drm/i915: respect the VBT minimum backlight brightness

Historically we've exposed the full backlight PWM duty cycle range to
the userspace, in the name of "mechanism, not policy". However, it turns
out there are both panels and board designs where there is a minimum
duty cycle that is required for proper operation. The minimum duty cycle
is available in the VBT.

The backlight class sysfs interface does not make any promises to the
userspace about the physical meaning of the range
0..max_brightness. Specifically there is no guarantee that 0 means off;
indeed for acpi_backlight 0 usually is not off, but the minimum
acceptable value.

Respect the minimum backlight, and expose the range acceptable to the
hardware as 0..max_brightness to the userspace via the backlight class
device; 0 means the minimum acceptable enabled value. To switch off the
backlight, the user must disable the encoder.

As a side effect, make the backlight class device max brightness and
physical PWM modulation frequency (i.e. max duty cycle)
independent. This allows a follow-up patch to virtualize the max value
exposed to the userspace.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: s/BUG_ON/WARN_ON/]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 3 insertions, 2 deletions

diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 8fc68c783228..bb9042bde7dd 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -165,6 +165,7 @@ struct intel_panel {
 	struct {
 		bool present;
 		u32 level;
+		u32 min;
 		u32 max;
 		bool enabled;
 		bool combination_mode;	/* gen 2/4 only */
@@ -969,8 +970,8 @@ void intel_pch_panel_fitting(struct intel_crtc *crtc,
 void intel_gmch_panel_fitting(struct intel_crtc *crtc,
 			      struct intel_crtc_config *pipe_config,
 			      int fitting_mode);
-void intel_panel_set_backlight(struct intel_connector *connector, u32 level,
-			       u32 max);
+void intel_panel_set_backlight_acpi(struct intel_connector *connector,
+				    u32 level, u32 max);
 int intel_panel_setup_backlight(struct drm_connector *connector);
 void intel_panel_enable_backlight(struct intel_connector *connector);
 void intel_panel_disable_backlight(struct intel_connector *connector);