[alsa-devel] 答复: 答复: A bug about cache inconsistency report

[Takashi Iwai]

On Wed, 08 Aug 2018 11:54:57 +0200,
Hans Hu(SH-RD) wrote:
> 
> > > >Then the next step would be to fake sg-buffer from this straight
> > > >buffer.  Revert the above, and modify sgbuf.c to the following:
> > > >- Allocate a large continuous buffer
> > > >- Assign each page in this large buffer
> > >
> > > >If this still works, it's not about vmap, but it just means that
> > > >the physically ordered pages do matter -- implicitly showing that
> > > >the snooping behavior isn't properly turned on / off on the controller.
> > >
> > > To fake SG-buffer, I did this test: restore all the codes to the original, then added some codes in snd_malloc_sgbuf_pages() like below, the result is badly niose.
> >
> > >Not really what I meant.  Rather something like below (totally untested).
> >
> > [Hans :] I know what you mean now and complete code like below, but the result is still noise.
> 
> >OK, so indeed the vmapped address does seem matter.  Interesting.
> >What kind of user access does produce the noise?  Does it via aplay mmap mode, too?
> 
> >In anyway, if the vmap is a problem, it might be worked around a patch like below (again totally untested and not sure whether it's correct).
> 
> [Hans :] As I know, In the hardware layer HDAC’s stream have two data transport path : non-snoop & snoop; In the software layer ALSA-Driver have two data transport path : mmap & not mmap(test shows, it is dependent on wav's format or mmap_flag in aplay.c). When hardware at non-snoop mode, without hardware module's help, software must mark the mem to uncacheable type: when mmap used, the mark action happened at pcm_mmap_prepare(), the not-mmap mode's mark action happened at __mark_pages_wc(). And when at not-mmap mode, the vmapped address directly used in snd_pcm_lib_write_transfer() -> copy_from_user().

Well, that's usually no problem regarding that cache coherency.
At least it hasn't been any issue with Intel and AMD CPUs.
Does the problem happen with Intel CPU instead of VIA?

> Yes, of course, this path combinations(non-snoop + not-mmap) unlikely be used by most music players.
> 
> [Hans :] I complete the patch in your last mail like below, not used the vunmap & re-vmap way(it has a little problem). Result is OK now(no noise anymore).

What problem did you get with vunmap / vmap?


Takashi