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

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Date:   Wed, 22 Jun 2022 08:45:12 -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
Subject: Re: [PATCHv6 1/2] i2c: designware: introduce a custom scl recovery
 for SoCFPGA platforms



On 6/21/22 16:37, Wolfram Sang wrote:
> On Mon, Jun 20, 2022 at 06:01:08PM -0500, Dinh Nguyen wrote:
>> The I2C pins on the SoCFPGA platforms do not go through a GPIO module,
>> thus cannot be recovered by the default method of by doing a GPIO access.
>> Only a reset of the I2C IP block can a recovery be successful, so this
>> change effectively resets the I2C controller, NOT any attached clients.
> 
> I am afraid here is a serious misunderstanding. The I2C bus recovery
> procedure is a documented mechanism how to get a stalled bus back in the
> case that a client device holds SDA low. This mechanism consists of 9
> SCL pulses. A reset of the IP core is *not a recovery*. If SocFPGA
> cannot togle SCL in some way, it cannot do recovery and
> adap->bus_recovery_info should be NULL. Or did I miss something?

 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.

> 
>> +static int i2c_socfpga_scl_recovery(struct i2c_adapter *adap)
>> +{
>> +	struct i2c_bus_recovery_info *bri = adap->bus_recovery_info;
>> +
>> +	bri->prepare_recovery(adap);
>> +	bri->unprepare_recovery(adap);
>> +
>> +	return 0;
>> +}
> 
> See, this function is named scl_recovery, but there is no SCL involved.
> This is why I think there is the misunderstanding here.
> 

I understand your point here. Perhaps just call it i2c_socfpga_recovery()?

Dinh