At Mon, 4 Feb 2013 17:50:27 +0100, Matthieu CASTET wrote:
Hi,
on a armv5 soc, we have audio corruption while using a usb-audio sound card. After analysis, it comes from alsa vmalloc mapping.
Yeah, it's a known issue since long time ago...
in the current kernel vmalloc mapping used by alsa are cached.
This cause coherency problem on armv5 architecture because the cache are VIVT :
- alsa kernel driver map a buffer with vmalloc. The memory attribute is cached
- userspace map vmalloc buffer. The memory attribute is cached
But because the cache are VIVT, these two mapping don't share the same cache line and won't see the same data. This corruption can be hidden by context switch because cache are flushed on context switch.
On x86 or armv7, the cache are PIPT and there is no such problem.
There was a commit "pcm - Call pgprot_noncached() for vmalloc'ed buffers" (c32d977b8157bf67cdf47729ce7dd054a26eb534), that made the userspace mapping not cached, but this caused issues because the kernel vmalloc mapping was still cached.
Instead of revering this commit, we could have made the kernel vmalloc noncached (see attached patch). It fix the issue on armv5.
But on some architecture (armv7 for example) you can't have mapping for the same physical memory cached (normal cached) and noncached (device memory). So the vmalloc and userspace mapping will conflict with the kernel direct RAM mapping.
A workaround could be to use writecombine protection instead of noncached. This work because writecombine is mapped on armv7 as L_PTE_MT_BUFFERABLE (normal uncached) that is compatible with cached memory (the same trick is used for dmacoherent). But that's architecture specific may not work on others architectures.
What's the best way the fix this issue on all architecture ?
I don't think there is no proper way to fix in the driver side unless a generic API for mmappable (noncached or such) vmalloc variant.
A simple workaround for now would be to give an option (either arch-specific ifdef or Kconfig) to make snd-usb-audio driver using the normal page allocation instead of vmalloc.
Takashi
Matthieu [2 vmalloc_uncached.diff <text/x-diff (7bit)>] commit f3d13b6752e20cdd193d0265cc6b8f1d814dd869 Author: Matthieu CASTET matthieu.castet@parrot.com Date: Wed Jan 30 15:27:00 2013 +0100
make snd_pcm_lib_alloc_vmalloc_buffer memory uncacheddiff --git a/sound/core/pcm_memory.c b/sound/core/pcm_memory.c index 69e01c4..1740d39 100644 --- a/sound/core/pcm_memory.c +++ b/sound/core/pcm_memory.c @@ -425,7 +425,7 @@ int _snd_pcm_lib_alloc_vmalloc_buffer(struct snd_pcm_substream *substream, return 0; /* already large enough */ vfree(runtime->dma_area); }
- runtime->dma_area = __vmalloc(size, gfp_flags, PAGE_KERNEL);
- runtime->dma_area = __vmalloc(size, gfp_flags, pgprot_noncached(PAGE_KERNEL)); if (!runtime->dma_area) return -ENOMEM; runtime->dma_bytes = size;