linux-kernel - Re: [PATCH v1] dmaengine: tegra-apb: Handle DMA_PREP

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Message-ID: <20190521045545.GP15118@vkoul-mobl>
Date:   Tue, 21 May 2019 10:25:45 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Jon Hunter <jonathanh@...dia.com>
Cc:     Dmitry Osipenko <digetx@...il.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Ben Dooks <ben.dooks@...ethink.co.uk>,
        dmaengine@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] dmaengine: tegra-apb: Handle DMA_PREP_INTERRUPT flag
 properly

On 08-05-19, 10:24, Jon Hunter wrote:
> 
> On 05/05/2019 19:12, Dmitry Osipenko wrote:
> > The DMA_PREP_INTERRUPT flag means that descriptor's callback should be
> > invoked upon transfer completion and that's it. For some reason driver
> > completely disables the hardware interrupt handling, leaving channel in
> > unusable state if transfer is issued with the flag being unset. Note
> > that there are no occurrences in the relevant drivers that do not set
> > the flag, hence this patch doesn't fix any actual bug and merely fixes
> > potential problem.
> > 
> > Signed-off-by: Dmitry Osipenko <digetx@...il.com>
> 
> >From having a look at this, I am guessing that we have never really
> tested the case where DMA_PREP_INTERRUPT flag is not set because as you
> mentioned it does not look like this will work at all!

That is a fair argument
> 
> Is there are use-case you are looking at where you don't set the
> DMA_PREP_INTERRUPT flag?
> 
> If not I am wondering if we should even bother supporting this and warn
> if it is not set. AFAICT it does not appear to be mandatory, but maybe
> Vinod can comment more on this.

This is supposed to be used in the cases where you submit a bunch of
descriptors and selectively dont want an interrupt in few cases...

Is this such a case?

Thanks
~Vinod