Re: [PATCH v3 07/10] arm/tegra: Add PWFM controller device tree probing

[Thierry Reding <thierry.reding-RM9K5IK7kjKj5M59NBduVrNAH6kLmebB@public.gmane.org>]

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* Stephen Warren wrote:
> Thierry Reding wrote at Saturday, March 03, 2012 3:54 PM:
> > I'm confused. If I know exactly that the hardware is Tegra30 (which it
> > definitely should be if I include tegra30.dtsi), then why list "tegra20-pwm"
> > as compatible?
> > 
> > Or did you mean to list tegra30-pwm as compatible value in the PWM driver?
> 
> Standard practice is to list the exact model of the HW as the first entry
> in compatible in the .dts file:
> 
>     nvidia,tegra30-pwm
> 
> This is so that the DT always describes exactly which HW model is actually
> present, so that if HW-model-specific WARs/... are required in the future,
> the DT already lists that information up-front.
> 
> Then additionally list any older HW models that this HW is also compatible
> with:
> 
>     nvidia,tegra20-pwm
> 
> 
> This allows the driver to list just nvidia,tegra20-pwm but still bind
> to DT nodes that are for later HW.
> 
> So, in other words, you end up with the following in the .dts/.dtsi file:
> 
>     compatible = "nvidia,tegra30-pwm", "nvidia,tegra20-pwm";

Okay, that makes sense now. For some reason I thought you were suggesting to
put the same into tegra20.dtsi as well. I'll add nvidia,tegra30-pwm to the
compatible list in the driver and list both values in the tegra30.dtsi.
Thanks for explaining.

> > > Could you also write binding documentation, in particular explaining
> > > what the two pwm-cells are specifically for Tegra:
> > >
> > > Documentation/devicetree/bindings/pwm/nvidia,tegra20-pwm.txt
> > >
> > > (although perhaps that'd be part of the previous patch which implements
> > > the driver)
> > 
> > Actually for Tegra the values would be those documented in the generic
> > binding because Tegra uses of_pwm_simple_xlate(). Does it still make sense to
> > add a Tegra-specific binding?
> 
> There should still be a Tegra-specific binding. Without it, there's no
> definite way to know whether the "standard" properties actually apply,
> or someone simply forgot to document it.

Understood. I assume that both the Tegra20 and Tegra30 variants should have
explicit bindings then as well, right?

Thierry

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