linux-kernel - Re: [RFC PATCH v3 2/2] pwm: imx: Configure output to GPIO in disabled state

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Date:   Wed, 12 Dec 2018 13:12:55 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Vokáč Michal <Michal.Vokac@...ft.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Lukasz Majewski <l.majewski@...ess.pl>,
        Fabio Estevam <fabio.estevam@....com>,
        Lothar Waßmann <LW@...o-electronics.de>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [RFC PATCH v3 2/2] pwm: imx: Configure output to GPIO in
 disabled state

On Wed, Dec 12, 2018 at 11:42:17AM +0000, Vokáč Michal wrote:
> On 12.12.2018 09:01, Uwe Kleine-König wrote:
> > On Thu, Dec 06, 2018 at 01:41:31PM +0000, Vokáč Michal wrote:
> >> Normally the PWM output is held LOW when PWM is disabled. This can cause
> >> problems when inverted PWM signal polarity is needed. With this behavior
> >> the connected circuit is fed by 100% duty cycle instead of being shut-off.
> >>
> >> Allow users to define a "pwm" and a "gpio" pinctrl states. The pwm pinctrl
> >> state is selected when PWM is enabled and the gpio pinctrl state is
> >> selected when PWM is disabled. In the gpio state the new pwm-gpios GPIO is
> >> configured as input and the internal pull-up resistor is used to pull the
> >> output level high.
> >>
> >> If all the pinctrl states and the pwm-gpios GPIO are not correctly
> >> specified in DT the PWM work as usual.
> >>
> >> As an example, with this patch a PWM controlled backlight with inversed
> >> signal polarity can be used in full brightness range. Without this patch
> >> the backlight can not be turned off as brightness = 0 disables the PWM
> >> and that in turn set PWM output LOW, that is full brightness.
> >>
> >> Inverted output of the PWM with "default" and with "pwm"+"gpio" pinctrl:
> >>
> >> +--------------+------------+---------------+----------- +-------------+
> >> | After reset  | Bootloader |   PWM probe   |     PWM    |     PWM     |
> >> | 100k pull-up |            |               | enable 30% |   disable   |
> >> +--------------+------------+---------------+------------+-------------+
> >> | pinctrl      |    none    |    default    |   default  |   default   |
> >> | out H   __________________                   __   __                 |
> >> | out L                     \_________________/  \_/  \_/\____________ |
> >> |                                           ^    ^    ^                |
> >> +--------------+------------+---------------+------------+-------------+
> >> | pinctrl      |    none    |      gpio     |     pwm    |     gpio    |
> >> | out H   __________________________________   __   __   _____________ |
> >> | out L                                     \_/  \_/  \_/              |
> >> |                                           ^    ^    ^                |
> >> +----------------------------------------------------------------------+
> >>
> >> Signed-off-by: Michal Vokáč <michal.vokac@...ft.com>
> >> ---
> >> Changes in v3:
> >>   - Commit message update.
> >>   - Minor fix in code comment (Uwe)
> >>   - Align function arguments to the opening parentheses. (Uwe)
> >>   - Do not test devm_pinctrl_get for NULL. (Thierry)
> >>   - Convert all messages to dev_dbg() (Thierry)
> >>   - Do not actively drive the pin in gpio state. Configure it as input
> >>     and rely solely on the internal pull-up. (Thierry)
> >>
> >> Changes in v2:
> >>   - Utilize the "pwm" and "gpio" pinctrl states.
> >>   - Use the pwm-gpios signal to drive the output in "gpio" pinctrl state.
> >>   - Select the right pinctrl state in probe.
> >>
> >>   drivers/pwm/pwm-imx.c | 77 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >>   1 file changed, 77 insertions(+)
> >>
> 
> [ snip ]
> 
> > On thing I noticed while looking at the rcar driver is: This doesn't
> > wait for the current period to end. Is this supposed to happen? Also for
> > the enable case the hardware is configured for the desired duty cycle
> > and only then the pinctrl is switched to pwm. Both might result in a
> > spike that is not desired.
> 
> The behavior should not change from how imx-pwm was working before.
> When PWM is disabled the output is immediately gated off (put into
> the idle state) independently on the period. I measured this.

Oh really, I wasn't aware of that. This is another bug in the imx pwm
then (I think).
 
> For the enable case you would certainly not get any additional spikes.

Yes, there is a possibility for a spike: If you configure for say 40%:
                   _    _
pwm output   : ___/ \__/ \__
muxing       : GPIO| PWM_
actual output: ____/\__/ \__

> The worst thing that may happen is that the first period will be
> slightly shorter depending on how long it takes to test the pinctrl
> and switch the muxing. And this is unavoidable, it would happen even
> if you wait for the start of a period. The test + muxing still takes
> some time.

You could first configure for duty_cycle 0 and a short period, then mux
to PWM and then configure the correct duty cycle. Ditto for disable.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |