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    SubjectRe: [PATCH] drm/i915/opregion: work around buggy firmware that provides 8+ output devices
    On Wed, 12 Feb 2014, Chris Wilson <> wrote:
    > On Wed, Feb 12, 2014 at 11:05:40AM +0800, Aaron Lu wrote:
    >> The ACPI table on ASUS UX302LA has more than 8 output devices under the
    >> graphics controller device node. The problem is, the real active output
    >> device, the LCD panel, is listed the last. The result is, the LCD's
    >> device id doesn't get recorded in the active device list CADL array and
    >> when the _DCS control method for the LCD device is executed, it returns
    >> 0x1d, meaning it is not active. This affects the hotkey delivery ASL
    >> code that will not deliver a notification if the output device is not
    >> active on backlight hotkey press.
    >> I don't see a clean way to solve this problem since the operation region
    >> spec doesn't allow more than 8 output devices so we have no way of
    >> storing all these output devices. The fact that output devices that have
    >> _BCM control method usually means they have a higher possibility of being
    >> used than those who don't made me choose a simple way to work around
    >> the buggy firmware by replacing the last entry in CADL array with the one
    >> that has _BCM control method. There is no specific reason why the last
    >> entry is picked instead of others.
    > Another possibility is that the connector list is in rough priority
    > order so might be useful for sorting the CADL array.
    > Since the CADL should only be a list of currently active devices, we
    > could just bite the bullet and repopulate it correctly after every
    > setcrtc.

    Agreed. Per spec,

    DIDL: Writes - Graphics driver writes to this field once during its
    initialization after determining platform supported connectors.

    CPDL: Writes - Graphics driver writes to this field on every monitor
    detection process.

    CADL: Writes - Graphics driver writes to this field on every mode set
    process and during boot.

    And for all of the above: Writes - System BIOS POST or ASL code should
    not write to these fields.


    Jani Nikula, Intel Open Source Technology Center

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