Re: An driver error when I using aplay!

[Takashi Iwai <tiwai@suse.de>]

At Tue, 8 Jun 2004 17:40:58 +0100,
Russell King wrote:
> 
> On Tue, Jun 08, 2004 at 05:48:45PM +0200, Takashi Iwai wrote:
> > At Mon, 7 Jun 2004 19:04:16 +0100,
> > Russell King wrote:
> > > > > They also _may_ only affect the userspace mapping of
> > > > > that memory and not kernel space, so this doesn't help the control
> > > > > or status mmaped pages.
> > > > 
> > > > Oh yes, then how about to use a DMA page for the control/status mmap,
> > > > too?
> > > 
> > > Oddly that's what I did on VIVT caches to get Alsa working a few months
> > > ago, but its very unclean and the way I did it, it exposes the "dma"
> > > address in the actual page - which would be a major security problem
> > > if I let the code out.  As I said back then, I didn't have a clean
> > > solution for this.
> > 
> > Well, i386 and x86-64 provide a function change_page_attr() for this
> > purpose.  But it's also a horrible arch-spec hack.
> > 
> > How about to introduce a function like alloc_pages_noncached()?
> 
> Just a quick response on this point while I wait for a rebuild (because
> I don't have time to think about the rest of your reply at the moment -
> and then I'll probably forget about it by the end of tonight, sorry...)

Hehe, I have also a quite limited FIFO queue ;)

> I think you'll find that alloc_pages_noncached() is unimplementable on
> x86 and other architectures.  Some architectures are unable to turn the
> cache on and off on a per-page basis, while others can.  Some need to
> ensure that the relative mappings of two pages fall on the same cache
> colour.  Some need reprogramming of some hardware depending on where
> stuff is mapped.  The list goes on and on...

Yes.

> Basically, I think you'll find that this part of the ALSA API isn't
> universally implementable across all kernel architectures.

Right.  Remember that now the control/status mmap isn't mandatory any
more.  We can live without it safely.  But still it's nicer to use it
as much as possible.

That is, the scenario is:  alloc_pages_noncached() may fail - we
accept it.  Then the mmap returns the error, and alsa-lib tries
non-mmap behavior.

--
Takashi Iwai <tiwai@suse.de>		ALSA Developer - www.alsa-project.org


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