Highmem issues with MMC filesystem

[Russell King - ARM Linux]

On Thu, Mar 18, 2010 at 01:15:58PM +0200, saeed bishara wrote:
> >> The only conclusion I came to so far is that ARMv5 where highmem works
> >> just fine in all cases has VIVT cache whereas ARMv6 has VIPT cache.
> >> And the problem with VIPT caches occurs when direct DMA is involved,
> >> otherwise there is no problem if PIO or NFS is used.  Sprinkling some
> >> flush_cache_all() in a few places makes things work, but this is not a
> >> satisfactory solution.
> >
> > This sounds like the problem we had with the DMA API.  Since that's now
> > fixed, there shouldn't be a problem with the latest (-rc) kernels, or
> > a kernel with my old streaming DMA patches applied.
> The failure happens also on 2.6.34.rc1,  as Nico said, it looks like
> that buffers that are subject to DMA remain dirty, as I understand it,
> for vipt nonaliasing cpu's, the kernel doesn't clean user space cache
> lines. if I force kmap_atomic/kunmap_atomic to highmem pages that are
> not mapped by the kernel (kmap_high_get returns null), then the issue
> disappears.

In no case does the kernel ever clean user space cache lines for DMA;
that's not the responsibility of the DMA API.