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

From: Jani Nikula <jani.nikula@linux.intel.com>
To: Hans de Goede <hdegoede@redhat.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: Thu, 10 Oct 2019 16:35:56 +0300	[thread overview]
Message-ID: <878spshgeb.fsf@intel.com> (raw)
In-Reply-To: <6669486b-fd2d-276a-645f-ae374bf2578a@redhat.com>

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.

BR,
Jani.

-- 
Jani Nikula, Intel Open Source Graphics Center
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