Re: [PATCH] [RFC] ASoC: OMAP: fix OMAP1510 broken PCM pointer callback

[Jarkko Nikula <jhnikula@gmail.com>]

On Mon, 29 Jun 2009 00:08:59 +0200
Janusz Krzysztofik <jkrzyszt@tis.icnet.pl> wrote:

> AFAIK, both CSSA_L and CDSA_L DMA registers are static. Loaded by CPU
> with 16 LSB of initial source and destination port addresses
> respectively, they are never updated by the DMA engine itself. That's
> why they can't be used for transfer progress indication unless
> updated by CPU.
> 
> The old omap-alsa driver was just updating them, intentionally or
> not, by reprogramming and restarting DMA every PCM period. That's why
> calculating PCM pointers from CSSA_L/CDSA_L worked.
> 
Thanks for finding out this. Good to know that OMAP low-level DMA code
now in this respect is as good as HW allows it.

> ASoC OMAP driver transfers whole PCM buffer with single DMA transfer,
> so it doesn't need to update DMA source/destination port address
> after initial playback/capture setup, even if restarting DMA, and
> actually never does this. Calculating PCM pointers from CSSA_L/CDSA_L
> registers without updating them every period would then be wrong.
> 
> For capture, reading CPC, that follows destination port address
> progress, just works fine (for both old and new driver). For
> playback, similar hardware functionality seems to be missing, so it
> has to be emulated in software if required.
> 
Kind an odd HW behaviour but that can happen. Probably it would be good
to explain this also in function omap_get_dma_src_pos.

Mark, I think this is fair to queue a fix for 2.6.31.

Acked-by: Jarkko Nikula <jhnikula@gmail.com>