[Intel-gfx] [PATCH] backlight: Don't read back backlight setting from kernel on DPMS off

[Hans de Goede]

We've several reports from users where the backlight comes up turned off
on starting X, when using video.use_native_backlight=1 (true dmi quirks, will
be the default for 3.16), in combination with having an external display
plugged in:

https://bugzilla.redhat.com/show_bug.cgi?id=1032978
https://bugzilla.redhat.com/show_bug.cgi?id=1103806

This seems to be caused by /sys/class/backlight/intel_backlight/brightness
reading back 0 when re-initializing the outputs. Unlike
/sys/class/backlight/acpi_video0/brightness which is used without the
video.use_native_backlight=1 param, which keeps returning the value from before

Here is an excerpt from Xorg.log when this happens:

[28225]: (II) intel(0): resizing framebuffer to 3286x1080
[28225]: (II) intel(0): switch to mode 1366x768 at 59.8 on pipe 0 using LVDS1, position (0, 0), rotation normal
[28225]: (II) intel(0): switch to mode 1920x1080 at 60.0 on pipe 0 using HDMI2, position (1366, 0), rotation normal
[28225]: (II) intel(0): DPMS off mode 3, saved backlight level 0
^^^ This is an extra debug line I added, mode == the mode parameter to
 xxx_output_dpms_backlight, saved backlight level is the value of
 backlight_active_level after the xxx_output_backlight_get call.

Note how backlight_active_level becomes 0 here.

[28225]: (II) intel(0): switch to mode 1366x768 at 59.8 on pipe 1 using LVDS1, position (0, 0), rotation normal
[28225]: (II) intel(0): DPMS on mode 0, setting backlight to 0

And here we restore the backlight to backlight_active_level which now is 0.

This commit fixes this by not reading back the backlight setting from the
kernel on DPMS off.

Signed-off-by: Hans de Goede <hdegoede at redhat.com>
---
 src/sna/sna_display.c   | 3 ---
 src/uxa/intel_display.c | 3 ---
 2 files changed, 6 deletions(-)

diff --git a/src/sna/sna_display.c b/src/sna/sna_display.c
index 13dbf64..c9d4b08 100644
--- a/src/sna/sna_display.c
+++ b/src/sna/sna_display.c
@@ -2323,9 +2323,6 @@ sna_output_dpms_backlight(xf86OutputPtr output, int oldmode, int mode)
 			sna_output_backlight_set(output,
 						   sna_output->backlight_active_level);
 	} else {
-		/* Only save the current backlight value if we're going from on to off. */
-		if (oldmode == DPMSModeOn)
-			sna_output->backlight_active_level = sna_output_backlight_get(output);
 		sna_output_backlight_set(output, 0);
 	}
 }
diff --git a/src/uxa/intel_display.c b/src/uxa/intel_display.c
index 95ddcc8..62902f4 100644
--- a/src/uxa/intel_display.c
+++ b/src/uxa/intel_display.c
@@ -963,9 +963,6 @@ intel_output_dpms_backlight(xf86OutputPtr output, int oldmode, int mode)
 			intel_output_backlight_set(output,
 						   intel_output->backlight_active_level);
 	} else {
-		/* Only save the current backlight value if we're going from on to off. */
-		if (oldmode == DPMSModeOn)
-			intel_output->backlight_active_level = intel_output_backlight_get(output);
 		intel_output_backlight_set(output, 0);
 	}
 }
-- 
2.0.0