Re: KMS backlight ABI proposition

From: Jani Nikula <jani.nikula@linux.intel.com>
To: "Daniel Thompson" <daniel.thompson@linaro.org>,
	"Stéphane Marchesin" <stephane.marchesin@gmail.com>
Cc: "Hans de Goede" <hdegoede@redhat.com>,
	"Martin Gräßlin" <mgraesslin@kde.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>
Subject: Re: KMS backlight ABI proposition
Date: Fri, 24 Feb 2017 13:41:53 +0200	[thread overview]
Message-ID: <878tovx1by.fsf@intel.com> (raw)
In-Reply-To: <08f3ec93-fda7-ee53-d290-12ecd42dcaac@linaro.org>

On Fri, 24 Feb 2017, Daniel Thompson <daniel.thompson@linaro.org> wrote:
> On 24/02/17 08:43, Jani Nikula wrote:
>  >> - "Some PWM based backlight allow adjusting the PWM modulation
>  >> frequency." you don't need a motivation for *why* I would want to
>  >> change the mod freq on the fly, actually in my experience you
>  >> shouldn't since this can lead to flickery backlights.
>  >
>  > The modulation frequency is usually an OEM design choice. The higher
>  > the
>  > frequency, less flicker, but also fewer user distinguishable levels of
>  > backlight (signal rise/fall times come into play). Occasionally there
>  > have been requests to be able to adjust the frequency. We can of course
>  > decide that's an implementation detail and not let userspace change it.
>
> In principle the backlight max level could also change during monitor 
> hotplug; recent versions DDC/CI include a backlight properties. We have 
> no driver for this right now but conversation is starting (and proper 
> backlight support in DRM connectors will probably help it happen faster).
>
> I'm afraid I have no idea if this is "real" or now. Right now I have no 
> clue how many monitors implement it. For regular desktop monitors which 
> mirror old school brightness/contrast in their UI then 0 wouldn't 
> surprise me much ;-)

Thanks for the reminder. I think so far this as worked by using the DDC
I2C device nodes directly. And it really works already to some extent,
on some displays. It would be nice to have this controlled the same way
as embedded diplays.

Another interesting aspect with DDC/CI brightness control, one that it
shares with eDP DPCD and DSI DCS brightness controls, is that it could
all happen within DRM, mostly in a generic, driver independent
manner. There would not be a need for a backlight class device or sysfs
API at all, except for legacy needs.

BR,
Jani.

-- 
Jani Nikula, Intel Open Source Technology Center
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