linux-kernel - Re: [PATCH 1/3] dmaengine: add new dma API for max_segment

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Date:	Mon, 06 Jun 2011 11:05:01 +0530
From:	"Koul, Vinod" <vinod.koul@...el.com>
To:	Shawn Guo <shawn.guo@...escale.com>
Cc:	Shawn Guo <shawn.guo@...aro.org>, dan.j.williams@...el.com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	patches@...aro.org
Subject: Re: [PATCH 1/3] dmaengine: add new dma API for max_segment_number

On Thu, 2011-06-02 at 13:56 +0800, Shawn Guo wrote:
> On Wed, Jun 01, 2011 at 01:12:24PM +0530, Koul, Vinod wrote:
> > On Thu, 2011-05-26 at 14:31 +0800, Shawn Guo wrote:
> > > Like dma_set(get)_max_seg_size for max_segment_size, the patch adds
> > > max_segment_number into device_dma_parameters and creates the
> > > corresponding dmaengine API dma_set(get)_max_seg_number for it.
> > > 
> > > Signed-off-by: Shawn Guo <shawn.guo@...aro.org>
> > > ---
> > >  include/linux/device.h      |    1 +
> > >  include/linux/dma-mapping.h |   15 +++++++++++++++
> > >  2 files changed, 16 insertions(+), 0 deletions(-)
> > Can you explain me why this change is required, the patch description
> > isn't helping for this.
> > 
> > If you satisfy me with above then I am okay with it, but I need ACK from
> > Dan before applying...
> > 
> I recently saw you people are trying to sort out the slave dma api
> document.  This reminds me there is small piece missing, which the
> patch here is trying to add.
> 
> Here is the user story.  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
> not 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.
> 
> Right now, there is well defined dma api (dma_get_max_seg_size) for
> mmc to query max_seg_size from dma driver, but the one for max_segs is
> missing.  That's why mxs-mmc driver has to hard-code it.
> 
> The mxs-mmc is just one example to demonstrate the need of the new
> api, and I'm sure there are other mmc host drivers (mxcmmc on imx-dma
> is another example) and possibly even other dmaengine users need this
> new api to know the maximum segments that dma driver can handle per
> dma call.
> 
Looks fine, care to add above in your patch description.
Few years down it will be really helpful to know why this was added.

-- 
~Vinod

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