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

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Message-ID: <CAL_Jsq+EyHtC7THLCJcwgtVtVESMOdmi3T3bBxgOqJt6BCtCHQ@mail.gmail.com>
Date:   Wed, 18 Jan 2017 20:51:08 -0600
From:   Rob Herring <robh@...nel.org>
To:     Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Cc:     Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Marek Vasut <marek.vasut@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Guochun Mao <guochun.mao@...iatek.com>,
        Richard Weinberger <richard@....at>,
        Russell King <linux@...linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-mediatek@...ts.infradead.org,
        Cyrille Pitchen <cyrille.pitchen@...el.com>,
        Brian Norris <computersforpeace@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v1 2/2] arm: dts: mt2701: add nor flash node

On Wed, Jan 18, 2017 at 5:38 PM, Thomas Petazzoni
<thomas.petazzoni@...e-electrons.com> wrote:
> Hello,
>
> On Wed, 18 Jan 2017 16:20:10 -0600, Rob Herring wrote:
>
>> > > Rob, Mark, any opinion?
>> >
>>
>> Sigh, is how to do compatibles really not yet understood?
>
> Well, it seems like not everyone necessarily understands what is the
> best strategy to adopt (me included).
>
>> > I agree that a clarification would be good. There are really two
>> > options:
>> >
>> >  1. Have two compatible strings in the DT, the one that matches the
>> >     exact SoC where the IP is found (first compatible string) and the
>> >     one that matches some other SoC where the same IP is found (second
>> >     compatible string). Originally, Linux only supports the second
>> >     compatible string in its device driver, but if it happens that a
>> >     difference is found between two IPs that we thought were the same,
>> >     we can add support for the first compatible string in the driver,
>> >     with a slightly different behavior.
>>
>> This. And no wildcards in the compatible string.
>
> OK. So it means that today we do something like:
>
>         compatible = "baz,foo-12", "baz,foo-00";
>
> and support only baz,foo-00 in the driver. If tomorrow we discover
> that there is in fact a difference between the two IP blocks, we can
> add support for baz,foo-12 in the driver, and handle the differences.
>
> But then, the DT still contains:
>
>         compatible = "baz,foo-12", "baz,foo-00";
>
> and therefore pretends that the IP block is compatible with
> "baz,foo-00" which is in fact *not* the case. It was a mistake to
> consider it as compatible. So we keep living with a DT that has
> incorrect information.

I wouldn't say it's a mistake necessarily. The old compatible would
probably work to some extent. I'd assume it was tested to some level.
Or it could be other changes exposing a difference.

>> >  2. Have a single compatible string in the DT, matching the exact SoC
>> >     where the IP is found. This involves adding immediately this
>> >     compatible string in the corresponding driver.
>>
>> I wouldn't object to this from a DT perspective as I have no clue
>> generally if IP blocks are "the same" or not. Subsystem maintainers will
>> object though.
>
> Knowing if IP blocks are "the same" is in fact not necessarily trivial.
> What appears to be identical IP blocks today might be discovered later
> as actually having subtle differences (sometimes not even visible in
> the datasheet).

Yes, I know. That's exactly when you should have multiple compatibles.
Trying to guarantee things are the same is just going to get you in
trouble. You only need to figure out if blocks are obviously different
and only drop the old compatible in that case.

>> > I've not really been able to figure out which of the two options is the
>> > most future-proof/appropriate.
>>
>> They are both future-proof. #2 has the disadvantage of requiring a
>> kernel update for a new SoC.
>
> Which is generally anyway needed because a new SoC will almost always
> require some new drivers, adjusting pin-muxing or clock drivers, etc.

Yes, but you don't want to have to update every single driver.

Rob