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

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Message-ID: <74494dda-e0cd-aa73-7e58-e4359c1ba292@microchip.com>
Date:   Tue, 5 Apr 2022 13:58:20 +0000
From:   <Codrin.Ciubotariu@...rochip.com>
To:     <michael@...le.cc>
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

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.