Comment 8 for bug 397617

(In reply to comment #7)
Hi, Jesse
    In the UMS 2D driver methods are provided to change the backlight.(Kernel, native, legacy, combox). And the default mode is the kernel, which is the ACPI I/F. Of course the backlight mode can be switched in xrandr tool.
    When the kernel backlight mode is used, it is consistent regardless of the backlight is changed by ACPI I/F or xrandr.
    But if the legacy/native backlight mode is used, it is inconsistent between xrandr tool and ACPI I/F. This is very obvious on the opregion platform. For example: When the brightness is changed by using ACPI I/F, the xrandr can report the correct brightness. But if the brightness is changed by using xrandr, the ACPI I/F will report the wrong brightness. This is not what we expected.

    In the KMS driver the native backlight I/F is used. And now it is not exported to the userland. It means that the backlight can't be controlled by the xrandr tool. Of course it is easy to expose the native backlight property to userland(in KMS mode). But after the backlight property is exposed to userland, the inconsistency appears again.
    How about exposing the ACPI backlight I/F to userland in the drmmode_dislay.c when the KMS mode is used? It means that the ACPI backlight I/F is hook up in 2D driver when it exists. Only when there is no ACPI I/F, other backlight control mode is exposed to xrandr. (For example: legacy/combo/native).
> Yes, in addition to adding backlight resource code to drmmode_display.c in the
> 2D driver.

> In many cases it isn't, and we need to use platform specific i2c commands.
   Yes. The platform specific i2c command is useful in most cases. But how about always using the ACPI I/F if there exists the ACPI I/F? What I say is that we don't use the other backlight control mode if there exists the ACPI backlight I/F. Only when there is no ACPI backlight I/F, the other backlight control mode will be exposed to userland.

   Thanks.
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