[BUG] pgprot_noncached() is -NOT- safe for mapping vmalloc buffers into userspace

[Takashi Iwai]

At Fri, 25 Mar 2011 20:15:33 +1100,
Benjamin Herrenschmidt wrote:
> 
> > > We
> > > must also make sure we don't go down that path for vmalloc memory
> > > though.
> >
> > Yes.
> 
> I haven't actually checked, but I assume that the test
> 
> substream->dma_buffer.dev.type == SNDRV_DMA_TYPE_DEV
> 
> In snd_pcm_default_mmap() takes care of that, please correct me if
> I'm wrong in which case we'll need something else there.

Well, in the case of usb-audio, it's not handled via
dma_mmap_coherent(), as the page isn't allocated via
dma_alloc_coherent() but vmalloc().

The bad commit was to overcome some problems on SH platform, IIRC,
when it's used with dmix -- i.e. concurrent accesses on the mmapped
buffer from multiple processes.  But, this looks obviously like a
wrong approach.

Actually, the buffer allocated there in usb-audio is an intermediate
buffer, that isn't directly transferred to hardware.  We may need a
bit more consideration what is the best way to solve that issue (if
it's still really present).

> > Your patch looks good.  Thanks for taking care of this! 
> 
> Are you taking care of sending it upstream ?

I'll apply the patch to remove vmalloc pgprot thingy surely to sound
tree and include in the next pull request.

Others should be sent through arch maintainers (PPC and ARM), right?


thanks,

Takashi