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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: Dmitry Frank <mail@dmitryfrank.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Dmitry Frank 2017-04-19 13:36:18 +03:00 committed by Rafael J. Wysocki
parent d485ef4307
commit 592c8095b8
1 changed files with 45 additions and 2 deletions
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47
drivers/acpi/acpi_video.c
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@ -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) {