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

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Date:	Wed, 7 May 2014 15:50:16 -0700
From:	Christopher Freeman <cfreeman@...dia.com>
To:	Lars-Peter Clausen <lars@...afoo.de>
CC:	Stephen Warren <swarren@...dotorg.org>,
	Laxman Dewangan <ldewangan@...dia.com>,
	Stephen Warren <swarren@...dia.com>,
	"vinod.koul@...el.com" <vinod.koul@...el.com>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 3/3] dma: tegra: avoid int overflow for transferred
 count

On Wed, May 07, 2014 at 12:15:39PM -0700, Lars-Peter Clausen wrote:
> 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.
>
What each driver receives remains unchanged here.  bytes_transferred is only ever read modulo bytes_requested in all cases (audio, spi, uart)  This shifts that part of the calculation to the assignment.  I guess this is a roundabout way of saying it's not any more wrong than it could have possibly been before.  We can rename "bytes_transferred" to something like "residue" or "segment_residue" though.
 
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