Highmem issues with MMC filesystem

[Nicolas Pitre]

On Wed, 17 Mar 2010, Hemanth V wrote:

> 
> ----- Original Message ----- From: "Nicolas Pitre" <nico at fluxnic.net>
> To: "Hemanth V" <hemanthv at ti.com>
> Cc: <linux-mmc at vger.kernel.org>; <pierre at ossman.eu>
> Sent: Wednesday, March 17, 2010 7:19 PM
> Subject: Re: Highmem issues with MMC filesystem
> 
> 
> > On Wed, 17 Mar 2010, Hemanth V wrote:
> > 
> > > Hi All,
> > > 
> > > We are trying to enable highmem support for OMAP based boards, while doing
> > > so
> > > we are
> > > unable to boot poky UI if the filesystem is in a MMC card.
> > > 
> > > The poky UI comes up fine if the filesystem is in NAND or NFS,
> > > while with MMC it gives errors like segmentation faults/syntax error while
> > > there are none.
> > > Seems like read from MMC is getting corrupted.
> > 
> > Yes, this is a known issue with highmem on ARM architectures v6 and
> > later, related to some cache mishandling.  This is not a MMC issue as
> > the problem occurs when booting from SATA too. The problem is still
> > investigated.
> > 
> 
> Nicolas, Russel are there any more details available which might help
> in investigating this further.

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.


Nicolas