[alsa-devel] Allocating DMA buffer for non-PCM

[Takashi Iwai]

At Thu, 14 Feb 2013 18:26:53 +0100,
Adrian Knoth wrote:
> 
> On 02/14/2013 06:14 PM, Takashi Iwai wrote:
> 
> >> 	/* allocate level buffer */
> >> 	err = snd_dma_alloc_pages(SNDRV_DMA_TYPE_DEV_SG,
> >> 			snd_dma_pci_data(hdspm->pci),
> >> 			MADIFX_LEVEL_BUFFER_SIZE, &hdspm->dmaLevelBuffer);
> >> 	if (err < 0) {
> >>     /* error */ [..]
> >>     }
> >>
> >>     hdspm->level_buffer = snd_sgbuf_get_ptr(&(hdspm->dmaLevelBuffer), 0);
> > 
> > hdspm->level_buffer = (u32*)hdspm.dmaLevelBuffer.area;
> 
> Looks good, TNX.
> 
> >> This used to work on my development machine (kernel 3.6.x), but now a
> >> kernel 3.2.0 user reports a NULL pointer dereference of
> >> hdspm->level_buffer, so apparently, snd_sgbuf_get_ptr() returned NULL
> >> for him.
> > What's level_buffer?
> 
> The card does hardware metering and stores RMS/peak values in a DMA
> buffer. I want to later pass this to userspace to show signal levels,
> either via memcpy()ing the DMA buffer or maybe via mmap(). But since I
> no longer have access to such a card, work on this is on halt atm.
> 
> > Is a SG-buffer for the audio stream?
> 
> The audio buffers use
> 
> 	     snd_pcm_lib_preallocate_pages_for_all(pcm,
> 						   SNDRV_DMA_TYPE_DEV_SG,
> 						   snd_dma_pci_data(hdspm->pci),
> 
> 
> so they're SG, yes.

But isn't the level meter buffer a single page?  If the buffer is only
for peak meters, it can't be that big even for multi-channel cards.
If so, SNDRV_DMA_TYPE_DEV_SG is utterly nonsense.  Otherwise, if it's
a SG buffer, you'll have to some code to set up TLB in anyway.


Takashi