[PATCH v3 6/6] Documentation: dmaengine: document DMA_CTRL_ACK

Robert Jarzmik robert.jarzmik at free.fr
Thu May 14 13:31:39 PDT 2015

Maxime Ripard <maxime.ripard at free-electrons.com> writes:

> Hi Robert,
>>    * DMA_CTRL_ACK
>> -    - Undocumented feature
>> -    - No one really has an idea of what it's about, besides being
>> -      related to reusing the DMA transaction descriptors or having
>> -      additional transactions added to it in the async-tx API
>> -    - Useless in the case of the slave API
>> +    - if set, the TX transfer can be reused after being completed.
> Your sentences should start with an upper-case letter.

>> +    - there is a guarantee the TX won't be freed until it is acked
>> +      by async_tx_ack()
> We never talked about what a "TX transfer" is in the
> documentation. That should be documented.
I'll remove the "TX", the transfer is widely used in this documentation.

>> +    - as a consequence, if a device driver wants to skip the dma_map_sg() and
>> +      dma_unmap_sg() because the DMA'd data wasn't used, it can resubmit the
>> +      transfer right after its completion.
> From a provider PoV, the data should always be mapped and / or
> allocated in coherent way, so I'm not sure how does that's relevant in
> the provider doc.
It's relevant because it says the mapping/unmapping is not necessary _between_ 2
submission of a transfer. This implies it is mapped, as it is a requirement for
the very first submission.

> Also, we should still mention that it's !slave API only.
Certainly not, it's fully slave API, that's the purpose of this patch.


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