linux-kernel - Re: [PATCH] ARM: dts: exynos: Add a placeholder for a MAC address

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Date:   Thu, 1 Oct 2020 11:16:48 +0200
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Lukasz Stelmach <l.stelmach@...sung.com>
Cc:     Rob Herring <robh+dt@...nel.org>, Kukjin Kim <kgene@...nel.org>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bartłomiej Żołnierkiewicz 
        <b.zolnierkie@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH] ARM: dts: exynos: Add a placeholder for a MAC address

On Thu, 1 Oct 2020 at 10:36, Lukasz Stelmach <l.stelmach@...sung.com> wrote:
>
> It was <2020-10-01 czw 09:22>, when Krzysztof Kozlowski wrote:
> > On Wed, Sep 30, 2020 at 04:31:51PM +0200, Łukasz Stelmach wrote:
> >> Add a placeholder for a MAC address. A bootloader may fill it
> >> to set the MAC address and override EEPROM settings.
> >>
> >> Signed-off-by: Łukasz Stelmach <l.stelmach@...sung.com>
> >> ---
> >>  arch/arm/boot/dts/exynos5422-odroidxu3.dts | 18 ++++++++++++++++++
> >>  1 file changed, 18 insertions(+)
> >>
> >> diff --git a/arch/arm/boot/dts/exynos5422-odroidxu3.dts b/arch/arm/boot/dts/exynos5422-odroidxu3.dts
> >> index db0bc17a667b..9f7f3eacb750 100644
> >> --- a/arch/arm/boot/dts/exynos5422-odroidxu3.dts
> >> +++ b/arch/arm/boot/dts/exynos5422-odroidxu3.dts
> >> @@ -70,3 +70,21 @@ &pwm {
> >>  &usbdrd_dwc3_1 {
> >>      dr_mode = "peripheral";
> >>  };
> >> +
> >> +&usbhost2 {
> >> +    #address-cells = <1>;
> >> +    #size-cells = <0>;
> >> +
> >> +    hub@1 {
> >> +            compatible = "usb8087,0024";
> >> +            reg = <1>;
> >> +            #address-cells = <1>;
> >> +            #size-cells = <0>;
> >> +
> >> +            ethernet: usbether@1 {
> >> +                    compatible = "usb0c45,6310";
> >> +                    reg = <1>;
> >> +                    mac-address = [00 00 00 00 00 00]; /* Filled in by a bootloader */
> >
> > Why do you need a placeholder? U-Boot can just append the
> > address/property.
>
> Several other dts files have such placeholder
>
> git grep 'mac-addr.*\[00 00 00 00 00 00\]' arch/arm/boot/dts/ | wc -l
> 26

Yeah, but if this is cargo cult, then it is not a good reason. First of all,
git grep '[^-]mac-addr.*\[.*00 00 00 00 00 00.*\]' arch/arm/boot/dts/
gives just seven boards. Not a big number to make a standard. :)

The meaning of mac-address, I think, is not a placeholder, but:
"should be used in cases where the MAC address assigned to the device
by the boot program is different from the local-mac-address property."

I think you actually wanted a local-mac-address and the majority of
DTSes use it. Not mac-address.

>
> I can see two scenarios where this patch may be helpful. Another
> (simple) boot loading code might be used.

OK, good point. However other bootloader still has to adhere to the DT
specification and the bindings.

> The value may also be
> customized during build time and used without any support of a
> bootloader.

Yes, with an overlay. You do not need a placeholder for this and
actually having a placeholder instead of using overlays is the wrong
approach for customization of boots/builds.

> Finding and filling a placeholder would be easier in u-boot too.

U-Boot already has the code for updating mac-address or
local-mac-address so how is it easier for him? You mean that it cannot
find an ethernet node here?

> And it serves as a reference how to configure a USB device in a dts

Great places for references are examples in bindings.

Best regards,
Krzysztof