linux-kernel - Re: [PATCH] toshiba_acpi: Fix blank screen at boot if transflective backlight is supported

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Date:	Fri, 20 Nov 2015 16:46:07 -0700
From:	Azael Avalos <coproscefalo@...il.com>
To:	Darren Hart <dvhart@...radead.org>
Cc:	"platform-driver-x86@...r.kernel.org" 
	<platform-driver-x86@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org,
	Fabian Koester <fabian.koester@...ngnow.com>
Subject: Re: [PATCH] toshiba_acpi: Fix blank screen at boot if transflective
 backlight is supported

Hi Darren,

2015-11-20 16:16 GMT-07:00 Darren Hart <dvhart@...radead.org>:
> On Sun, Nov 15, 2015 at 08:32:47PM -0700, Azael Avalos wrote:
>> If transflective backlight is supported and the brightness is zero
>> (lowest brightness level), the set_lcd_brightness function will activate
>> the transflective backlight, making the LCD appear to be turned off.
>>
>> This patch fixes the issue by incrementing the brightness level, and
>> by doing so, avoiding the activation of the tranflective backlight.
>
> So... I'm not sure this is a bug or even wrong behavior. The lowest setting on
> my Thinkpad Yoga 12 is also "off" in Linux. Same is true for my Mac Book Pro (in
> Mac OS X).

Yeah, and even on "regular" (with non transflective backlight) Toshibas
zero is also "off" and no problem is observed.

>
> From what I can tell, what this patch does is merely disable the lowest setting.

Nope, we are just not letting the driver enable transflective backlight
and continue using the "normal" brightness range.

>
> What is it about this behavior that is considered to be wrong?
>

The problem is with laptops with transflective backlight support.

The code for transflective backlight is fused with the LCD backlight
code (go figure...), so if a Toshiba laptop with transflective support
is detected, the max brightness levels are raised by one.

A regular laptop's brightness levels are 0-7, but on a laptop with
transflective support, the brightness levels are 0-8, zero being
"activate transflective backlight" and 1-8 the regular brightness
levels.

So, when a laptop with transflective support AND an initial
brightness level of zero gets probed for backlight support,
the set_lcd_brightness function activates the transfective
backlight, as the brightness levels are still not incremented,
and thus, the screen goes dark.

I hope this clarifies a bit the situation here.

Cheers
Azael

-- 
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