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?
[Hans :] I don't have enough Intel/AMD machine here, up to now, only find one Intel's : HW: mother board is Dell 042P49, HDA controller is 8086:1c20, codec is cx20641.
And the same issue is seen on that machine?
[Hans :] Yes
Thanks,
Hans
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?
[Hans :] if patch like below, even the new vmapped address new_area be assigned to dmab->area, it will not use this time, but next. This time used dmab->area still the last one, but the old one had been vunmap, so Oops reported.
That's bad. But the problem isn't as you guessed, I guess. The allocator re-uses the pre-allocated buffer if available, and this makes things inconsistent. I'll cook later.
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