hatch@...el.com
Subject: Re: [PATCHv6 1/2] i2c: designware: introduce a custom scl recovery
 for SoCFPGA platforms



On 7/12/22 07:41, Dinh Nguyen wrote:
> Hi Wolfram,
> 
> On 6/22/22 15:07, Wolfram Sang wrote:
>>
>>>  From the original code, the first mechanism to a recovery is to 
>>> acquire a
>>> GPIO for the SCL line and send the 9 SCL pulses, after that, it does 
>>> a reset
>>> of the I2C module. For the SOCFPGA part, there is no GPIO line for 
>>> the SCL,
>>> thus the I2C module cannot even get a reset. This code allows the 
>>> function
>>> to reset the I2C module for SOCFPGA, which is the 2nd part of the 
>>> recovery
>>> process.
>>
>> The second part is totally useless if the client device is holding SDA
>> low. Which is exactly the situation that recovery tries to fix. As I
>> said, if you can't control SCL, you don't have recovery.
>>
> 
> This is recovery of the master and not the slave.  We have a customer 
> that is the using I2C with the signals routed through the FPGA, and thus 
> are not GPIO. During a timeout, with this code, the driver is able to 
> recover the master.
> 

Adding a bit more, because of patch:

ca382f5b38f3 ("i2c: designware: add i2c gpio recovery option")

the driver is now not able to reset the controller at all because it has 
placed a strict dependency on getting a GPIO. Before this patch, during 
a timeout, there was a simple call to i2c_dw_init_master(), which 
ultimately resets the master.

Dinh