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    SubjectRe: [PATCH] acpi: video: fix reversed indexed BQC
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    On Fri, Aug 2, 2013 at 1:56 AM, Aaron Lu <aaron.lu@intel.com> wrote:
    > On 08/02/2013 02:44 PM, Felipe Contreras wrote:

    >> The initial _BCM commands don't work, so the level remains at 100%.
    >> Since the level is max_level, acpi_video_bqc_quirk() tries with the
    >> first level, which is 0, and 0 happens to be the index of 100.
    >>
    >> So _BQC is returning 100, which is not the index of 0 (what we tested
    >> for), but actually 100.
    >>
    >> I think the current code is correct, but acpi_video_bqc_quirk() should
    >> be testing br->levels[3], or anything other than 0/100 which can be
    >> easily confused.
    >>
    >> If so, the code would find that _BQC doesn't work on this machine (in
    >> win8 mode)... at least initially. My guess is that it only starts to
    >> work after acpi_video_bus_start_devices() is called.
    >>
    >> Forcing br->flags._BQC_use_index = 0 seems to work.
    >
    > Seems ASUS machines tend to have this issue:
    > https://bugzilla.kernel.org/show_bug.cgi?id=52951
    > https://bugzilla.kernel.org/show_bug.cgi?id=56711

    I don't see any real solution for the ACPI driver.

    > I have a patch to enhance the quirk some time ago:
    > https://github.com/aaronlu/linux/commit/0a3d2c5b59caf80ae5bb1ca1fda0f7bf448b38c9

    I think this is unnecessarily complicated; the comment makes it clear
    that the purpose is to find out if _BQC is working, and this does the
    trick:

    From 2bfa401b0a50fcde292ac0eb60cb6f857caf2fc6 Mon Sep 17 00:00:00 2001
    From: Felipe Contreras <felipe.contreras@gmail.com>
    Date: Fri, 2 Aug 2013 02:27:44 -0500
    Subject: [PATCH] acpi: video: improve quirk check

    If the _BCL package is descending, the first level (br->levels[2]) will
    be 0, and if the number of levels matches the number of steps, we might
    confuse a returned level to mean the index.

    For example:

    current_level = max_level = 100
    test_level = 0
    returned level = 100

    In this case 100 means the level, not the index, and _BCM failed. But if
    the _BCL package is descending, the index of level 0 is also 100, so we
    assume _BQC is indexed, when it's not.

    The solution is simple; test anything other than the first level (e.g.
    1).

    Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
    ---
    drivers/acpi/video.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/drivers/acpi/video.c b/drivers/acpi/video.c
    index 0ec434d..e1284b8 100644
    --- a/drivers/acpi/video.c
    +++ b/drivers/acpi/video.c
    @@ -689,7 +689,7 @@ static int acpi_video_bqc_quirk(struct
    acpi_video_device *device,
    * Some systems always report current brightness level as maximum
    * through _BQC, we need to test another value for them.
    */
    - test_level = current_level == max_level ? br->levels[2] : max_level;
    + test_level = current_level == max_level ? br->levels[3] : max_level;

    result = acpi_video_device_lcd_set_level(device, test_level);
    if (result)
    --
    1.8.3.4
    --
    Felipe Contreras


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