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

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Date:	Fri, 05 Aug 2016 08:32:56 +0200
From:	Robert Jarzmik <robert.jarzmik@...e.fr>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	Sinan Kaya <okaya@...eaurora.org>,
	Russell King - ARM Linux <linux@...linux.org.uk>,
	Vinod Koul <vinod.koul@...el.com>,
	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

Lars-Peter Clausen <lars@...afoo.de> writes:

> On 08/04/2016 06:08 PM, Sinan Kaya wrote:
> [...]
>> The other way is I can feed this information to what Dave just introduced
>> as part of the callback mechanism and not touch this.
>
> Use the callback mechanism. It is a lot easier to implement correctly than
> the tx_status() mechanism.
>
>> The main discussion here is that 
>> 
>> "tx_status does not indicate whether the final transaction is successful or
>> not" whether the driver has the capability to determine error or not.
>
> tx_status() is supposed to be able to indicate whether a transfer failed or
> not. But in my opinion this feature is broken by design and implementing it
> correctly is very difficult without creating memory leaks. Which is probably
> why so few drivers actually implement it.

I think you can implement the error reporting by remembering the "last" cookie
where an error occurred such as in :
 - e093bf60ca49 ("dmaengine: pxa: handle bus errors")
   => see the part of the commit beginning with "As dma_cookie_status() ..."

The point about error reporting was already discussed  with Vinod in here:
   https://lkml.org/lkml/2016/4/13/471
   => Vinod and I were seeing the reporting can be improved

Yet the description made by Russell of the DMA API semantics can be implemented
and used to find whether a specific tx failed or not, it's rather the "in
progress" part I find misleading.

Cheers.

-- 
Robert