linux-kernel - Re: [PATCH 5/5] drm/i915: Replace "Broadcast RGB" with "RGB quantization range" property

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Date:   Wed, 15 Apr 2020 10:33:25 +0300
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Yussuf Khalil <dev@...345.net>, Daniel Vetter <daniel@...ll.ch>
Cc:     Simon Ser <contact@...rsion.fr>, David Airlie <airlied@...ux.ie>,
        "dri-devel\@lists.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>
Subject: Re: [PATCH 5/5] drm/i915: Replace "Broadcast RGB" with "RGB quantization range" property

On Tue, 14 Apr 2020, Yussuf Khalil <dev@...345.net> wrote:
> On Tue, 2020-04-14 at 14:34 +0200, Daniel Vetter wrote:
>> On Tue, Apr 14, 2020 at 02:21:06PM +0300, Jani Nikula wrote:
>> > On Tue, 14 Apr 2020, Jani Nikula <jani.nikula@...ux.intel.com>
>> > wrote:
>> > > On Mon, 13 Apr 2020, Simon Ser <contact@...rsion.fr> wrote:
>> > > > On Monday, April 13, 2020 11:40 PM, Yussuf Khalil <
>> > > > dev@...345.net> wrote:
>> > > > 
>> > > > > DRM now has a globally available "RGB quantization range"
>> > > > > connector
>> > > > > property. i915's "Broadcast RGB" that fulfils the same
>> > > > > purpose is now
>> > > > > considered deprecated, so drop it in favor of the DRM
>> > > > > property.
>> > > > 
>> > > > For a UAPI point-of-view, I'm not sure this is fine. Some user-
>> > > > space
>> > > > might depend on this property, dropping it would break such
>> > > > user-space.
>> > > 
>> > > Agreed.
>> > > 
>> > > > Can we make this property deprecated but still keep it for
>> > > > backwards
>> > > > compatibility?
>> > > 
>> > > Would be nice to make the i915 specific property an "alias" for
>> > > the new
>> > > property, however I'm not sure how you'd make that happen.
>> > > Otherwise
>> > > juggling between the two properties is going to be a nightmare.
>> > 
>> > Ah, the obvious easy choice is to use the property and enum names
>> > already being used by i915 and gma500, and you have no problem.
>> > Perhaps
>> > they're not the names you'd like, but then looking at the total
>> > lack of
>> > consistency across property naming makes them fit right in. ;)
>> 
>> Yeah if we don't have contradictory usage across drivers when
>> modernizing
>> these properties, then let's just stick with the names already there.
>> It's
>> not pretty, but works better since more userspace/internet howtos
>> know how
>> to use this stuff.
>> -Daniel
>
> Note that i915's "Broadcast RGB" isn't the same as gma500's: i915 has an
> "Automatic" option, whereas gma500 does not.

Adding "Automatic" option that just defaults to "Full" in gma500 does
not break existing userspace, but allows for extending it to do more
clever things later.

> Also, radeon has a property called
> "output_csc" that fulfills a similar purpose. Looking at the code, though, it
> seems that radeon does not adhere to the standard correctly (or I am missing
> something).
>
> An alternative would be to leave the existing driver-specific properties and
> change them to be pseudo-aliases for the "RGB quantization range" property.
> This can be done by letting the drivers read from and write to the new property
> when user-space tries to read or modify the driver's property. This way we could
> retain full backwards compatibility for all drivers equally.
>
> What do you think?

I'm obviously biased and predisposed to avoid adding extra complexity to
i915 when it's not necessary. We'd have *two* connector properties for
the same thing until the end of time, even if one is an alias for the
other.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center