[PATCH v2 1/3] dmaengine: add new dma API for max_segment_number

[FUJITA Tomonori]

On Mon, 6 Jun 2011 10:14:10 +0100
Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:

> On Mon, Jun 06, 2011 at 05:06:03PM +0900, FUJITA Tomonori wrote:
> > max_segs isn't unrelated with the dma mapping API. I explained above,
> > IOMMUs doesn't increase the number of segments (could decrease the
> > number of segments by merging).
> > 
> > The limitation about the number of segment already lives elsewhere
> > (e.g. queue's limits.max_segments).
> 
> I think you're missing the point entirely.
> 
> Lets take the problem at hand: you have two devices.  One of them is
> handled by the DMA engine code.  One of them is a block device.
> 
> The block layer needs to know the various parameters of what is
> allowable for DMA, including such things as the maximum size of a
> segment, and the _number_ of segments that can be placed into any
> one request.
> 
> As the DMA provider is _entirely_ separate and unknown to the block
> device driver, the block device driver has no way to sanely provide
> these parameters to the block layer - they are not a property of the
> block device driver, but of the DMA provider.

struct device_dma_parameters is used for a property of the block
device drivers (and scsi HBA drivers, etc). Not DMA provider. Right?

The drivers calls dma_set_seg_boundary() and the subsystems call
dma_get_seg_boundary to set the value to queues.

This patch is trying to use struct device_dma_parameters in a
different way. It adds a new DMA parameter but for the DMA parameter
for a different layer. I'm not sure about different-layer stuff in 
one structure and using similar API.