linux-kernel - Re: [PATCH v1 3/3] dma: tegra: avoid int overflow for transferred count

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Message-ID: <536A865B.1060007@metafoo.de>
Date:	Wed, 07 May 2014 21:15:39 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	Christopher Freeman <cfreeman@...dia.com>, ldewangan@...dia.com,
	swarren@...dia.com, vinod.koul@...el.com, dan.j.williams@...el.com,
	dmaengine@...r.kernel.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 3/3] dma: tegra: avoid int overflow for transferred
 count

On 05/07/2014 06:38 PM, Stephen Warren wrote:
> On 05/06/2014 03:22 PM, Christopher Freeman wrote:
>> bytes_transferred will overflow during long audio playbacks.  Since
>> the driver only ever consults this value modulo bytes_requested, store the
>> value modulo bytes_requested.
>
> The audio driver may only interpret the value modulo bytes_requested,
> but what about other drivers such as the high-speed UART (and SPI?) drivers?
>
> What is the dmaengine API's design requirement here, and what do other
> dmaengine drivers do. If it's to store the modulo, then I'm fine with
> this change.

Yep, this part of the API. The residue should be between transfer length and 
0. While 0 is special and should only be returned if the transfer has 
finished. For cyclic transfers this means it should never be zero. So if 
transferred_bytes is incremented modulo length and residue is length - 
transferred_bytes you get the correct result.

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