Re: Does the i915 VBT tell us if a panel is an OLED panel?

From: Sean Paul <sean@poorly.run>
To: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Christian Kellner <ckellner@redhat.com>, Intel-gfx@lists.freedesktop.org
Subject: Re: Does the i915 VBT tell us if a panel is an OLED panel?
Date: Fri, 11 Oct 2019 15:30:23 -0400	[thread overview]
Message-ID: <20191011193023.GV85762@art_vandelay> (raw)
In-Reply-To: <878spshgeb.fsf@intel.com>

On Thu, Oct 10, 2019 at 04:35:56PM +0300, Jani Nikula wrote:
> On Thu, 10 Oct 2019, Hans de Goede <hdegoede@redhat.com> wrote:
> > Hi Jani,
> >
> > During plumbers I had some discussions with Daniel about supporting
> > OLED screens. Userspace may need to know that a panel is OLED for 2
> > reasons:
> >
> > 1) To avoid screen burn-in
> > 2) OLED screens do not have a classic backlight, so in some cases
> > some sort of brightness/contrast emulation through gamma tables may
> > be necessary to still allow the user to control the brightness.
> 
> I'd think most OLED displays have a native way to control
> brightness. Some eDP panels can use the eDP PWM pin to control
> brightness, although it does not directly drive an actual backlight, and
> some others use the eDP standard DPCD brightness control
> methods. Similarly, OLED DSI displays have DCS commands for this.
> 
> Often I've seen various content adaptive brightness settings combined
> with the OLED brightness control, so it can be more power efficient than
> using gamma.
> 
> > The idea we've discussed is to add a property on the drm_connector
> > (details to be filled in) which indicates that the panel is an OLED
> > panel.
> >
> > This has lead to the question: "how do we know the panel is OLED"?
> >
> > Do you know if this info is coded into the VBT somewhere?
> 
> Not AFAICT. But there is the indication of the brightness control
> method, and one option is the eDP AUX interface. I fathom it's entirely
> possible for panels to use the eDP AUX interface for controlling an LCD
> backlight, so this does not directly translate to OLED.
> 
> However, the DisplayID spec has Display Device Data block (0x0c) that
> contains Display Device Technology byte, including a value for Organic
> LED/OEL. I haven't actually checked any OLED displays if they have this
> or not, and we don't currently parse it in drm, but this seems like a
> better option than VBT. Moreover, this is usable also for regular DP,
> which should be as important as eDP for the burn-in avoidance.

One datapoint: I have an eDP OLED panel and it does not seem to have
DisplayID extensions.

Sean

> 
> BR,
> Jani.
> 
> 
> -- 
> Jani Nikula, Intel Open Source Graphics Center

-- 
Sean Paul, Software Engineer, Google / Chromium OS
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