> > - Previously, only USB transfers to unaddressable memory needed
> > to go through the bounce buffer, now all of them do, which may
> > impact runtime performance for USB endpoints that do a lot of
> > transfers.
> >
> > On the upside, the local_mem support uses write-combining buffers,
> > which should be a bit faster for transfers to the device compared to
> > normal uncached coherent memory as used in dmabounce.
> >
>
> Talking from past experience using this trick on a NXP ARM9 SoC ~10
> years ago, using on-chip SRAM for USB DMA likely results in a
> significant performance boost, even without write combining, although
> the exact scenario obviously matters.
Right, that makes sense, but it won't help here because there is
no SRAM. One detail I noticed is that the localmem pool normally
gets mapped as WC, which is what I did in the new code as well, but
dma_alloc_flags(..., DMA_ATTR_WRITE_COMBINE) does not always
honor this flag. I think it will do it here because a GFP_KERNEL
allocation should be served by the remap_allocator, while
GFP_ATOMIC allocations would be served by pool_allocator_alloc(),
which ignores the flag.
Arnd
