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>
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@ -73,6 +73,10 @@ module_param(report_key_events, int, 0644);
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MODULE_PARM_DESC(report_key_events,
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"0: none, 1: output changes, 2: brightness changes, 3: all");
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/*
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* Whether the struct acpi_video_device_attrib::device_id_scheme bit should be
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* assumed even if not actually set.
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*/
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static bool device_id_scheme = false;
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module_param(device_id_scheme, bool, 0444);
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@ -144,7 +148,15 @@ struct acpi_video_device_attrib {
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the VGA device. */
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u32 pipe_id:3; /* For VGA multiple-head devices. */
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u32 reserved:10; /* Must be 0 */
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u32 device_id_scheme:1; /* Device ID Scheme */
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/*
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* The device ID might not actually follow the scheme described by this
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* struct acpi_video_device_attrib. If it does, then this bit
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* device_id_scheme is set; otherwise, other fields should be ignored.
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*
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* (but also see the global flag device_id_scheme)
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*/
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u32 device_id_scheme:1;
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};
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struct acpi_video_enumerated_device {
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@ -728,7 +740,33 @@ static int acpi_video_bqc_quirk(struct acpi_video_device *device,
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/*
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* Some systems always report current brightness level as maximum
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* through _BQC, we need to test another value for them.
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* through _BQC, we need to test another value for them. However,
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* there is a subtlety:
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*
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* If the _BCL package ordering is descending, the first level
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* (br->levels[2]) is likely to be 0, and if the number of levels
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* matches the number of steps, we might confuse a returned level to
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* mean the index.
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*
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* For example:
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*
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* current_level = max_level = 100
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* test_level = 0
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* returned level = 100
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*
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* In this case 100 means the level, not the index, and _BCM failed.
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* Still, if the _BCL package ordering is descending, the index of
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* level 0 is also 100, so we assume _BQC is indexed, when it's not.
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*
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* This causes all _BQC calls to return bogus values causing weird
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* behavior from the user's perspective. For example:
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*
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* xbacklight -set 10; xbacklight -set 20;
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*
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* would flash to 90% and then slowly down to the desired level (20).
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*
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* The solution is simple; test anything other than the first level
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* (e.g. 1).
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*/
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test_level = current_level == max_level
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? br->levels[ACPI_VIDEO_FIRST_LEVEL + 1]
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@ -789,6 +827,11 @@ int acpi_video_get_levels(struct acpi_device *device,
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goto out;
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}
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/*
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* Note that we have to reserve 2 extra items (ACPI_VIDEO_FIRST_LEVEL),
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* in order to account for buggy BIOS which don't export the first two
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* special levels (see below)
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*/
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br->levels = kmalloc((obj->package.count + ACPI_VIDEO_FIRST_LEVEL) *
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sizeof(*br->levels), GFP_KERNEL);
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if (!br->levels) {
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