Re: [RFC 0/2] ACPI: video: prefer native over vendor

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 11/9/22 14:51, Rafael J. Wysocki wrote:
> On Sat, Nov 5, 2022 at 4:17 PM Hans de Goede <hdegoede@redhat.com> wrote:
>>
>> Hi,
>>
>> On 11/5/22 15:52, Hans de Goede wrote:
>>> Hi Rafael, Matthew,
>>>
>>> Here is a second attempt at always registering only a single
>>> /sys/class/backlight device per panel.
>>>
>>> This first round of testing has shown that native works well even on
>>> systems so old that the don't have acpi_video backlight control support.
>>>
>>> This patch series makes native be preferred over vendor, which should
>>> avoid the problems seen with the 6.1 changes before the fixes.
>>>
>>> ATM there is one known model where this will cause a regression,
>>> the Sony Vaio PCG-FRV3 from 2003. I plan to add a DMI quirk for that
>>> in the next version of this series, but I'm waiting for some more
>>> testing (to check that the vendor interface does actually work) first.
>>>
>>> I will also do another blogpost, focussing on asking users to see
>>> if they have a laptop which provides a combination of vendor + native
>>> backlight interfaces, which may be impacted by this series. This is
>>> the main reason why this is a RFC for now.
>>
>> The blogpost requesting testing of laptops with a combination
>> of vendor + native backlight interfaces can be found here:
>>
>> https://hansdegoede.dreamwidth.org/27024.html
> 
> The patches in this series look reasonable to me,

Thanks.

> even though I'm not
> sure if the assumption that the Windows 8 hardware certification
> requirements were always followed is not overly optimistic.

After the second patch in the series the only remaining
prefer_native_over_acpi_video() and thus the only acpi_osi_is_win8()
call is guarded by a (video_caps & ACPI_VIDEO_BACKLIGHT) check as before:

        if ((video_caps & ACPI_VIDEO_BACKLIGHT) &&
             !(native_available && prefer_native_over_acpi_video()))
                return acpi_backlight_video;

So even if the assumption is wrong, there is no behavior change
other then the intended behavior change already caused by the second
patch.

The last part of __acpi_video_get_backlight_type() which contains
the basic (non special case) heuristics looks like this after
this series:

        /* Use ACPI video if available, except when native should be preferred. */
        if ((video_caps & ACPI_VIDEO_BACKLIGHT) &&
             !(native_available && prefer_native_over_acpi_video()))
                return acpi_backlight_video;

        /* Use native if available */
        if (native_available)
                return acpi_backlight_native;

        /* No ACPI video/native (old hw), use vendor specific fw methods. */
        return acpi_backlight_vendor;

Which is also a bit more KISS / easier to follow then before and
I hope that this will work well.

Regards,

Hans