Re: [PATCH] ACPI: Disable Windows 8 compatibility for some Lenovo ThinkPads

From: Aaron Lu <aaron.lu@intel.com>
To: joeyli <jlee@suse.com>
Cc: Ben Jencks <ben@bjencks.net>,
	Seth Forshee <seth.forshee@canonical.com>,
	Len Brown <lenb@kernel.org>, "Rafael J. Wysocki" <rjw@sisk.pl>,
	linux-acpi@vger.kernel.org, Matthew Garrett <mjg59@srcf.ucam.org>
Subject: Re: [PATCH] ACPI: Disable Windows 8 compatibility for some Lenovo ThinkPads
Date: Mon, 22 Apr 2013 18:08:17 +0800	[thread overview]
Message-ID: <51750C11.7060600@intel.com> (raw)
In-Reply-To: <1366597102.23707.92.camel@linux-s257.site>

On 04/22/2013 10:18 AM, joeyli wrote:
> 於 五，2013-04-19 於 11:15 +0800，Aaron Lu 提到：
>> On 04/03/2013 03:04 PM, Ben Jencks wrote:
>>> On 04/02/2013 09:00 AM, Seth Forshee wrote:
>>>> On Tue, Apr 02, 2013 at 05:08:23PM +0800, Aaron Lu wrote:
>>>>>
>>>>> I really wondered, how Windows handled this, it should have the same
>>>>> problem, unless they are not using the acpi video interface?
>>>>
>>>> I can only guess.
>>>>
>>>> I think I remember reading that Windows 8 does smooth backlight
>>>> transitions, so it may well hit every intermediate brightness value.
>>>> Lenovo could also be supplying a driver which rounds values to the
>>>> nearest working value or uses some other interface or something else.
>>>
>>> Just checked; Windows 8 doesn't use the ACPI interface. It seems to have
>>> access to at least 100 distinct brightness levels.
>>
>> I just came across a document on win8 backlight control, it has words
>> like this:
>> "
>> In Windows 8, the primary mechanism by which a platform should expose
>> its display brightness control functionality is the Windows Display
>> Driver Model (WDDM) miniport Device Driver Interfaces (DDI).
>> "
>> So looks like, on win8, ACPI interface is not used for these systems.
>>
>> The link for the document is here:
>> http://msdn.microsoft.com/en-US/library/windows/hardware/jj159305
>>
>> -Aaron
>>
> 
> Per WDDM document, OEM/ODM should keep the ACPI methods (_BQL, _BCM,
> _BQC) available for compliant to the OS that doesn't support WDDM, e.g.
> XP, Vista.

Thanks for the information, so this suggests that acpi interface is
mostly used for pre-win8 OSes?(does win7 support WDDM?)

One interesting thing is, if they do not support acpi interface, why
they even bother to expose this interface in acpi table?

-Aaron
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