[PATCH v3] dmaengine: add new dma API for max_segment_number

On Wed, 15 Jun 2011 20:08:41 +0800
Shawn Guo [off-list ref] 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.

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.

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 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.

As I wrote earlier, SCSI midlayer sets this parameter in a different
way (storing this in SCSI specific data structure).

If you add a new generic API for this parameter, please make sure that
the API works well for the others that already do the same
differently. That is, please send this to linux-scsi too (and discuss
with SCSI maintainer).