linux-kernel - Re: [PATCH] dma: Revert "dmaengine: mic_x100: add missing spin

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Date:	Mon, 4 Jan 2016 15:35:34 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Vinod Koul <vinod.koul@...el.com>,
	Ashutosh Dixit <ashutosh.dixit@...el.com>
Cc:	linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org,
	Sudeep Dutt <sudeep.dutt@...el.com>,
	Nikhil Rao <nikhil.rao@...el.com>,
	Siva Yerramreddy <yshivakrishna@...il.com>,
	Saurabh Sengar <saurabh.truth@...il.com>
Subject: Re: [PATCH] dma: Revert "dmaengine: mic_x100: add missing
 spin_unlock"

On 01/04/2016 04:35 AM, Vinod Koul wrote:
> On Tue, Dec 22, 2015 at 07:35:23PM -0800, Ashutosh Dixit wrote:
>> This reverts commit e958e079e254 ("dmaengine: mic_x100: add missing
>> spin_unlock").
>>
>> The above patch is incorrect. There is nothing wrong with the original
>> code. The spin_lock is acquired in the "prep" functions and released
>> in "submit".
> 
> And going by dmaengine sematics, I do not think that is entrely right.
> 
> A user may choose to prepare multiple desciptors and then sumbit later,
> looking at code I do not see how that will work.

The DMAengine API actually mandates that prep and submit must always be
called in pairs, without any other DMAengine calls in between. The patch is
correct.

Quoting from Documentation/dmaengine/client.txt:

   Once a descriptor has been obtained, the callback information can be
   added and the descriptor must then be submitted.  Some DMA engine
   drivers may hold a spinlock between a successful preparation and
   submission so it is important that these two operations are closely
   paired.
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