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

On Wed, 8 Jun 2011 13:05:57 -0700
Dan Williams [off-list ref] wrote:

Perhaps, but this sounds like the reverse of what happens today where
scsi device drivers with knowledge of their own hardware will tell the
midlayer/subsystem the restriction.  The change with regard to this
patch is that the scsi device driver (for example) will recognize that
the device it is driving will not be a bus master and will arrange to
allocate a dma channel from dmaengine.  When said scsi driver reports
the dma restrictions to the subsystem it will borrow the parameters
from the dma channel, not the scsi device.  So yes, I still think it
should be whatever the dma channel says.

Although, you've been doing scsi work longer than I, so maybe I'm
overlooking something...?

Are there any cases today where the subsystem imposes tighter
restrictions on the dma geometry than what the device reports?  Even

Yeah, there are already lots.

SCSI-mid layer set the max sg entries to 128 on many architectures
(see SCSI_MAX_SG_CHAIN_SEGMENTS).

SCSI HBA drivers stores the max sg entries in scsi_host structure.

SCSI-mid layer sets the tighter one (see __scsi_alloc_queue).

	blk_queue_max_segments(q, min_t(unsigned short, shost->sg_tablesize,
					SCSI_MAX_SG_CHAIN_SEGMENTS));

As you know, modern SCSI HBA can handle more sg entries than 128.

if that were the case it would be same situation that the scsi device
driver reports maximum parameters, but the subsystem opts for
something tighter.  Whether the maximal parameters come from the scsi
device or the dma channel is moot.

If only you convert all the SCSI HBA drivers to store the limit of sg
entries in struct device_dma_parameters and use the proposed API.

I can't find any description in this patchset about how we will
convert software that already set the limit of max sg entries in a
different way.

I don't think that this patchset needs to convert the SCSI (and other
software layers setting the limit of max sg entries). But I think that
we need a new rule, the data structure, and the API about how and
where everyone sets the dma restrictions and tells them to the upper
software layers.