[PATCH] aoa i2sbus: Stop Apple i2s DMA gracefully

[Takashi Iwai]

At Thu, 08 Feb 2007 14:12:37 +0100,
Johannes Berg wrote:
> 
> From: Paul Mackerras <paulus at samba.org>
> 
> This fixes the problem of getting extra bytes inserted at the
> beginning of a recording when using the Apple i2s interface and DBDMA
> controller.  It turns out that we can't just abort the DMA; we have to
> let it stop at the end of a command, and then wait for the S7 bit to
> be set before turning off the DBDMA controller.  Doing that for
> playback doesn't seem to be necessary, but doesn't hurt either.
> 
> We use the technique used by the Darwin driver: make each transfer
> command branch to a stop command if the S0 status bit is set.  Thus we
> can ask the DMA controller to stop at the end of the current command
> by setting S0.
> 
> The interrupt routine now looks at and clears the status word of the
> DBDMA command ring.  This is necessary so it can know when the DBDMA
> controller has seen that S0 is set, and so when it should look for the
> DBDMA controller being stopped and S7 being set.  This also ended up
> simplifying the calculation in i2sbus_pcm_pointer.
> 
> Tested on a 15 inch albook.
> 
> Signed-off-by: Paul Mackerras <paulus at samba.org>
> 
> I modified this patch and added the suspend/resume bits to it to get my
> powermac into a decent state when playing sound across suspend to disk
> that has a different bitrate from what the firmware programs the
> hardware to.
> 
> I also added the SNDRV_PCM_INFO_JOINT_DUPLEX flag because it seemed the
> right thing to do and I was looking at the info stuff.
> 
> Signed-off-by: Johannes Berg <johannes at sipsolutions.net>

Thanks, applied to ALSA tree now.  Should be merged to 2.6.21.


Takashi