[alsa-devel] [PATCH 1/4] ALSA: memalloc: Allow NULL device for SNDRV_DMA_TYPE_CONTINOUS type

[Sridharan, Ranjani]

>
> > >
> > Hi Takashi,
> >
> > I have a question about the usage of snd_dma_alloc_pages() with the
> > SNDRV_DMA_TYPE_DEV type. I am working on adding a platform-device for
> > audio which is a child device of the top-level PCI device in SOF.
> > When I use the handle for the platform-device as the "dev" argument for
> > snd_dma_alloc_pages(), the dma alloc fails on some platforms (ex: Ice
> > Lake). But it works fine if I use the top-level PCI device instead.
> > Why would that be? Are there any restrictions to what devices can be
> > used for dma alloc?
>
> This pretty much depends on the device.  Basically the ALSA memalloc
> stuff simply calls dma_alloc_coherent() if the buffer type is
> SNDRV_DMA_TYPE_DEV, and the rest goes deeply into the code in
> kernel/dma/*.
>
> My wild guess is that the significant difference in your case is about
> the DMA coherence mask set on the device.  As default the platform
> device keeps 32bit DMA while the modern PCI drivers often sets up
> 64bit DMA mask.
>

Thanks, Takashi. So, in this case, would you recommend to always use the
PCI device for dma alloc?

Thanks,
Ranjani