Re: snd_pcm_lib_preallocate_pages_for_all

[Takashi Iwai <tiwai@suse.de>]

At Mon, 5 Jul 2004 10:00:39 +0100,
Russell King wrote:
> 
> On Mon, Jul 05, 2004 at 11:56:25AM +0300, Bo Henriksen wrote:
> > It now crashes again. I have tried to preallocate the buffer using
> > dma_alloc_coherent and free it with dma_free_coherent. This succeeds and
> > I am using this in other drivers as well. I guess I am missing something
> > else?
> 
> Yes, ALSA assumes that memory returned from dma_alloc_coherent is
> kernel direct mapped memory.  ALSA is wrong in that assumption.
> Hopefully ALSA people can help you further.

The simplest solution is to disable mmap on your driver.
(i.e. removing SNDRV_PCM_INFO_MMAP from info field of hw struct).

The ALSA core part believes that the buffer is mmappable as long as
that flag is set, regardless whether it's allocated via
dma_alloc_coherent() or anything else.

I have a hack on my local tree to adapt the new dma_to_*() macros for
ARM, but it's not clean enough yet for merging to the main ALSA tree.

I hope in future the kernel will have a support function for easier
mmap as Russell proposed on lkml.


Takashi


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