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

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Message-Id: <20110622023929W.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 22 Jun 2011 02:44:47 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	matthew@....cx
Cc:	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, linux@....linux.org.uk,
	vinod.koul@...el.com, gregkh@...e.de,
	fujita.tomonori@....ntt.co.jp, dan.j.williams@...el.com,
	cjb@...top.org, per.forlin@...aro.org
Subject: Re: [PATCH v3 RESEND] dma-mapping: add new API for
	max_segment_number

On Fri, 17 Jun 2011 06:40:35 -0600
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.

SCSI HBA drivers stores the max number of sg entries in
host->sg_tablesize (and scsi-ml tells the block layer about the
limit)? So if we have the generic API to handle the value, scsi HBA
drivers (and scsi-ml) could use it too?
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