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

From: Jani Nikula <jani.nikula@linux.intel.com>
To: Sean Paul <sean@poorly.run>
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: Mon, 14 Oct 2019 11:12:04 +0300	[thread overview]
Message-ID: <87d0ezd9uz.fsf@intel.com> (raw)
In-Reply-To: <20191011193023.GV85762@art_vandelay>

On Fri, 11 Oct 2019, Sean Paul <sean@poorly.run> wrote:
> 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.

Does the DPCD say it supports backlight control through DP AUX?

BR,
Jani.

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

-- 
Jani Nikula, Intel Open Source Graphics Center
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