Re: [PATCH 2/2] ARM: dts: exynos: use Exynos5420 dedicated USB2 PHY compatible

[Krzysztof Kozlowski]

On Tue, Jan 26, 2021 at 11:44:26PM +0100, Arnd Bergmann wrote:
> On Fri, Nov 20, 2020 at 12:10 PM Marek Szyprowski
> <m.szyprowski@xxxxxxxxxxx> wrote:
> > On 20.11.2020 12:05, Krzysztof Kozlowski wrote:
> > > On Fri, Nov 20, 2020 at 09:56:37AM +0100, Marek Szyprowski wrote:
> > >> USB2.0 PHY in Exynos5420 differs from Exynos5250 variant a bit, so use the
> > >> recently introduced dedicated compatible for Exynos5420.
> > >>
> > >> Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> > >> ---
> > >>   arch/arm/boot/dts/exynos54xx.dtsi | 6 +++---
> > >>   1 file changed, 3 insertions(+), 3 deletions(-)
> > >>
> > >> diff --git a/arch/arm/boot/dts/exynos54xx.dtsi b/arch/arm/boot/dts/exynos54xx.dtsi
> > >> index fe9d34c23374..2ddb7a5f12b3 100644
> > >> --- a/arch/arm/boot/dts/exynos54xx.dtsi
> > >> +++ b/arch/arm/boot/dts/exynos54xx.dtsi
> > >> @@ -188,7 +188,7 @@
> > >>                      compatible = "samsung,exynos4210-ehci";
> > >>                      reg = <0x12110000 0x100>;
> > >>                      interrupts = <GIC_SPI 71 IRQ_TYPE_LEVEL_HIGH>;
> > >> -                    phys = <&usb2_phy 1>;
> > >> +                    phys = <&usb2_phy 0>;
> > >>                      phy-names = "host";
> > >>              };
> > >>
> > >> @@ -196,12 +196,12 @@
> > >>                      compatible = "samsung,exynos4210-ohci";
> > >>                      reg = <0x12120000 0x100>;
> > >>                      interrupts = <GIC_SPI 71 IRQ_TYPE_LEVEL_HIGH>;
> > >> -                    phys = <&usb2_phy 1>;
> > >> +                    phys = <&usb2_phy 0>;
> > >>                      phy-names = "host";
> > >>              };
> > >>
> > >>              usb2_phy: phy@12130000 {
> > >> -                    compatible = "samsung,exynos5250-usb2-phy";
> > >> +                    compatible = "samsung,exynos5420-usb2-phy";
> > > The DTS change will wait till PHY driver adjustements get merged... or
> > > if the difference is not critical, maybe using both compatibles (5420
> > > and 5250) would have sense?
> >
> > It won't work easily with both compatibles, because in the 5420 variant
> > I've also changed the PHY indices (5420 has no device and second hsic
> > phy). IMHO the dts change can wait for the next release.
> 
> I see this made it into the pull request now, but I had not been aware
> of the change earlier, and I'm slightly annoyed to have received it this
> way:
> 
> - This is clearly an incompatible change to the dtb, and you all
>    noticed that because it would cause a bisection problem. As
>    a general rule, if a dts change does not work across bisection,
>    we should not merge it at all, because it causes problems for
>    anyone with external dts or dtb files.

Hi Arnd,

No, it does not create a bisection problem. The driver change adding new
compatible is already in v5.11-rc1.

> 
> - It would likely have been possible to define the new binding in
>   a backward-compatible way. I don't see a reason why the index
>   values in the binding had to change here, other than a slight
>   inconvenience for the driver.

It does not matter since it's a new compatible and old one is not
affected. Nothing got broken before this patch, nothing got broken after
applying it via samsung-soc. No backwards compatibility is affected.

> 
> - If the change was really unavoidable, I would have expected
>   a long explanation about why it had to be done in both the
>   commit message and in the tag description for the pull
>   request.
> 
> I've dropped the pull request for now, maybe this can still
> be sorted out with another driver change that makes the
> new compatible string backward-compatible.

It's a different hardware. New hardware does not have to be compatible
with old hardware. However old DTB is still doing fine (although with
the original issue not fixed).

Best regards,
Krzysztof