Re: [PATCH 2/4] PCI: rcar-gen2: Add device tree support for r8a7793

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Hi Bjorn,

On Fri, Dec 4, 2015 at 10:26 PM, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> On Tue, Dec 01, 2015 at 04:24:30PM +0900, Simon Horman wrote:
>> Simply document new compat strings.
>> There appears to be no need for a driver updates.
>>
>> Signed-off-by: Simon Horman <horms+renesas@xxxxxxxxxxxx>
>> ---
>>  Documentation/devicetree/bindings/pci/pci-rcar-gen2.txt | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/Documentation/devicetree/bindings/pci/pci-rcar-gen2.txt b/Documentation/devicetree/bindings/pci/pci-rcar-gen2.txt
>> index b19be08a8113..7c231b3e5872 100644
>> --- a/Documentation/devicetree/bindings/pci/pci-rcar-gen2.txt
>> +++ b/Documentation/devicetree/bindings/pci/pci-rcar-gen2.txt
>> @@ -8,6 +8,7 @@ OHCI and EHCI controllers.
>>  Required properties:
>>  - compatible: "renesas,pci-r8a7790" for the R8A7790 SoC;
>>             "renesas,pci-r8a7791" for the R8A7791 SoC;
>> +           "renesas,pci-r8a7793" for the R8A7793 SoC;
>
> What's the benefit of adding a string here if the driver doesn't check
> for it?  Since the driver doesn't look for it, there's no way to test
> anything.

If we ever discover a difference between PCI on r8a7793 compared to PCI on
other SoCs of the R-Car Gen2 family, we can handle that difference in the
driver.

> It doesn't seem like this file is an authoritative source of names, so

Yes it is. When adding compatible values to a DTS, checkpatch.pl will check
for their existence in the binding documentation. Hence we always add the
compatible values to the DT binding docs, before we start using them.

> if we add it here, there's the possibility the r8a7793 will be
> canceled or renamed, and then we'd have to update this if the driver
> ever did need an r8a7793-specific quirk.  If we waited until the

I don't understand what you mean here.

> driver actually needs a quirk, then we'd know exactly what name to
> look for and we could update the driver, DT, and doc all together.

If we update driver, DT, and doc only after we discover the need for a quirk,
it's already too late, due to stable DT rules. Hence we always document and
use SoC-specific compatible values, sometimes combined with family-specific
compatible values. The driver only matches to the as least specific value as
possible.

I hope this clears up your worries.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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