linux-kernel - Re: [PATCH 08/18] ASoC: cs35l56: Fix default SDW TX mixer registers

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Date: Tue, 30 Jan 2024 11:04:54 +0000
From: Richard Fitzgerald <rf@...nsource.cirrus.com>
To: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        <broonie@...nel.org>, <tiwai@...e.com>
CC: <alsa-devel@...a-project.org>, <linux-sound@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <patches@...nsource.cirrus.com>
Subject: Re: [PATCH 08/18] ASoC: cs35l56: Fix default SDW TX mixer registers

On 29/01/2024 17:15, Pierre-Louis Bossart wrote:
> 
> 
> On 1/29/24 17:27, Richard Fitzgerald wrote:
>> Patch the SDW TX mixer registers to silicon defaults.
>>
>> CS35L56 is designed for SDCA and a generic SDCA driver would
>> know nothing about these chip-specific registers. So the
>> firmware sets up the SDW TX mixer registers to whatever audio
>> is relevant on a specific system.
>>
>> This means that the driver cannot assume the initial values
>> of these registers. But Linux has ALSA controls to configure
>> routing, so the registers can be patched to silicon default and
>> the ALSA controls used to select what audio to feed back to the
>> host capture path.
> 
> humm, which has the precedence then?
> a) the values set by firmware
> b) the default values set by the driver?
> 
> Also if the firmware touches those registers shouldn't they be marked as
> 'volatile'?
>

The firmware was designed to work with Windows, so it looks a bit
strange if you are coming at it from ALSA. There's not really any
defined 'precedence'. The firmware will setup the feedback monitor paths
to something that satisfies SDCA and Windows expectations.

We don't care about that in Linux, the firmware on the Intel DSP
probably isn't running the same algorithms for Linux, and we have ALSA
controls to configure those paths. So we patch the mixers back to their
silicon defaults and take over complete control of them.

The firmware only writes them during its power-up sequence so they
will only change when we are rebooting the firmware or coming out of
low-power standby, which is under the control of the driver. When that
happens regmap will re-apply the patch and then sync up the registers
again. The firmware won't touch them after boot, so we can avoid having
to mark them volatile (which would mean implementing our own manual
caching of the settings).

> 
>> Backport note:
>> This won't apply to kernels older than v6.6.
>>
>> Signed-off-by: Richard Fitzgerald <rf@...nsource.cirrus.com>
>> Fixes: e49611252900 ("ASoC: cs35l56: Add driver for Cirrus Logic CS35L56")
>> ---
>>   sound/soc/codecs/cs35l56-shared.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/sound/soc/codecs/cs35l56-shared.c b/sound/soc/codecs/cs35l56-shared.c
>> index 35789ffc63af..a812abf90836 100644
>> --- a/sound/soc/codecs/cs35l56-shared.c
>> +++ b/sound/soc/codecs/cs35l56-shared.c
>> @@ -12,6 +12,15 @@
>>   #include "cs35l56.h"
>>   
>>   static const struct reg_sequence cs35l56_patch[] = {
>> +	/*
>> +	 * Firmware can change these to non-defaults to satisfy SDCA.
>> +	 * Ensure that they are at known defaults.
>> +	 */
>> +	{ CS35L56_SWIRE_DP3_CH1_INPUT,		0x00000018 },
>> +	{ CS35L56_SWIRE_DP3_CH2_INPUT,		0x00000019 },
>> +	{ CS35L56_SWIRE_DP3_CH3_INPUT,		0x00000029 },
>> +	{ CS35L56_SWIRE_DP3_CH4_INPUT,		0x00000028 },
>> +
>>   	/* These are not reset by a soft-reset, so patch to defaults. */
>>   	{ CS35L56_MAIN_RENDER_USER_MUTE,	0x00000000 },
>>   	{ CS35L56_MAIN_RENDER_USER_VOLUME,	0x00000000 },