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

Pawel MOLL pawel.moll at st.com
Fri Sep 26 13:25:17 CEST 2008

> > IIRC, a similar buffer handling (via vmalloc) is used in video
> > drivers.  I suppose they don't work as well, right?

Well, our FB driver is aware of the problem. I can't say much about
other ones...

> > DRM driver uses __vmalloc() with PAGE_KERNEL_NOCACHE, but it's only
> > for PPC32 non-coherent.

Hm. PPC with non-coherent cache?

 * Only on coherent architectures, we can mmap the status and the control records
 * for effcient data transfer.  On others, we have to use HWSYNC ioctl...
#if defined(CONFIG_X86) || defined(CONFIG_PPC) || defined(CONFIG_ALPHA)
 * mmap status record
static struct page * snd_pcm_mmap_status_nopage(struct vm_area_struct *area,
                                                unsigned long address, int *type)

> x86 does it also, via its PAGE_AGP definition.
> There are not that many platforms that define PAGE_KERNEL_NOCACHE though,
> so this gets a bit messy..

Oh, bloody hell, I haven't realised that!

> Yeah, that's not sexy, but maybe the only working case right now
> > (better with arch-specific ifdefs).

The question is what ifdefs to use ;-) Because the cached/uncached
decision must be consistent between snd_pcm_default_mmap() and buffer

What a mess! ;-)


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