Re: [PATCH v4 10/16] dt-bindings: phy: tegra-xusb: Add nvidia,pmc prop

[Thierry Reding]

On Mon, Oct 19, 2020 at 04:40:46PM -0500, Rob Herring wrote:
> On Fri, Oct 16, 2020 at 09:07:20PM +0800, JC Kuo wrote:
> > This commit describes the "nvidia,pmc" property for Tegra210 tegra-xusb
> > PHY driver. It is a phandle and specifier referring to the Tegra210
> > pmc@7000e400 node.
> > 
> > Signed-off-by: JC Kuo <jckuo@xxxxxxxxxx>
> > ---
> > v4:
> >    new change to document "nvidia,pmc" prop
> > 
> >  .../devicetree/bindings/phy/nvidia,tegra124-xusb-padctl.txt      | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/phy/nvidia,tegra124-xusb-padctl.txt b/Documentation/devicetree/bindings/phy/nvidia,tegra124-xusb-padctl.txt
> > index 38c5fa21f435..ea559baeb546 100644
> > --- a/Documentation/devicetree/bindings/phy/nvidia,tegra124-xusb-padctl.txt
> > +++ b/Documentation/devicetree/bindings/phy/nvidia,tegra124-xusb-padctl.txt
> > @@ -54,6 +54,7 @@ For Tegra210:
> >  - avdd-pll-uerefe-supply: PLLE reference PLL power supply. Must supply 1.05 V.
> >  - dvdd-pex-pll-supply: PCIe/USB3 PLL power supply. Must supply 1.05 V.
> >  - hvdd-pex-pll-e-supply: High-voltage PLLE power supply. Must supply 1.8 V.
> > +- nvidia,pmc: phandle and specifier referring to the Tegra210 pmc@7000e400 node.
> 
> 'Tegra210 pmc@7000e400' is kind of specific. Going to update this for 
> every address and chip?
> 
> If there's only one PMC, you can just find the compatible PMC node. Then 
> you don't need a DT update.

I did advise against doing the lookup by compatible string because I
think it has a couple of downsides. On one hand it's going to create an
additional maintenance burden on the XUSB pad controller driver because
the PMC is usually not compatible between SoC versions, so for every new
SoC generation we'll have to add the PMC compatible string to the XUSB
pad controller driver.

On the other hand, this is new functionality and we can easily gate that
on the existence of this phandle, so it doesn't impact DT backwards
compatibility. That also has the advantage of explicitly listing the
dependency between the PMC and the XUSB pad controller in DT, so if we
ever do get around to come up with an even better mechanism to resolve
these dependencies than deferred probe, we do have the data already
available.

Thierry

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