linux-kernel - Re: [PATCH] DMAENGINE: add a slave buffer prep call

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Message-ID: <20100812075641.GX27749@pengutronix.de>
Date:	Thu, 12 Aug 2010 09:56:41 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Linus Walleij <linus.ml.walleij@...il.com>
Cc:	Rabin Vincent <rabin@....in>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] DMAENGINE: add a slave buffer prep call

On Wed, Aug 11, 2010 at 09:02:04PM +0200, Linus Walleij wrote:
> 2010/8/11 Rabin Vincent <rabin@....in>:
> 
> > On Tue, Aug 10, 2010 at 11:46:06PM +0200, Linus Walleij wrote:
> >> This makes it possible for engines to implement slave transfers
> >> to be done to/from a simple kmalloc():ed memory buffer and not
> >> just from scatterlists.
> >
> > Why is this needed?  Drivers can just pass in a single-entry scatterlist
> > to the existing API to achieve the same functionality, and a couple of
> > them already do so.
> 
> Because of the overhead, simply. Especially if you want to trigger
> many jobs after each other. (This is necessary in device/slave-DMA
> since every transaction may fail...) It's not just constructing the
> sg-headers and freeing them again and again, it's also list traversals
> here and there since the driver must assume it can be a linked sglist and
> then two other list traversals for each
> dma_map_sg()/dma_unmap_sg() pair and ... yeah that's basically
> it.
> 
> And the number of extra code lines needed.
> 
> Then it's something conceptwise of creating a list that you know
> is just always one element that is just not elegant, like taking a queue
> numer and standing in queue when there is only one customer but
> hey, maybe it's just me.
> 
> One way of achieving it for all present drivers is to wrap the passed
> buffer in a single sglist and pass to the sglist function if the single
> buffer call is not supported in the driver. Maybe it'd be better if I
> coded up the patch like that so all driver can rely on this function
> to be present?

+1 for this approach as it seems easier to implement and has benefits
for all drivers.

Sascha


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