At Fri, 25 Mar 2011 09:16:48 +1100, Benjamin Herrenschmidt wrote: > > Hi Takashi ! > > While working on endian-fixing xHCI with Matt (CC), we discovered the > source of our problems with usb-audio on a board we were working on. > > c32d977b8157bf67cdf47729ce7dd054a26eb534 > "ALSA: pcm - Call pgprot_noncached() for vmalloc'ed buffers" > > I'm afraid that this is totally bogus :-) > > I don't know on what arch it is safe to have the same memory be mapped > cachable in the kernel (and accessed via this cached mapping) and > non-cachable in userspace, but I can confidently say that wherever it > works it does so by accident. > > In the case of usb-audio, what we observed is that the user application > was writing samples using an uncached mapping, so directly to memory, > which does -not- invalidate conflicting cache lines on the way, an the > kernel would then memcpy those data to the USB buffers using a cached > mapping (vmalloc) and essentially get stale stuff from the cache instead > of the real samples. > > Worse, on some processors, it's actually -illegal- to create (and even > more to -access-) a conflicting mapping of a page of memory, ie, have it > mapped cached somewhere and uncached somewhere else. It will lockup some > processors and afaik, some x86 as well. > > In fact, cache coherent architectures often don't support mapping memory > uncached -at-all- so something like snd_pcm_lib_mmap_noncached() > shouldn't exist, or at least be under arch control. There's no case > where it's "always safe". There will almost always be a cache alias in > the linear mapping unless special arch specific sauce has been applied. I see. I'll take your removal patch. It should be applied to stable kernel, too, right? > Now, there's another problem on top of that, which is that > snd_pcm_default_mmap() will not work properly the "other way around" on > powerpc, where the mapping -needs- to be uncached bcs you are running on > a non cache coherent embedded CPU and trying to mmap DMA memory, but > that's something that needs fixing inside powerpc by properly defining > dma_mmap_coherent() & ARCH_HAS_DMA_MMAP_COHERENT (I thought we had added > it a while back but it's not upstream, patch must have got lost). Mea culpa. I moved to a different team and have had little time for the upstream development since then... > We > must also make sure we don't go down that path for vmalloc memory > though. Yes. Your patch looks good. Thanks for taking care of this! Takashi -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
