linux-kernel - Re: [PATCH v1 2/2] arm: dts: mt2701: add nor flash node

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Date:   Fri, 13 Jan 2017 17:28:25 +0100
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Marek Vasut <marek.vasut@...il.com>
Cc:     Matthias Brugger <matthias.bgg@...il.com>,
        Guochun Mao <guochun.mao@...iatek.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Richard Weinberger <richard@....at>,
        Cyrille Pitchen <cyrille.pitchen@...el.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        linux-mtd@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] arm: dts: mt2701: add nor flash node

On Fri, 13 Jan 2017 17:13:55 +0100
Marek Vasut <marek.vasut@...il.com> wrote:

> On 01/13/2017 04:12 PM, Matthias Brugger wrote:
> > 
> > 
> > On 13/01/17 15:17, Boris Brezillon wrote:  
> >> On Fri, 13 Jan 2017 15:13:29 +0800
> >> Guochun Mao <guochun.mao@...iatek.com> wrote:
> >>  
> >>> Add Mediatek nor flash node.
> >>>
> >>> Signed-off-by: Guochun Mao <guochun.mao@...iatek.com>
> >>> ---
> >>>  arch/arm/boot/dts/mt2701-evb.dts |   25 +++++++++++++++++++++++++
> >>>  arch/arm/boot/dts/mt2701.dtsi    |   12 ++++++++++++
> >>>  2 files changed, 37 insertions(+)
> >>>
> >>> diff --git a/arch/arm/boot/dts/mt2701-evb.dts
> >>> b/arch/arm/boot/dts/mt2701-evb.dts
> >>> index 082ca88..85e5ae8 100644
> >>> --- a/arch/arm/boot/dts/mt2701-evb.dts
> >>> +++ b/arch/arm/boot/dts/mt2701-evb.dts
> >>> @@ -24,6 +24,31 @@
> >>>      };
> >>>  };
> >>>
> >>> +&nor_flash {
> >>> +    pinctrl-names = "default";
> >>> +    pinctrl-0 = <&nor_pins_default>;
> >>> +    status = "okay";
> >>> +    flash@0 {
> >>> +        compatible = "jedec,spi-nor";
> >>> +        reg = <0>;
> >>> +    };
> >>> +};
> >>> +
> >>> +&pio {
> >>> +    nor_pins_default: nor {
> >>> +        pins1 {
> >>> +            pinmux = <MT2701_PIN_240_EXT_XCS__FUNC_EXT_XCS>,
> >>> +                 <MT2701_PIN_241_EXT_SCK__FUNC_EXT_SCK>,
> >>> +                 <MT2701_PIN_239_EXT_SDIO0__FUNC_EXT_SDIO0>,
> >>> +                 <MT2701_PIN_238_EXT_SDIO1__FUNC_EXT_SDIO1>,
> >>> +                 <MT2701_PIN_237_EXT_SDIO2__FUNC_EXT_SDIO2>,
> >>> +                 <MT2701_PIN_236_EXT_SDIO3__FUNC_EXT_SDIO3>;
> >>> +            drive-strength = <MTK_DRIVE_4mA>;
> >>> +            bias-pull-up;
> >>> +        };
> >>> +    };
> >>> +};
> >>> +
> >>>  &uart0 {
> >>>      status = "okay";
> >>>  };
> >>> diff --git a/arch/arm/boot/dts/mt2701.dtsi
> >>> b/arch/arm/boot/dts/mt2701.dtsi
> >>> index bdf8954..1eefce4 100644
> >>> --- a/arch/arm/boot/dts/mt2701.dtsi
> >>> +++ b/arch/arm/boot/dts/mt2701.dtsi
> >>> @@ -227,6 +227,18 @@
> >>>          status = "disabled";
> >>>      };
> >>>
> >>> +    nor_flash: spi@...14000 {
> >>> +        compatible = "mediatek,mt2701-nor",
> >>> +                 "mediatek,mt8173-nor";  
> >>
> >> Why define both here? Is "mediatek,mt8173-nor" really providing a
> >> subset of the features supported by "mediatek,mt2701-nor"?
> >>  
> > 
> > I think even if the ip block is the same, we should provide both
> > bindings, just in case in the future we find out that mt2701 has some
> > hidden bug, feature or bug-feature. This way even if we update the
> > driver, we stay compatible with older device tree blobs in the wild.
> > 
> > We can drop the mt2701-nor in the bindings definition if you want. 

Oh, sorry, I misunderstood. What I meant is that if you want to
list/support all possible compatibles, maybe you should just put one
compatible in your DT and patch your driver (+ binding doc) to define
all of them.

> 
> This exactly. We should have a DT compat in the form:
> compatible = "vendor,<soc>-block", "vendor,<oldest-compat-soc>-block";
> Then if we find a problem in the future, we can match on the
> "vendor,<soc>-block" and still support the old DTs.

Not sure it's only in term of whose IP appeared first. My understanding
is that it's a way to provide inheritance. For example:

	"<soc-vendor>,<ip-version>", "<ip-vendor>,<ip-version>";

or

	"<soc-vendor>,<full-featured-ip-version>","<soc-vendor>,<basic-feature-ip-version>";

BTW, which one is the oldest between mt8173 and mt2701? :-)