Question regarding back compatibility for DT

Question regarding back compatibility for DT

[Daniel Lezcano]

Hi Rob, dt people,

is it acceptable a driver is changed using a new DT binding but becomes
incompatible with the old DT files ?

IOW, is it possible to change the DT and the driver without supporting the old
DT format, forcing an update of the DT and the kernel ?

Thanks.

  -- Daniel

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Re: Question regarding back compatibility for DT

[Warner Losh]

On Tue, Mar 21, 2017 at 8:58 AM, Daniel Lezcano
<daniel.lezcano-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
> Hi Rob, dt people,
>
> is it acceptable a driver is changed using a new DT binding but becomes
> incompatible with the old DT files ?
>
> IOW, is it possible to change the DT and the driver without supporting the old
> DT format, forcing an update of the DT and the kernel ?

I've seen new properties many times added, even though the new driver
wouldn't work quite right on old DT files. I've also seen people
create a new hardware compatibility name for a totally new binding.
Finally, I've seen the bindings just randomly change for no reason
with no concern for backwards compat. The last really sucks if you
have a driver that's not in the Linux tree (say, because it's a
FreeBSD driver).

Warner

> Thanks.
>
>   -- Daniel
>
> --
>
>  <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs
>
> Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
> <http://twitter.com/#!/linaroorg> Twitter |
> <http://www.linaro.org/linaro-blog/> Blog
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Re: Question regarding back compatibility for DT

[Rob Herring]

On Tue, Mar 21, 2017 at 11:16 AM, Warner Losh <imp-uzTCJ5RojNnQT0dZR+AlfA@public.gmane.org> wrote:
> On Tue, Mar 21, 2017 at 8:58 AM, Daniel Lezcano
> <daniel.lezcano-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
>> Hi Rob, dt people,
>>
>> is it acceptable a driver is changed using a new DT binding but becomes
>> incompatible with the old DT files ?
>>
>> IOW, is it possible to change the DT and the driver without supporting the old
>> DT format, forcing an update of the DT and the kernel ?
>
> I've seen new properties many times added, even though the new driver
> wouldn't work quite right on old DT files. I've also seen people
> create a new hardware compatibility name for a totally new binding.
> Finally, I've seen the bindings just randomly change for no reason
> with no concern for backwards compat. The last really sucks if you
> have a driver that's not in the Linux tree (say, because it's a
> FreeBSD driver).

Hopefully that is getting better? If not, how do we make it better? I
try to prevent it through reviews, but I have have no visibility into
what FreeBSD does and doesn't care about.

The common suggestion is moving the bindings out of the kernel. The
biggest issue I have with that is that we would lose the review of all
the kernel subsystem maintainers. They may be biased toward Linux, but
they know their class of h/w and no other project has the breath of
h/w support.

Rob
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Re: Question regarding back compatibility for DT

[Rob Herring]

On Tue, Mar 21, 2017 at 10:58 AM, Daniel Lezcano
<daniel.lezcano-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
> Hi Rob, dt people,
>
> is it acceptable a driver is changed using a new DT binding but becomes
> incompatible with the old DT files ?
>
> IOW, is it possible to change the DT and the driver without supporting the old
> DT format, forcing an update of the DT and the kernel ?

If you want a global rule, then no it is not okay. However, it depends
on the platform and it's users. I just ask that breaking compatibility
and why is highlighted in the commit message and the platform
maintainer is okay with it.

Rob
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Re: Question regarding back compatibility for DT

[Daniel Lezcano]

On Tue, Mar 21, 2017 at 11:27:37AM -0500, Rob Herring wrote:
> On Tue, Mar 21, 2017 at 10:58 AM, Daniel Lezcano
> <daniel.lezcano-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
> > Hi Rob, dt people,
> >
> > is it acceptable a driver is changed using a new DT binding but becomes
> > incompatible with the old DT files ?
> >
> > IOW, is it possible to change the DT and the driver without supporting the old
> > DT format, forcing an update of the DT and the kernel ?
> 
> If you want a global rule, then no it is not okay. However, it depends
> on the platform and it's users. I just ask that breaking compatibility
> and why is highlighted in the commit message and the platform
> maintainer is okay with it.

Thanks for the clarification. I will ask the maintainer if breaking the
compatibility is ok.

That shows how important is to wisely choose the right binding.

  -- Daniel


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