Date:   Mon, 4 Sep 2023 14:47:49 +0100
From:   Stefan Binding <sbinding@...nsource.cirrus.com>
To:     Takashi Iwai <tiwai@...e.de>
CC:     Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
        <alsa-devel@...a-project.org>, <linux-kernel@...r.kernel.org>,
        <patches@...nsource.cirrus.com>,
        Vitaly Rodionov <vitalyr@...nsource.cirrus.com>
Subject: Re: [PATCH v1] ALSA: hda: cs35l41: Support mute notifications for
 CS35L41 HDA


On 04/09/2023 13:29, Takashi Iwai wrote:
> On Mon, 04 Sep 2023 14:00:20 +0200,
> Stefan Binding wrote:
>>
>> On 29/08/2023 15:23, Takashi Iwai wrote:
>>> On Tue, 29 Aug 2023 16:18:12 +0200,
>>> Stefan Binding wrote:
>>>> On 25/08/2023 13:13, Takashi Iwai wrote:
>>>>> On Fri, 25 Aug 2023 14:05:25 +0200,
>>>>> Stefan Binding wrote:
>>>>>> From: Vitaly Rodionov <vitalyr@...nsource.cirrus.com>
>>>>>>
>>>>>> Some laptops require a hardware based mute system, where when a hotkey
>>>>>> is pressed, it forces the amp to be muted.
>>>>>>
>>>>>> For CS35L41, when the hotkey is pressed, an acpi notification is sent
>>>>>> to the CS35L41 Device Node. The driver needs to handle this notification
>>>>>> and call a _DSM function to retrieve the mute state.
>>>>>>
>>>>>> Since the amp is only muted during playback, the driver will only mute
>>>>>> or unmute if playback is occurring, otherwise it will save the mute
>>>>>> state for when playback starts.
>>>>>>
>>>>>> Only one handler can be registered for the acpi notification, but all
>>>>>> amps need to receive that notification, we can register a single handler
>>>>>> inside the Realtek HDA driver, so that it can then notify through the
>>>>>> component framework.
>>>>>>
>>>>>> Signed-off-by: Vitaly Rodionov <vitalyr@...nsource.cirrus.com>
>>>>>> Signed-off-by: Stefan Binding <sbinding@...nsource.cirrus.com>
>>>>> We don't do normally in this way.  The ACPI hot key handling is done
>>>>> via user-space, and user-space daemon triggers the mute of the
>>>>> system.
>>>>>
>>>>> Can't the ACPI notify the key event on those machines?
>>>> This feature is not the "normal" mute button on a keyboard, it is a
>>>> custom request
>>>> from a manufacturer which only mutes the audio on the speakers.
>>>> On previous generations, this was achieved using a GPIO controlled by
>>>> the BIOS/EC.
>>>> However, since CS35L41 does not have such GPIO, we must control it by
>>>> other means.
>>>>
>>>> Our solution, which we have to share with the Windows driver, it to use ACPI
>>>> notifications to tell the driver to mute the amps when the shortcut is
>>>> pressed.
>>>>
>>>> Does this seem like a valid exception to the typical approach?
>>> It's still the question whether we have to do this inevitably in the
>>> kernel in a way like that.  It sounds quite unusual.  Why this must be
>>> handled directly?  IOW, what's the difference from the "normal" mute
>>> button?
>>>
>>> And, even if we take this approach, it leaves the device muted without
>>> exposing it to user-space.  Then user wouldn't know what happens.
>>>
>>>
>>> thanks,
>>>
>>> Takashi
>> We spoke to the ODM for this system to get a more detailed explanation
>> of this feature.
>> The keyboard shortcut enables something called "Unobtrusive
>> Mode". According to their explanation:
>>
>> - Unobtrusive mode is distinct to normal mute, as it only mutes the speakers
>> - There is no requirement to update the volume controls, as the screen
>> backlight will be off anyway in this mode
>> - All other unobtrusive mode functions are enabled without user-space
>> dependencies, and they would prefer not to make speaker mute an
>> exception
> Thanks, it gives a bit better clue.
> The remaining question is rather the exact behavior of this
> "unobtrusive mode".  How is it triggered, and what's the exact
> expectation?  e.g. It must secretly mute the speaker?  That is, it
> must not  expose the mixer state change to user-space?  Or is it tied
> with the normal mixer state and user may unmute again?
>
>
> Takashi
 From what we understand, unobtrusive mode, which is activated by a 
keyboard shortcut (not a single key), performs several operations, such as:
- muting the speaker (headphones remain unmuted)
- dimming/shutting down the LCD backlight
- turning off keyboard backlight and any keyboard LEDs
Apart from muting the speaker, all of these operations are done in 
hardware, as the keyboard shortcut still works in the BIOS.
Previous laptops with this feature appear to use a GPIO to mute the 
speaker, and we are informed that on those laptops userspace was not 
informed of the mute.
Since CS35L41 does not have a GPIO mute, we had to use a different 
solution, involving ACPI notifications, which request the driver to mute.
The same mechanism is used in Windows.
Our understanding is that it is not intended for the mute to be 
overridden by userspace.
Similarly, on previous laptops, userspace could not override this mute, 
since it was not informed of it.

Thanks,
Stefan

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