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

[Rob Herring <robh@kernel.org>]

On Thu, Jan 19, 2017 at 2:14 AM, Boris Brezillon
<boris.brezillon@free-electrons.com> wrote:
> Hi Rob,
>
> On Wed, 18 Jan 2017 20:51:08 -0600
> Rob Herring <robh@kernel.org> wrote:
>
>> On Wed, Jan 18, 2017 at 5:38 PM, Thomas Petazzoni
>> <thomas.petazzoni@free-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.
>
> One last question and I'm done: is something like that acceptable?
>
>         compatible = "<vendor>,<old-soc>","<vendor>,<new-soc>";
>
> This can happen when someone adds support for an unsupported feature
> on a brand new SoC, and then someone else use the same driver for an
> older SoC embedding the same IP but still wants to add a new compatible
> just in case these 2 IPs appear to be slightly different.

Yes, it's old and new compatible strings in this case and it's newest
compatible string first.

> Here the order of compat strings is no longer following a clear rule
> like 'most-specific compatible first' or 'newest IP/SoC version first',
> it's completely dependent on the order these IPs were supported in the
> OS (Linux). I'm perfectly fine with that BTW, just want to make sure
> this is authorized.

I guess we should say "newest compatible for IP first" instead. There
are some exceptions where we add fallbacks later on, but that falls
under the most-specific part.

It's order that the bindings are defined, not Linux support really,
but in practice those are the same.

Rob