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

[Takashi Iwai]

At Fri, 15 Jan 2010 09:04:17 +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.
> > 
> 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.

Thanks for heading up about this.

It seems, however, that pgprot_noncached() is still dangerous to apply
unconditionally, according to MIPS guys.  Some MIPS platforms may freeze
when this is set to invalid pages.

So, this should be applied still in an arch-dependent manner...
Oh well.


thanks,

Takashi