linux-kernel - Re: [PATCH v4 2/3] backlight: dwi_bl: Add Apple DWI backlight driver

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Message-ID: <Z4AEK1LzFeR1DnPk@aspen.lan>
Date: Thu, 9 Jan 2025 17:15:23 +0000
From: Daniel Thompson <danielt@...nel.org>
To: Nick Chan <towinchenmi@...il.com>
Cc: Lee Jones <lee@...nel.org>, Jingoo Han <jingoohan1@...il.com>,
	Pavel Machek <pavel@....cz>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Helge Deller <deller@....de>,
	Hector Martin <marcan@...can.st>, Sven Peter <sven@...npeter.dev>,
	Alyssa Rosenzweig <alyssa@...enzweig.io>,
	dri-devel@...ts.freedesktop.org, linux-leds@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fbdev@...r.kernel.org, asahi@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 2/3] backlight: dwi_bl: Add Apple DWI backlight driver

On Thu, Jan 09, 2025 at 11:52:56PM +0800, Nick Chan wrote:
>
> Daniel Thompson 於 2025/1/8 下晝6:52 寫道:
> > On Wed, Dec 11, 2024 at 07:34:38PM +0800, Nick Chan wrote:
> >> Add driver for backlight controllers attached via Apple DWI 2-wire
> >> interface, which is found on some Apple iPhones, iPads and iPod touches
> >> with a LCD display.
> >>
> >> Although there is an existing apple_bl driver, it is for backlight
> >> controllers on Intel Macs attached via PCI, which is completely different
> >> from the Samsung-derived DWI block.
> >>
> >> Signed-off-by: Nick Chan <towinchenmi@...il.com>
> >> ---
> >>  drivers/video/backlight/Kconfig  |  12 +++
> >>  drivers/video/backlight/Makefile |   1 +
> >>  drivers/video/backlight/dwi_bl.c | 122 +++++++++++++++++++++++++++++++
> > I'd rather this was called apple_dwi_bl.c to match that config options,
> > etc.
> Still trying to determine the type of the backlight control, I think
> it is linear however and I will send a new version when I am more
> certain.

If you've got an physicalinstance of the backlight then you can get a
general idea by comparing levels: 512, 1024, 1536 and 2047.

A linear backlight will feel make the gaps feel uneven (e.g. the
difference between 1536 and 2047 will look very different to the
difference difference between 512 and 1024). A logarithmic backligt
will make the perceived brightness intervals between the above all
feel pretty similar.


Daniel.