Re: [PATCH 1/3] dmaengine: add new dma API for max_segment_number

From: Koul, Vinod
Date: Mon Jun 06 2011 - 02:09:59 EST

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@xxxxxxxxxx>
> > > ---
> > > 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.


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