linux-kernel - Re: [PATCH v3 RESEND] dma-mapping: add new API for max_segment

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Date:	Fri, 17 Jun 2011 20:09:44 +0200
From:	Per Forlin <per.forlin@...aro.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Shawn Guo <shawn.guo@...aro.org>, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-mmc@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	vinod.koul@...el.com, gregkh@...e.de,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Dan Williams <dan.j.williams@...el.com>, cjb@...top.org
Subject: Re: [PATCH v3 RESEND] dma-mapping: add new API for max_segment_number

On 17 June 2011 14:40, Matthew Wilcox <matthew@....cx> wrote:
> On Thu, Jun 16, 2011 at 08:30:53PM +0800, Shawn Guo wrote:
>> Here is the user story that tells the need of the new api.  The
>> mxs-mmc is the mmc host controller for Freescale MXS architecture.
>> There are a pair of  mmc host specific parameters max_seg_size and
>> max_segs that mxs-mmc host driver needs to tell mmc core, so that
>> mmc core can know how big each data segment could be and how many
>> segments could be handled one time in a scatter list by host driver.
>>
>> The mxs-mmc driver is one user of dmaengine mxs-dma, and it will call
>> mxs-dma to transfer data in scatter list.  That is to say mxs-mmc has
>> no idea of what max_seg_size and max_segs should be, because they are
>> all mxs-dma capability parameters, and mxs-mmc needs to query them
>> from mxs-dma.
>
> This approach would make sense if mxs-mmc were generic, but it's tied to
> mxs-dma, so it can just as well call mxs-dma to find out how many segments
> it supports.
>
mxs-dma use the generic dmaengine.h. The patch is not only intended
for mxs-mmc. MMCI and other mmc host drivers need a way to query
max_seg_number.
It would be possible to store this number within drivers/dmaengine but
this would not work for omap_hsmmc that doesn't use drivers/dmaengine

Regards,
Per
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