Re: [PATCH] i2c: at91: use dma safe buffers

[<Codrin.Ciubotariu@microchip.com>]

On 05.04.2022 17:08, Michael Walle wrote:
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> Am 2022-04-05 15:58, schrieb Codrin.Ciubotariu@microchip.com:
>> On 05.04.2022 14:09, Michael Walle wrote:
>>> Am 2022-04-05 12:02, schrieb Codrin.Ciubotariu@microchip.com:
>>>> On 05.04.2022 12:38, Michael Walle wrote:
>>>>> Am 2022-04-05 11:23, schrieb Codrin.Ciubotariu@microchip.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().

You are right, when dev->use_dma && (dev->buf_len <= AT91_I2C_DMA_THRESHOLD)

got it. Thanks!

Best regards,
Codrin