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

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Message-ID: <cbe8352c-c1c7-12a7-c658-82e7ffee0be8@gmail.com>
Date:   Wed, 8 May 2019 15:37:14 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Jon Hunter <jonathanh@...dia.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Vinod Koul <vkoul@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Ben Dooks <ben.dooks@...ethink.co.uk>
Cc:     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

08.05.2019 12:24, Jon Hunter пишет:
> 
> 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!
> 
> Is there are use-case you are looking at where you don't set the
> DMA_PREP_INTERRUPT flag?

No. I just noticed it while was checking whether we really need to
handle the BUSY bit state for the Ben's "accurate reporting" patch.

> 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.

The warning message will be also okay if it's not mandatory.