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

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Date:   Wed, 18 Jan 2017 16:20:10 -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, Guochun Mao <guochun.mao@...iatek.com>,
        Richard Weinberger <richard@....at>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.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
Subject: Re: [PATCH v1 2/2] arm: dts: mt2701: add nor flash node

On Tue, Jan 17, 2017 at 02:36:50PM +1100, Thomas Petazzoni wrote:
> Hello,
> 
> (Side note: you guys should learn about stripping irrelevant parts of
> an e-mail when replying!)
>
> On Mon, 16 Jan 2017 09:40:32 +0100, Boris Brezillon wrote:
> 
> > > 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 ;-).
> > 
> > Rob, Mark, any opinion?
>

Sigh, is how to do compatibles really not yet understood?
 
> 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. 

>  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.

> 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. 

Rob