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

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Message-ID: <20190201041750.GM4635@vkoul-mobl>
Date:   Fri, 1 Feb 2019 09:47:50 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Federico Vaga <federico.vaga@...n.ch>
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 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
> 
> 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..?

-- 
~Vinod