linux-kernel - Re: [PATCH v7 11/11] pwm: Add Raspberry Pi Firmware based PWM bus

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Date:   Thu, 11 Mar 2021 14:41:10 +0100
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     f.fainelli@...il.com, linux-kernel@...r.kernel.org,
        linux-pwm@...r.kernel.org, bcm-kernel-feedback-list@...adcom.com,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        wahrenst@....net, linux-input@...r.kernel.org,
        dmitry.torokhov@...il.com, gregkh@...uxfoundation.org,
        devel@...verdev.osuosl.org, p.zabel@...gutronix.de,
        linux-gpio@...r.kernel.org, linus.walleij@...aro.org,
        linux-clk@...r.kernel.org, sboyd@...nel.org,
        linux-rpi-kernel@...ts.infradead.org, bgolaszewski@...libre.com,
        andy.shevchenko@...il.com
Subject: Re: [PATCH v7 11/11] pwm: Add Raspberry Pi Firmware based PWM bus

On Thu, 2021-03-11 at 14:18 +0100, Uwe Kleine-König wrote:
> Hello Nicolas,
> 
> On Thu, Mar 11, 2021 at 02:01:00PM +0100, Nicolas Saenz Julienne wrote:
> > On Wed, 2021-03-10 at 12:50 +0100, Uwe Kleine-König wrote:
> > > On Mon, Jan 18, 2021 at 01:32:44PM +0100, Nicolas Saenz Julienne wrote:
> > 
> > [...]
> > 
> > > > +	/*
> > > > +	 * This sets the default duty cycle after resetting the board, we
> > > > +	 * updated it every time to mimic Raspberry Pi's downstream's driver
> > > > +	 * behaviour.
> > > > +	 */
> > > > +	ret = raspberrypi_pwm_set_property(rpipwm->firmware, RPI_PWM_DEF_DUTY_REG,
> > > > +					   duty_cycle);
> > > > +	if (ret) {
> > > > +		dev_err(chip->dev, "Failed to set default duty cycle: %pe\n",
> > > > +			ERR_PTR(ret));
> > > > +		return ret;
> > > 
> > > This only has an effect for the next reboot, right?
> > 
> > It effects all reboots until it's further changed.
> > 
> > > If so I wonder if it is a good idea in general. (Think: The current PWM
> > > setting enables a motor that makes a self-driving car move at 100 km/h.
> > > Consider the rpi crashes, do I want to car to pick up driving 100 km/h at
> > > power up even before Linux is up again?)
> > 
> > I get your point. But this isn't used as a general purpose PWM. For now the
> > interface is solely there to drive a PWM fan that's arguably harmless. This
> > doesn't mean that the RPi foundation will not reuse the firmware interface for
> > other means in the future. In such case we can always use a new DT compatible
> > and bypass this feature (the current DT string is
> > 'raspberrypi,firmware-poe-pwm', which is specific to this use-case).
> > 
> > My aim here is to be on par feature wise with RPi's downstream implementation.
> 
> Just because the downstream kernel does it should not be the (single)
> reason to do that. My gut feeling is: For a motor restoring the PWM
> config on reboot is bad and for a fan it doesn't really hurt if it
> doesn't restart automatically. So I'd prefer to to drop this feature.

Fair enough, I'll remove it then.

Regards,
Nicolas


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