Re: [PATCH v5] Documentation: gpu: Mention the requirements for new properties

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Fri, 9 Jul 2021 10:02:28 +0200
Daniel Vetter <daniel@ffwll.ch> wrote:

> On Fri, Jul 09, 2021 at 10:24:44AM +0300, Pekka Paalanen wrote:
> > On Tue,  6 Jul 2021 18:12:44 +0200
> > Maxime Ripard <maxime@cerno.tech> wrote:
> >   
> > > New KMS properties come with a bunch of requirements to avoid each
> > > driver from running their own, inconsistent, set of properties,
> > > eventually leading to issues like property conflicts, inconsistencies
> > > between drivers and semantics, etc.
> > > 
> > > Let's document what we expect.  
> > 
> > ...
> >   
> > > Changes from v4:
> > >   - Changes suggested by Pekka
> > > 
> > > Changes from v3:
> > >   - Roll back to the v2
> > >   - Add Simon and Pekka in Cc
> > > 
> > > Changes from v2:
> > >   - Take into account the feedback from Laurent and Lidiu to no longer
> > >     force generic properties, but prefix vendor-specific properties with
> > >     the vendor name
> > > 
> > > Changes from v1:
> > >   - Typos and wording reported by Daniel and Alex
> > > ---
> > >  Documentation/gpu/drm-kms.rst | 30 ++++++++++++++++++++++++++++++
> > >  1 file changed, 30 insertions(+)
> > > 
> > > diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
> > > index 87e5023e3f55..47994890fd1e 100644
> > > --- a/Documentation/gpu/drm-kms.rst
> > > +++ b/Documentation/gpu/drm-kms.rst
> > > @@ -463,6 +463,36 @@ KMS Properties
> > >  This section of the documentation is primarily aimed at user-space developers.
> > >  For the driver APIs, see the other sections.
> > >  
> > > +Requirements
> > > +------------
> > > +
> > > +KMS drivers might need to add extra properties to support new features.
> > > +Each new property introduced in a driver need to meet a few
> > > +requirements, in addition to the one mentioned above:
> > > +
> > > +* It must be standardized, documenting:
> > > +
> > > +  * The full, exact, name string;
> > > +  * If the property is an enum, all the valid variants name;  
> > 
> > Hi,
> > 
> > "variant" feels a little off to me, I would have used "value name
> > strings".
> >   
> > > +  * What values are accepted, and what these values mean;
> > > +  * What the property does and how it can be used;
> > > +  * How the property might interact with other, existing properties.
> > > +
> > > +* It must provide a generic helper in the core code to register that
> > > +  property on the object it attaches to.
> > > +
> > > +* Its content must be decoded by the core and provided in the object's
> > > +  associated state structure. That includes anything drivers might want
> > > +  to precompute, like :c:type:`struct drm_clip_rect <drm_clip_rect>` for
> > > +  planes.
> > > +
> > > +* Its initial state must match the behavior prior to the property
> > > +  introduction. This might be a fixed value matching what the hardware
> > > +  does, or it may be inherited from the state the firmware left the
> > > +  system in during boot.  
> > 
> > I'd like to point out that this rule should apply also to
> > properties that already exist in general, but are newly exposed in a
> > driver for hardware that didn't expose the property before.  
> 
> I think we should just make this a very strong recommendation, and in
> general encourage people to use the tests against their driver?
> 
> Otherwise a small "I'll just enable this" thing can become a huge project.
> And in general I think grandfathering existing things in is the pragmatic
> choice.
> 
> But maybe that could be a follow-up patch?

Sure, I don't mind. Just saying now that it came to mind. Drivers do
not arbitrarily change behaviour without exposing more properties
either, right?


Thanks,
pq


> -Daniel
> 
> >   
> > > +
> > > +* An IGT test must be submitted where reasonable.
> > > +
> > >  Property Types and Blob Property Support
> > >  ----------------------------------------
> > >    
> > 
> > Regardless of my comments above:
> > 
> > Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
> > 
> > 
> > Thanks,
> > pq  
> 
> 
> 


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