linux-kernel - Re: [PATCH RESEND] i2c: tegra: Add SMBus block read function

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Message-ID: <f3db5152-0e1f-4258-4cfd-521828a243c2@gmail.com>
Date:   Sat, 12 Feb 2022 23:08:55 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Akhil R <akhilrajeev@...dia.com>,
        "christian.koenig@....com" <christian.koenig@....com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        Mikko Perttunen <mperttunen@...dia.com>,
        "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        "sumit.semwal@...aro.org" <sumit.semwal@...aro.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        Svyatoslav Ryhel <clamor95@...il.com>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>
Subject: Re: [PATCH RESEND] i2c: tegra: Add SMBus block read function

12.02.2022 19:20, Dmitry Osipenko пишет:
> 12.02.2022 19:15, Dmitry Osipenko пишет:
>> 11.02.2022 12:11, Akhil R пишет:
>>>> 10.02.2022 18:36, Akhil R пишет:
>>>>> Emulate SMBus block read using ContinueXfer to read the length byte
>>>>>
>>>>> Signed-off-by: Akhil R <akhilrajeev@...dia.com>
>>>>> ---
>>>>>  drivers/i2c/busses/i2c-tegra.c | 18 ++++++++++++++++--
>>>>>  1 file changed, 16 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
>>>>> index 03cea102ab76..2941e42aa6a0 100644
>>>>> --- a/drivers/i2c/busses/i2c-tegra.c
>>>>> +++ b/drivers/i2c/busses/i2c-tegra.c
>>>>> @@ -1233,6 +1233,11 @@ static int tegra_i2c_xfer_msg(struct tegra_i2c_dev
>>>> *i2c_dev,
>>>>>               return err;
>>>>>
>>>>>       i2c_dev->msg_buf = msg->buf;
>>>>> +
>>>>> +     /* The condition true implies smbus block read and len is already read */
>>>>> +     if (msg->flags & I2C_M_RECV_LEN && end_state !=
>>>> MSG_END_CONTINUE)
>>>>> +             i2c_dev->msg_buf = msg->buf + 1;
>>>>> +
>>>>>       i2c_dev->msg_buf_remaining = msg->len;
>>>>>       i2c_dev->msg_err = I2C_ERR_NONE;
>>>>>       i2c_dev->msg_read = !!(msg->flags & I2C_M_RD);
>>>>> @@ -1389,6 +1394,15 @@ static int tegra_i2c_xfer(struct i2c_adapter *adap,
>>>> struct i2c_msg msgs[],
>>>>>                       else
>>>>>                               end_type = MSG_END_REPEAT_START;
>>>>>               }
>>>>> +             /* If M_RECV_LEN use ContinueXfer to read the first byte */
>>>>> +             if (msgs[i].flags & I2C_M_RECV_LEN) {
>>>>> +                     ret = tegra_i2c_xfer_msg(i2c_dev, &msgs[i],
>>>> MSG_END_CONTINUE);
>>>>> +                     if (ret)
>>>>> +                             break;
>>>>> +                     /* Set the read byte as msg len */
>>>>> +                     msgs[i].len = msgs[i].buf[0];
>>>>> +                     dev_dbg(i2c_dev->dev, "reading %d bytes\n", msgs[i].len);
>>>>> +             }
>>>>>               ret = tegra_i2c_xfer_msg(i2c_dev, &msgs[i], end_type);
>>>>>               if (ret)
>>>>>                       break;
>>>>> @@ -1416,10 +1430,10 @@ static u32 tegra_i2c_func(struct i2c_adapter
>>>> *adap)
>>>>>  {
>>>>>       struct tegra_i2c_dev *i2c_dev = i2c_get_adapdata(adap);
>>>>>       u32 ret = I2C_FUNC_I2C | (I2C_FUNC_SMBUS_EMUL &
>>>> ~I2C_FUNC_SMBUS_QUICK) |
>>>>> -               I2C_FUNC_10BIT_ADDR | I2C_FUNC_PROTOCOL_MANGLING;
>>>>> +               I2C_FUNC_10BIT_ADDR | I2C_FUNC_PROTOCOL_MANGLING;
>>>>>
>>>>>       if (i2c_dev->hw->has_continue_xfer_support)
>>>>> -             ret |= I2C_FUNC_NOSTART;
>>>>> +             ret |= I2C_FUNC_NOSTART | I2C_FUNC_SMBUS_READ_BLOCK_DATA;
>>>>>
>>>>>       return ret;
>>>>>  }
>>>>
>>>> Please describe how this was tested.
>>> This is tested using an I2C EEPROM to emulate SMBus block read in which
>>> we read the first byte as the length of bytes to read. This is an expected
>>> feature for NVIDIA Grace chipset where there will be an actual SMBus device.
>>
>> We have several Tegra30+ tablets that have EC on SMBus. Svyatoslav tried
>> your I2C patch + [1] on Asus TF201 and reported that it breaks EC. Any
>> idea why it doesn't work?
>>
>> [1]
>> https://github.com/grate-driver/linux/commit/aa8d71f5a960ef40503e5448c622d62d1c53a2c0
> 
> Ah, I see now that I2C_FUNC_SMBUS_WRITE_BLOCK_DATA not supported, we
> should check again without the write then.

We also missed that i2c_smbus_read_i2c_block_data() populates the first
byte of the read data with the transfer size. So
i2c_smbus_read_block_data() actually works properly.

It's unclear to me what's the point of emulating
I2C_FUNC_SMBUS_READ_BLOCK_DATA within the driver if you could use
i2c_smbus_read_i2c_block_data().