Re: AW: EXTERNAL - [PATCH] Input: pwm-beeper - Support volume setting via sysfs

From: Jeff LaBundy <jeff@labundy.com>
To: Marek Vasut <marex@denx.de>
Cc: "Traut Manuel LCPF-CH" <Manuel.Traut@mt.com>,
	"linux-input@vger.kernel.org" <linux-input@vger.kernel.org>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Dmitry Torokhov" <dmitry.torokhov@gmail.com>,
	"Frieder Schrempf" <frieder.schrempf@kontron.de>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"linux-pwm@vger.kernel.org" <linux-pwm@vger.kernel.org>
Subject: Re: AW: EXTERNAL - [PATCH] Input: pwm-beeper - Support volume setting via sysfs
Date: Mon, 15 May 2023 09:25:28 -0500	[thread overview]
Message-ID: <ZGJA2M+V8ualidHH@nixie71> (raw)
In-Reply-To: <a5293af4-8d02-ed8f-52d1-722c71d47f37@denx.de>

Hi Marek and Traut,

On Mon, May 15, 2023 at 03:36:02PM +0200, Marek Vasut wrote:
> On 5/15/23 08:50, Traut Manuel LCPF-CH wrote:
> > Hi Marek,
> 
> Hi,
> 
> > > The PWM beeper volume can be controlled by adjusting the PWM duty cycle, expose volume setting via sysfs, so users can make the beeper quieter.
> > > This patch adds sysfs attribute 'volume' in range 0..50000, i.e. from 0 to 50% in 1/1000th of percent steps, this resolution should be sufficient.
> > > 
> > > The reason for 50000 cap on volume or PWM duty cycle is because duty cycle above 50% again reduces the loudness, the PWM wave form is inverted > wave form of the one for duty cycle below 50% and the beeper gets quieter the closer the setting is to 100% . Hence, 50% cap where the wave form yields the loudest result.
> > > 
> > >   Signed-off-by: Marek Vasut <marex@denx.de>
> > > ---
> > > An alternative option would be to extend the userspace input ABI, e.g. by using SND_TONE top 16bits to encode the duty cycle in 0..50000 range, and bottom 16bit to encode the existing frequency in Hz . Since frequency in Hz is likely to be below some 25 kHz for audible bell, this fits in 16bits just fine. Thoughts ?
> > 
> > I tend to not change existing user-space interfaces. I would prefer to have an additional event or using sysfs.
> 
> I am increasingly concerned about the race condition between change of
> volume (via sysfs) and frequency (via SND_TONE) . So I would be banking
> toward additional event, like SND_TONE_WITH_VOLUME or something along those
> lines.

This is just my $.02, but I don't see anything wrong with proposing an
_additive_ change to the ABI such as this. My only concern is that this
kind of change seems useful to any effect (e.g. SND_BEEP) and not limited
to only tones. Unless volume adjustment is less useful for those?

> 
> > > ---
> > > NOTE: This uses approach similar to [1], except it is much simpler.
> > >       [1] https://patchwork.kernel.org/project/linux-input/cover/20230201152128.614439-1-manuel.traut@mt.com/
> > 
> > This one is more complex, because the mapping between duty cycle and volume is not linear. Probably it depends also on the used beeper hardware which values are doing a significant change in volume. Therefore the patchset introduced a mapping between volume levels and duty cycle times in the device-tree to allow user-space applications to control the beeper volume hardware independently.
> 
> I wonder whether this mapping shouldn't be considered policy and left to
> userspace to deal with, instead of swamping the kernel or DT with it ?

I agree that the kernel need not try and linearize the values; in fact LEDs
already have the same problem. I still feel however that imposing a unique
maximum value (e.g. 50,000) is inappropriate because the range should remain
the same regardless of the underlying HW implementation (PWM, class A/B, etc.).

> 
> -- 
> Best regards,
> Marek Vasut

Kind regards,
Jeff LaBundy