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Date:   Mon, 16 Jan 2017 17:09:13 +0100
From:   Marek Vasut <marek.vasut@...il.com>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....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>,
        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 01/16/2017 09:40 AM, Boris Brezillon wrote:
> On Sun, 15 Jan 2017 01:23:48 +0100
> Marek Vasut <marek.vasut@...il.com> wrote:
> 
>> On 01/14/2017 09:29 AM, Boris Brezillon wrote:
>>> 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.  
>>
>> But then you grow the table in the driver, is that what we want if we
>> can avoid that ?
> 
> The space you save by not growing the mtk_nor_of_ids table is lost in
> your dtbs, so I'm not sure the size argument is relevant here. Also,
> note that distros are shipping a lot of dtbs, and you're likely to have
> several boards based on the mt2701 SoC, so, for this specific use case,
> it's better to make the in-driver of-id table grow than specifying 2
> compatibles in the DT. But as I said, I'm not sure we should rely on
> this argument to decide which approach to choose (we're talking about a
> few bytes here).
> 
>>
>>>>>>    
>>>>>>>> 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.  
>>
>> Well this is OK I guess, but then you can also use "mediatek,mt8173-nor"
>> as the oldest supported compatible and be done with it, no ? It looks a
>> bit crappy though, I admit that ...
>>
> 
> Let's stop bikeshedding and wait for DT maintainers feedback
> before taking a decision ;-).

+1 :)

> Rob, Mark, any opinion?
> 


-- 
Best regards,
Marek Vasut

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