On Thu, 06 Nov 2014, Arnd Hannemann arnd@arndnet.de wrote:
Hi,
thanks for your quick response.
Am 06.11.2014 um 10:39 schrieb Jani Nikula:
On Thu, 06 Nov 2014, Arnd Hannemann arnd@arndnet.de wrote:
Hi,
I have a Thinkpad T440s (Haswell) connected to two additional Monitors via a Docking Station (MST).
During Bootup all three displays work, even when X is started. However, if the laptop display is turned off once (either because of power saving, or via xrandr), it fails to "come back". That is if I try to re-enable it the Display stays blank. I believe this used to work in 3.17.
Here is the xrandr Ouput of the edp, when its enabled (but staying blank): Screen 0: minimum 8 x 8, current 3840 x 1200, maximum 32767 x 32767 eDP1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 309mm x 175mm 1920x1080 60.0*+ 59.9
here is the debug output, while trying to enable it:
...
[ 416.538848] [drm:intel_edp_backlight_power] panel power control backlight disable
I'm happy to provide further input.
What does cat /sys/class/backlight/intel_backlight/bl_power say? What if
root@kallisto:~# cat /sys/class/backlight/intel_backlight/bl_power 1
you echo 0 there?
:-) Works my display comes back, when I echo 0 there.
Is user-space doing something wrong here?
If the userspace wishes to switch off backlight, then it's doing nothing wrong at all! ;)
Here's the story as I know it.
Once upon a time someone added the bl_power attribute to the sysfs class backlight interface. Even though the name implies a boolean backlight power, the values are in fact FB_BLANK_* from fb.h, and power on is FB_BLANK_UNBLANK, or 0. All the other values are various levels of blanking which make little sense to backlight, and thus any non-zero values mean power off. [1]
Until recently, intel_backlight of drm/i915 did not support bl_power at all. We ignored the attribute altogether. However changing bl_power from its default 0 did cause a backlight update hook to be called. In some edge cases doing this fixed some backlight issues by reprogramming the backlight intensity, and probably lead to the false assumption that bl_power needed to be set to 1 to enable power.
Now that we've enabled support for bl_power attribute (on eDP at least), the previously harmless, or sometimes even helpful, bl_power=1 actually does what it means. That is, switch off the backlight.
Please try this patch (untested) to find out the culprit.
diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c index bddc8b17a4d8..8cf5c6cdeef5 100644 --- a/drivers/video/backlight/backlight.c +++ b/drivers/video/backlight/backlight.c @@ -143,6 +143,7 @@ static ssize_t bl_power_store(struct device *dev, struct device_attribute *attr, rc = -ENXIO; mutex_lock(&bd->ops_lock); if (bd->ops) { + printk(KERN_INFO "bl_power %lu by %s\n", power, current->comm); pr_debug("set power to %lu\n", power); if (bd->props.power != power) { bd->props.power = power;
BR, Jani.
[1] http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Document... ("Read the documentation and you'll get it right" does not score well in Rusty's API design manifesto)