Re: DMA Engine Documentation: TX Descriptor and Submission

[Vinod Koul <vkoul@kernel.org>]

On 01-02-19, 09:59, Federico Vaga wrote:
> 
> 
> On February 1, 2019 4:17:50 AM UTC, Vinod Koul <vkoul@kernel.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..?
> 

Please wrap your replies within 80chars

> 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.  -- Inviato dal mio
> dispositivo Android con K-9 Mail. Perdonate la brevità.

Sure please feel free to send a patch :)

Mostly people ask these questions based on a driver they are working
off, and it makes sense to fix the assumptions by asking what they are
trying to achieve...

-- 
~Vinod