Date:   Mon, 29 Jul 2019 20:46:25 +0200
From:   Jernej Škrabec <jernej.skrabec@...l.net>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     linux-sunxi@...glegroups.com, Chen-Yu Tsai <wens@...e.org>,
        Mark Rutland <mark.rutland@....com>, linux-pwm@...r.kernel.org,
        devicetree <devicetree@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Maxime Ripard <mripard@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        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

Dne ponedeljek, 29. julij 2019 ob 20:40:41 CEST je Uwe Kleine-König 
napisal(a):
> On Mon, Jul 29, 2019 at 06:40:15PM +0200, Jernej Škrabec wrote:
> > Dne ponedeljek, 29. julij 2019 ob 18:24:28 CEST je Uwe Kleine-König
> > 
> > napisal(a):
> > > 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.
> > 
> > I think there's misunderstanding. I only argued that we can't use
> > 
> > compatible = "allwinner,sun50i-h6-pwm",
> > 
> > 	 "allwinner,sun5i-a10s-pwm";
> > 
> > as you suggested and only
> > 
> > compatible = "allwinner,sun50i-h6-pwm";
> > 
> > will work. Not because of driver itself (it can still use _optional()
> > variants), but because of DT binding, which should be able to validate H6
> > PWM node - reset and bus clock references are required in this case.
> 
> I think I understood. In my eyes there is no need to let validation of
> the DT bindings catch a missing "optional" property that is needed on
> H6.
> 
> You have to draw the line somewhere which information the driver has
> hard-coded and what is only provided by the device tree and just assumed
> to be correct by the driver. You argue the driver should know that 

No, in this thread I argue that DT validation tool, executed by

make ARCH=arm64 dtbs_check

should catch that. This is not a driver, but DT binding described in YAML.

Best regards,
Jernej

> if it
> cares for a "allwinner,sun50i-h6-pwm" device it should know (and check)
> that there is a clock named "bus" and a resets property that links to a
> reset controller. How is that different from checking that the base
> address is 0x0300a000 or that the "pwm" clock is the osc24M clock
> running at 24 MHz? This isn't checked in the driver or the dt schema.
> Still if the device tree got one of them wrong this yields an
> non-working pwm device that isn't catched in the driver.
> 
> Best regards
> Uwe

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