On Fri, 2011-03-25 at 11:12 +0100, Takashi Iwai wrote:

At Fri, 25 Mar 2011 20:15:33 +1100,
Benjamin Herrenschmidt wrote:

quoted

quoted

quoted

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().

Right, I just wanted to make sure I was right to assume that a page
allocated by vmalloc() was going to fail the above test in 
snd_pcm_default_mmap() and thus -not- get into dma_mmap_coherent()...
just double checking as I'm not totally familiar with the intricacies of
the pcm code :-)

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.

Is this a vivt architecture ? Maybe enforcing some restrictions on the
virtual addresses so they hit the same cache congruence classes ?

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).

Right. I wouldn't expect vmalloc stuff to hit HW in most cases anyways,
though I do wonder why you don't pass the buffer directly to the HCD and
avoid that intermediate step but that's a completely different
question :-)

quoted

quoted

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?

Well, I am ppc maintainer so that's sorted :-) I've CCed Russell for the
other, it's up to him, I have no specific dependency there, it's just an
easy cleanup I stumbled upon.

Cheers,
Ben.

thanks,

Takashi
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