The comment for acpi_video_bqc_quirk is by Felipe Contreras, taken from
the git history.
Signed-off-by: Dmitry Frank <mail@xxxxxxxxxxxxxxx>
---
drivers/acpi/acpi_video.c | 43 +++++++++++++++++++++++++++++++++++++++++--
1 file changed, 41 insertions(+), 2 deletions(-)
diff --git a/drivers/acpi/acpi_video.c b/drivers/acpi/acpi_video.c
index cfd3bed57bed..7fd16efa84c8 100644
--- a/drivers/acpi/acpi_video.c
+++ b/drivers/acpi/acpi_video.c
@@ -148,7 +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 force_device_id_scheme)
+ */
+ u32 device_id_scheme:1;
};
struct acpi_video_enumerated_device {
@@ -732,7 +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]
@@ -793,6 +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) {
--
2.11.0
--
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