linux-kernel - Re: [PATCH] i2c: at91: use dma safe buffers

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Date:   Tue, 05 Apr 2022 16:08:30 +0200
From:   Michael Walle <michael@...le.cc>
To:     Codrin.Ciubotariu@...rochip.com
Cc:     Nicolas.Ferre@...rochip.com, alexandre.belloni@...tlin.com,
        Claudiu.Beznea@...rochip.com, sumit.semwal@...aro.org,
        christian.koenig@....com, linux-i2c@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org, stable@...r.kernel.org
Subject: Re: [PATCH] i2c: at91: use dma safe buffers

Am 2022-04-05 15:58, schrieb Codrin.Ciubotariu@...rochip.com:
> On 05.04.2022 14:09, Michael Walle wrote:
>> Am 2022-04-05 12:02, schrieb Codrin.Ciubotariu@...rochip.com:
>>> On 05.04.2022 12:38, Michael Walle wrote:
>>>> Am 2022-04-05 11:23, schrieb Codrin.Ciubotariu@...rochip.com:
>>>>>> +       if (dev->use_dma) {
>>>>>> +               dma_buf = i2c_get_dma_safe_msg_buf(m_start, 1);
>>>>> 
>>>>> If you want, you could just dev->buf = i2c_get_dma_safe...
>>>> 
>>>> But where is the error handling in that case? dev->buf will
>>>> be NULL, which is eventually passed to dma_map_single().
>>>> 
>>>> Also, I need the dma_buf for the i2c_put_dma_safe_msg_buf()
>>>> call anyway, because dev->buf will be modified during
>>>> processing.
>>> 
>>> You still:
>>>       if (!dev->buf) {
>>>               ret = -ENOMEM;
>>>               goto out;
>>>       }
>>> 
>>> So, at91_do_twi_transfer()/dma_map_single() will not be called.
>> 
>> Ahh, I misunderstood you. Yes, but as I said, I need the dma_buf
>> temporary variable anyway, because dev->buf is modified, eg. see
>> at91_twi_read_data_dma_callback().
> at91_twi_read_data_dma_callback() is called as callback if
> dma_async_issue_pending(dma->chan_rx) is called.
> dma_async_issue_pending(dma->chan_rx) is called on
> at91_twi_read_data_dma(), which is called in at91_do_twi_transfer(),
> which we decided above to skip in case of error.

It is not about errors, you need the exact same pointer you
got from i2c_get_dma_safe_msg_buf() to be passed to
i2c_put_dma_safe_msg_buf(). And because (in some cases, it
isn't really obvious) the dev->buf will be advanced a few
bytes, I cannot pass dev->buf to i2c_put_dma_safe_msg_buf().

-michael