On 27 June 2011 12:02, Russell King - ARM Linux [off-list ref] wrote:

On Mon, Jun 27, 2011 at 11:42:52AM +0200, Per Forlin wrote:

quoted

Conclusion:
Working with mmc the relative cost of DSB is almost none. There seems
to be slightly higher number for mmc blocking requests with the DSB
patch compared to not having it.

These figures suggest that dsb is comparitively not heavy on the hardware
you're testing.

Yes, of course.

I think I'm going to apply the patch anyway - it certainly makes stuff
no worse, and if someone has a platform where dsb is more expensive,
then they should see a greater benefit from this change.

I agree.

The next thing to think about in DMA-land is whether we should total up
the size of the SG list and choose whether to flush the individual SG
elements or do a full cache flush. ?There becomes a point where the full
cache flush becomes cheaper than flushing each SG element individually.

Interesting.
I have seen such optimisations in hwmem (yet another memory manager
for multi media hardware). It would be nice to have such functionality
in dma-mapping, to be used by anyone

Regards,
Per