Re: [PATCH v4 6/6] ALSA: led control - add sysfs kcontrol LED marking layer

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 3/19/21 6:22 PM, Takashi Iwai wrote:
> On Fri, 19 Mar 2021 17:34:39 +0100,
> Hans de Goede wrote:
>>
>> Hi,
>>
>> On 3/17/21 6:29 PM, Jaroslav Kysela wrote:
>>> We need to manage the kcontrol entries association for the LED trigger
>>> from the user space. This patch adds a layer to the sysfs tree like:
>>>
>>> /sys/devices/virtual/sound/ctl-led/mic
>>>    + card0
>>>    |  + attach
>>>    |  + detach
>>>    |  ...
>>>    + card1
>>>       + attach
>>>       ...
>>>
>>> Operations:
>>>
>>>   attach and detach
>>>     - amixer style ID is accepted and easy strings for numid and
>>>       simple names
>>>   reset
>>>     - reset all associated kcontrol entries
>>>   list
>>>     - list associated kcontrol entries (numid values only)
>>>
>>> Additional symlinks:
>>>
>>> /sys/devices/virtual/sound/ctl-led/mic/card0/card ->
>>>   /sys/class/sound/card0
>>>
>>> /sys/class/sound/card0/controlC0/led-mic ->
>>>   /sys/devices/virtual/sound/ctl-led/mic/card0
>>>
>>> Signed-off-by: Jaroslav Kysela <perex@perex.cz>
>>
>> Thank you so much for this patch.
>>
>> I've given this new version a try, dropping my sound/soc/codecs/rt56??.c patches to set the access-flags directly.
>>
>> And with these 3 lines in /etc/rc.d/rc.local I get nicely working control of the mute
>> LED build into the (detachable) USB-keyboard's mute hot-key:
>>
>> modprobe snd_ctl_led
>> echo -n name="Speaker Channel Switch" > /sys/class/sound/card1/controlC1/led-speaker/attach
>> echo -n name="HP Channel Switch" > /sys/class/sound/card1/controlC1/led-speaker/attach
>>
>> This needs to be replaced by some UCM profile code doing the equivalent of course,
>> but for a proof-of-concept test of the kernel API this introduces the above will do.
> 
> IMO, that's the question: how we'll enable this in future.  If the
> binding of the control/mute mapping is provided via UCM, it's supposed
> to be changeable by each user.  Then the current sysfs permission
> doesn't fit.  OTOH, if it's 0666, it's accessible to all users even
> remotely, which is worse than the access with the normal sound device
> file.  Or if it's supposed to be changed via udev stuff or systemd?
> Or is it just for debugging?
> 
> Through a quick glance over the series, I'm fine to take those
> patches, but the only concern is the sysfs entries.  Basically, once
> when we use sysfs entries, it's set in stone.  So we should be very
> clear about our strategy how to deploy the control/mute mapping
> regarding using those sysfs entries.
> 
> OTOH, if the interface is thought for debugging or development
> purpose, it could be done in debugfs, which we can keep playing in
> further development, too.
> 
> And, BTW, the mute LED mode setup doesn't have to be sysfs entries;
> we'd need primarily only the flags for inverted LED behavior, and
> those are only two, so it could be simply module options.  Then it's
> even easier for users to set up than tweaking sysfs entries.

The flexibility offered by this new sysfs API is necessary for the ASoC
codec drivers, because Mark does not want to have which controls are
tied to the LED triggers hard-coded inside the codec drivers.

So as Jaroslav mentions in his reply, the plan is to have the UCM
profiles contain commands to setup the LED triggers to this new
sysfs API.

Regards,

Hans