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

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.

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.

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.