Commit 592c8095 authored by Dmitry Frank's avatar Dmitry Frank Committed by Rafael J. Wysocki
Browse files

ACPI / video: add comments about subtle cases 



The comment for acpi_video_bqc_quirk is by Felipe Contreras, taken from
the git history.

Signed-off-by: default avatarDmitry Frank <mail@dmitryfrank.com>
Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
parent d485ef43

drivers/acpi/acpi_video.c

+45 −2
Original line number Diff line number Diff line
@@ -73,6 +73,10 @@ module_param(report_key_events, int, 0644); 
MODULE_PARM_DESC(report_key_events,

	"0: none, 1: output changes, 2: brightness changes, 3: all");



/*

 * Whether the struct acpi_video_device_attrib::device_id_scheme bit should be

 * assumed even if not actually set.

 */

static bool device_id_scheme = false;

module_param(device_id_scheme, bool, 0444);
@@ -144,7 +148,15 @@ struct acpi_video_device_attrib { 
				   the VGA device. */

	u32 pipe_id:3;		/* For VGA multiple-head devices. */

	u32 reserved:10;	/* Must be 0 */

	u32 device_id_scheme:1;	/* Device ID Scheme */



	/*

	 * The device ID might not actually follow the scheme described by this

	 * struct acpi_video_device_attrib. If it does, then this bit

	 * device_id_scheme is set; otherwise, other fields should be ignored.

	 *

	 * (but also see the global flag device_id_scheme)

	 */

	u32 device_id_scheme:1;

};



struct acpi_video_enumerated_device {
@@ -728,7 +740,33 @@ 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.

	 * through _BQC, we need to test another value for them. However,

	 * there is a subtlety:

	 *

	 * If the _BCL package ordering is descending, the first level

	 * (br->levels[2]) is likely to 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.

	 * Still, if the _BCL package ordering is descending, the index of

	 * level 0 is also 100, so we assume _BQC is indexed, when it's not.

	 *

	 * This causes all _BQC calls to return bogus values causing weird

	 * behavior from the user's perspective.  For example:

	 *

	 * xbacklight -set 10; xbacklight -set 20;

	 *

	 * would flash to 90% and then slowly down to the desired level (20).

	 *

	 * The solution is simple; test anything other than the first level

	 * (e.g. 1).

	 */

	test_level = current_level == max_level

		? br->levels[ACPI_VIDEO_FIRST_LEVEL + 1]
@@ -789,6 +827,11 @@ int acpi_video_get_levels(struct acpi_device *device, 
		goto out;

	}



	/*

	 * Note that we have to reserve 2 extra items (ACPI_VIDEO_FIRST_LEVEL),

	 * in order to account for buggy BIOS which don't export the first two

	 * special levels (see below)

	 */

	br->levels = kmalloc((obj->package.count + ACPI_VIDEO_FIRST_LEVEL) *

	                     sizeof(*br->levels), GFP_KERNEL);

	if (!br->levels) {