linux-kernel - Re: [linux-sunxi] Re: [PATCH 4/6] pwm: sun4i: Add support for H6 PWM

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Message-ID: <20190729162428.bxuzgxg5sjqptlbp@pengutronix.de>
Date:   Mon, 29 Jul 2019 18:24:28 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Mark Rutland <mark.rutland@....com>, linux-pwm@...r.kernel.org,
        Jernej Škrabec <jernej.skrabec@...l.net>,
        devicetree <devicetree@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Maxime Ripard <mripard@...nel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        Rob Herring <robh+dt@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Sascha Hauer <kernel@...gutronix.de>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [linux-sunxi] Re: [PATCH 4/6] pwm: sun4i: Add support for H6 PWM

Hello,

On Tue, Jul 30, 2019 at 12:09:40AM +0800, Chen-Yu Tsai wrote:
> On Tue, Jul 30, 2019 at 12:07 AM Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> > On Mon, Jul 29, 2019 at 05:55:52PM +0200, Jernej Škrabec wrote:
> > > Dne ponedeljek, 29. julij 2019 ob 08:40:30 CEST je Uwe Kleine-König
> > > napisal(a):
> > > > On Fri, Jul 26, 2019 at 08:40:43PM +0200, Jernej Skrabec wrote:
> > > > > --- a/drivers/pwm/pwm-sun4i.c
> > > > > +++ b/drivers/pwm/pwm-sun4i.c
> > > > > @@ -331,6 +331,13 @@ static const struct sun4i_pwm_data
> > > > > sun4i_pwm_single_bypass = {>
> > > > >   .npwm = 1,
> > > > >
> > > > >  };
> > > > >
> > > > > +static const struct sun4i_pwm_data sun50i_pwm_dual_bypass_clk_rst = {
> > > > > + .has_bus_clock = true,
> > > > > + .has_prescaler_bypass = true,
> > > > > + .has_reset = true,
> > > > > + .npwm = 2,
> > > > > +};
> > > > > +
> > > > >
> > > > >  static const struct of_device_id sun4i_pwm_dt_ids[] = {
> > > > >
> > > > >   {
> > > > >
> > > > >           .compatible = "allwinner,sun4i-a10-pwm",
> > > > >
> > > > > @@ -347,6 +354,9 @@ static const struct of_device_id sun4i_pwm_dt_ids[] =
> > > > > {
> > > > >
> > > > >   }, {
> > > > >
> > > > >           .compatible = "allwinner,sun8i-h3-pwm",
> > > > >           .data = &sun4i_pwm_single_bypass,
> > > > >
> > > > > + }, {
> > > > > +         .compatible = "allwinner,sun50i-h6-pwm",
> > > > > +         .data = &sun50i_pwm_dual_bypass_clk_rst,
> > > >
> > > > If you follow my suggestion for the two previous patches, you can just
> > > > use:
> > > >
> > > >     compatible = "allwinner,sun50i-h6-pwm", "allwinner,sun5i-a10s-pwm";
> > > >
> > > > and drop this patch.
> > >
> > > Maxime found out that it's not compatible with A10s due to difference in bypass
> > > bit, but yes, I know what you mean.
> > >
> > > Since H6 requires reset line and bus clock to be specified, it's not compatible
> > > from DT binding side. New yaml based binding must somehow know that in order
> > > to be able to validate DT node, so it needs standalone compatible. However,
> > > depending on conclusions of other discussions, this new compatible can be
> > > associated with already available quirks structure or have it's own.
> >
> > I cannot follow. You should be able to specify in the binding that the
> > reset line and bus clock is optional. Then allwinner,sun50i-h6-pwm
> > without a reset line and bus clock also verifies, but this doesn't
> > really hurt (and who knows, maybe the next allwinner chip needs exactly
> > this).
> 
> It is not optional. It will not work if either the clocks or reset controls
> are missing. How would these be optional anyway? Either it's connected and
> thus required, or it's not and therefore should be omitted from the
> description.

[Just arguing about the clock here, the argumentation is analogous for
the reset control.]

>From the driver's perspective it's optional: There are devices with and
without a bus clock. This doesn't mean that you can just ignore this
clock if it's specified. It's optional in the sense "If dt doesn't
specify it, then assume this is a device that doesn't have it and so you
don't need to handle it." but not in the sense "it doesn't matter if
you handle it or not.".

Other than that I'm on your side. So for example I think it's not
optimal that gpiod_get_optional returns NULL if GPIOLIB=n or that
devm_reset_control_get_optional returns NULL if RESET_CONTROLLER=n
because this hides exactly the kind of problem you point out here.

Best regards
Uwe

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