[alsa-devel] am335x: mcasp in DIT mode

Yegor Yefremov yegorslists at googlemail.com
Mon Mar 4 09:05:10 CET 2013

On Mon, Mar 4, 2013 at 8:55 AM, Daniel Mack <zonque at gmail.com> wrote:
> Hi Vaibhav,
> Hi Yegor,
> On 04.03.2013 07:22, Bedia, Vaibhav wrote:
>> On Fri, Mar 01, 2013 at 16:23:53, Yegor Yefremov wrote:
>>> I've solved the problem with DIT/SPDIF mode (see the issue description
>>> here: http://e2e.ti.com/support/arm/sitara_arm/f/791/p/247447/870030.aspx).
>>> In davinci_mcasp_hw_params() the DIT or I2S params will be set in the
>>> beginning. DIT mode configures the DAVINCI_MCASP_TXMASK_REG and
>>> And here comes the problem:
>>> at the end of davinci_mcasp_hw_params() the
>>> davinci_config_channel_size() will touch the same registers again and
>>> thus overwrite the settings necessary for DIT. After I commented this
>>> routine I got the sound over S/PDIF and sii9022a HDMI transmitter and
>>> I could see the proper bits appearing on my oscilloscope.
>>> What were the best way to solve this problem?
>>> 1. execute  davinci_config_channel_size() only if not in DIT mode?
>>> 2. for DIT only change the DAVINCI_MCASP_TXMASK_REG according to channel width?
>>> 3. execute
>>> if (dev->op_mode == DAVINCI_MCASP_DIT_MODE)
>>>                 davinci_hw_dit_param(dev);
>>> else
>>>                 davinci_hw_param(dev, substream->stream);
>>> after davinci_config_channel_size()
>> AFAIK DIT mode was working on Davinci platforms some time back. Since AM335x
>> has the same hardware block I was surprised to see this bug report. Not having
>> a setup handy to test out DIT related changes, I looked at the commits on the
>> mcasp file to figure out what happened. I suspect one of the recent patches which
>> added 24bit support inadvertently broke the DIT support. Would it be possible
>> for you to do a git-bisect to find out what change it was? It would be good
>> to reference that change in the final patch.
> I agree, but I also believe it's not easy to do, given the number of
> different trees you need for AM33xx support in general.
> You could revert them one-by-one, but then again, you probably need most
> of them in order to make the driver work on AM33xx and in setups that
> differ from what the davici driver was originally written for.
>> Coming to the 3 options that you have listed I would suggest going with #1.
>> DIT mode requires the configuration to be 32 bit slot with rotation set to 24
>> but looks like the current implementation of davinci_config_channel_size()
>> doesn't handle this scenario very well.
> Hmm, if that's the case, I fail to see how the 24-bit patches you
> mentioned could possibly account for that, given that they don't touch
> the logical around this area. Unless I miss anything very obvious :) Can
> you be a bit more specific about which of these patches you suspect?

I think the DIT support was dead on arrival ;-) Look at the initial
commit: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/sound/soc/davinci/davinci-mcasp.c?id=b67f4487295560599f6cca55fb7e8773ff27f00a

Size adjustment is made after DIT mode settings and though
davinci_config_channel_size(dev, word_length) doesn't overwrite all
needed settings, it overwrites XSSZ value and thus disables 32-bit
slot. Do I see it right?

>> I think the driver needs a bit of
>> rework to ensure Tx and Rx paths don't end up overwriting the configuration
>> but that needs to be taken up separately.
> Yegor - could you send me a patch that fixes the problem for you? I can
> do regression tests on my platform.

I'll do.


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