[alsa-devel] [PATCH RFC v2 REPOST 3/8] ASoC: davinci-evm: HDMI audio support for TDA998x trough McASP I2S bus

Mark Brown broonie at kernel.org
Tue Jan 21 20:15:47 CET 2014

On Wed, Jan 15, 2014 at 01:27:21PM +0200, Jyri Sarha wrote:
> On 12/31/2013 03:25 PM, Mark Brown wrote:

> >>support. The only supported sample format is SNDRV_PCM_FORMAT_S32_LE.
> >>The 8 least significant bits are ignored.

> >Where does this constraint come from?

> From driver/gpu/drm/i2c/tda998x_drv.c. The driver configures CTS_N
> register statically to a value that works only with 4 byte samples.
> According to my tests it is possible to support 3 and 2 byte samples
> too by changing the CTS_N register value, but I am not sure if the
> configuration can be changed on the fly. My data sheet of the nxp
> chip is very vague about the register definitions, but I suppose the
> register configures some clock divider on the chip. HDMI supports
> only upto 24bit audio and the data sheet states that any extraneous
> least significant bits are ignored.

Hrm.  This sounds like it ought to be presenting as an ASoC CODEC

> >>+	snd_pcm_hw_constraint_minmax(runtime, SNDRV_PCM_HW_PARAM_CHANNELS,
> >>+				     2, 2);

> >Why not just set all this statically when registering the DAI?

> Because there is no relevant DAI to where to put these limitations.
> I did not want to add yet another dummy codec driver, but decided to
> use the already existing ASoC HDMI codec. By default the driver
> support all audio params supported by HDMI. The limitations are
> coming from NXP chip, the NXP driver, and because the chip is used
> in i2s mode. In other words the limitation is coming from machine
> setup, not from the DAIs.

OK, but it sounds like it's got register configuration as well?  That
really does sound like a device that ought to have a driver...

> No excuse for i initialization, I'll fix that. The casting is just
> to survive with just one kmalloc call instead of separate memory
> blobs for
> struct snd_pcm_hw_constraint_list and referred list of supported
> sample rates. I'll allocate a second blob, if that is easier to
> read.

Yes, please.  Unless it's something where memory is getting tight (eg,
due to be allocated a lot) it's more important that we can read the code
than that we save a few bytes.
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