linux-kernel - Re: [PATCH] dmaengine: qcom_hidma: release the descriptor before the callback

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Date:	Mon, 8 Aug 2016 14:38:52 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	Russell King - ARM Linux <linux@...linux.org.uk>,
	Sinan Kaya <okaya@...eaurora.org>,
	linux-arm-msm@...r.kernel.org, timur@...eaurora.org,
	linux-kernel@...r.kernel.org,
	Christopher Covington <cov@...eaurora.org>,
	dmaengine@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] dmaengine: qcom_hidma: release the descriptor before the
 callback

On Thu, Aug 04, 2016 at 05:59:30PM +0200, Lars-Peter Clausen wrote:
> On 08/04/2016 05:38 PM, Russell King - ARM Linux wrote:
> [...]
> > What you instead need to do is to find some way to record in your
> > driver that transaction 2 failed, and when dma_cookie_status() says
> > that a transaction has DMA_COMPLETE status, you need to look up to
> > see whether it failed.
> 
> In my opinion this is where the current API is broken by design. For each
> transfer that fails you need to store the cookie associated with that
> transfer in some kind of lookup table. Since there is no lifetime associated
> with a cookie entries in this table would need to be retained forever and it
> will grow unbound.

And how many drivers can report errors? And how many drivers can guarantee
DMA_COMPLETE implies transaction was succesful.

> Ideally we'd mark error reporting through this interface as deprecated and
> discourage new users of the interface. As far as I can see most of the few
> drivers that do return DMA_ERROR get it wrong anyway, e.g. return it
> unconditionally for all cookies when an error occurred for any of them.

Error reporting is quite tricky as detection is a problem. So yes if you
can do so, it is highly encouraged to report using new interface which is
better than client checking after callback.

Btw what is the behaviour after error? I would think that client will see an
error and report to upper layer while initiaite closure of transaction. So
does driver need to keep the state for a longer time :-)

-- 
~Vinod