linux-kernel - Re: [PATCH v3 0/3] input: pwm-beeper: add feature to set volume level

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Date:   Thu, 16 Feb 2017 15:44:01 -0600
From:   David Lechner <david@...hnology.com>
To:     Frieder Schrempf <frieder.schrempf@...eet.de>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     robh@...nel.org, pawel.moll@....com, ijc+devicetree@...lion.org.uk,
        galak@...eaurora.org, luis@...ethencourt.com,
        linux-input@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/3] input: pwm-beeper: add feature to set volume level

On 02/16/2017 03:15 PM, Frieder Schrempf wrote:
> Hello David,
>
> On 20.01.2017 20:11, David Lechner wrote:
>> On 01/19/2017 03:37 PM, Dmitry Torokhov wrote:
>>> On Thu, Jan 19, 2017 at 04:24:07PM +0100, Frieder Schrempf wrote:
>>>> Make the driver accept switching volume levels via sysfs.
>>>> This can be helpful if the beep/bell sound intensity needs
>>>> to be adapted to the environment of the device.
>>>>
>>>> The number of volume levels available and their values can
>>>> be specified via device tree (similar to pwm-backlight).
>>>>
>>>> The volume adjustment is done by changing the duty cycle of
>>>> the pwm signal.
>>>
>>> I wonder how this all will mesh up with beepers that have dedicated
>>> amplifiers (support is being added by David Lechner).
>>
>> This will work very well with it. I fact, it is a feature I would like
>> to have but I was not sure about a good way to implement it. Please Cc:
>> me on future revisions of this series and I will be glad to test it.
>
> I have just send v4.
>>
>> One thing that would be nice would be for a more generic way to change
>> the volume from userspace. Having a sysfs attribute on the platform
>> device will work, but it requires very specific knowledge from any
>> userspace program that wants to control the volume. Would it be possible
>> to add an alsa mixer volume control or something like that?
>
> An alsa mixer control for the pwm-beeper volume sounds nice, but
> unfortunately I have no idea how one would implement this, or if this is
> even possible.
>>


I know it is possible because I have done it. ;-)

https://github.com/ev3dev/lego-linux-drivers/blob/master/evb/evb_sound.c

The driver in the link above does PCM playback using PWM. But, it also 
included an input device similar to pwm-beeper and it includes an ALSA 
volume control specifically for the beeper.