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Date:   Sat, 14 Jan 2017 09:29:58 +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 18:33:40 +0100
Marek Vasut <marek.vasut@...il.com> wrote:

> On 01/13/2017 05:56 PM, Boris Brezillon wrote:
> > On Fri, 13 Jan 2017 17:44:12 +0100
> > Marek Vasut <marek.vasut@...il.com> wrote:
> >   
> >> On 01/13/2017 05:28 PM, Boris Brezillon wrote:  
> >>> 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.    
> >>
> >> Uh, what ? I lost you here :-)

I mean adding a new entry in the mtk_nor_of_ids table (in
mtk-quadspi.c) so that the mediatek,mt2701-nor compatible string can be
matched directly, and you won't need to define 2 compatible strings in
your device tree.

> >>  
> >>>> 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? :-)    
> >>
> >> And that's another thing and I agree with you, but I don't think that's
> >> what we're discussing in this thread. But (!), OT, I think we should
> >> codify the rules in Documentation/ . This discussion came up multiple
> >> times recently.
> >>
> >> And my question still stands, what do we put into the DT here, IMO
> >> compatible = "mediatek,mt2701-nor", "mediatek,mt8173-nor";  
> > 
> > I'd say
> > 
> > 	compatible = "mediatek,mt8173-nor";
> > 
> > because both compatible are referring to very specific IP version. It's
> > not the same as  
> 
> But then you don't have the ability to handle a block in this particular
> SoC in case there's a bug found in it in the future,
> so IMO it should be:
> 
> compatible = "mediatek,mt2701-nor", "mediatek,mt8173-nor";

Sorry again, I meant

	compatible = "mediatek,mt2701-nor";

> 
> > 	compatible = "mediatek,mt8173-nor", "mediatek,mt81xx-nor";  
> 
> This doesn't look right, since here we add two new compatibles ...

That was just an example to describe how compatible inheritance works
(at least that's my understanding of it), it does not apply to this
particular use case.

> 
> > where you clearly have a generic compatible which is overloaded by a
> > specific one.
> > 
> > But anyway, I'm not the one taking the decision here, let's wait for DT
> > maintainers reviews.
> >   
> >> and what goes into the binding document ? I guess both too ?  
> > 
> > If both exist, they should be both documented.
> >   
> 
> 

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