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

[Takashi Iwai]

At Fri, 26 Sep 2008 19:30:45 +0900,
Paul Mundt wrote:
> 
> 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.
> > 
> x86 does it also, via its PAGE_AGP definition.

Right, but it doesn't seem for pages allocated via vmalloc() as in
drm_scatter.c.

> There are not that many platforms that define PAGE_KERNEL_NOCACHE though,
> so this gets a bit messy..

Indeed.  We really need to have a generic API for this kind of
things...


thanks,

Takashi