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Date: Mon, 29 Jun 2009 09:04:03 +0300
From: Jarkko Nikula <jhnikula@gmail.com>
To: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Cc: Peter Ujfalusi <peter.ujfalusi@nokia.com>,
       Tony Lindgren <tony@atomide.com>,
       "alsa-devel@alsa-project.org" <alsa-devel@alsa-project.org>,
       "linux-omap@vger.kernel.org" <linux-omap@vger.kernel.org>,
       linux-kernel@vger.kernel.org, broonie@opensource.wolfsonmicro.com
Subject: Re: [PATCH] [RFC] ASoC: OMAP: fix OMAP1510 broken PCM pointer
 callback
Message-Id: <20090629090403.5bfd35c8.jhnikula@gmail.com>
In-Reply-To: <200906290008.59640.jkrzyszt@tis.icnet.pl>
References: <200906280021.05931.jkrzyszt@tis.icnet.pl>
	<20090628223732.53954402.jhnikula@gmail.com>
	<200906290008.59640.jkrzyszt@tis.icnet.pl>
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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>
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