linux-kernel - Re: DMA Engine Documentation: TX Descriptor and Submission

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Date:   Fri, 1 Feb 2019 09:59:22 +0000
From:   Federico Vaga <federico.vaga@...n.ch>
To:     Vinod Koul <vkoul@...nel.org>
CC:     <dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-doc@...r.kernel.org>
Subject: Re: DMA Engine Documentation: TX Descriptor and Submission



On February 1, 2019 4:17:50 AM UTC, Vinod Koul <vkoul@...nel.org> wrote:
>On 28-01-19, 09:47, Federico Vaga wrote:
>> Hi,
>> 
>> I have a new question concerning documentation.
>> 
>>
>https://www.kernel.org/doc/html/latest/driver-api/dmaengine/client.html
>> 
>> >From this document it is not really clear, at least to me, if
>clients can 
>> consider valid the `struct dma_async_tx_descriptor` after submission
>to the 
>> DMA engine.
>
>Nope they can't and should not touch the descriptor after submission.
>The client get cookie and that is supposed to be used

Good, thanks


>> 
>> Clients get a TX descriptor from a DMA engine using things like 
>> `dmaengine_prep_*`. These calls - may - allocate new descriptors and
>return 
>> them to the caller; this may include other structures which are not
>visible to 
>> clients. So, if my understanding is correct, this means that it's the
>DMA 
>> engine that, on TX completion, releases any TX descriptor allocated
>by 
>> `dmaengine_prep_*`. This implies that the pointer that the client is
>using 
>> must be considered invalid right after `dmaengine_submit()`.
>> If what I understood by reading the documentation and the code is
>correct, 
>> then I think that this should be mentioned in the Documentation.
>> If I'm wrong, please tell me where :)
>
>And what exactly are you trying to do here..?

What if I answer: "the right thing"? Joking. I just expressed my understanding of something which is not documented properly (my opinion). I wanted to be sure that the logic, the reasons behind are the ones that I understood. I will propose a patch to the documentation, unless you want to do it.
-- 
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