[alsa-devel] Audio buffer mmap-ing on sh arch vs. cache aliasing problem

[Paul Mundt]

On Fri, Sep 26, 2008 at 12:04:23PM +0200, Takashi Iwai wrote:
> At Thu, 25 Sep 2008 15:55:53 +0100,
> Pawel MOLL wrote:
> > 
> > > One thing we can try is a patch like below.  But, I'm not sure whether
> > > this is correct over all architectures, too.  At best, a generic API
> > > would be helpful for such a thing...
> > > +	area->vm_page_prot = pgprot_noncached(area->vm_page_prot);
> > 
> > Well, it's not enough, because the kernel mapping of buffer is still
> > cached... A hack below does the job, but it is not nice as well...
> 
> Yeah, that's not sexy, but maybe the only working case right now
> (better with arch-specific ifdefs).
> 
> IIRC, a similar buffer handling (via vmalloc) is used in video
> drivers.  I suppose they don't work as well, right?
> DRM driver uses __vmalloc() with PAGE_KERNEL_NOCACHE, but it's only
> for PPC32 non-coherent.
> 
To come back to this issue, we don't appear to have made any headway on
the vmalloc front, but pgprot_noncached() is now a standard interface
provided by all architectures, so the ifdef is no longer needed in the
snd_pcm_default_mmap() path.