From e1c412e75754ab7b7002f3e18a2652d999c40d4b Mon Sep 17 00:00:00 2001 From: Jani Nikula Date: Wed, 5 Nov 2014 14:46:31 +0200 Subject: [PATCH] drm/i915: safeguard against too high minimum brightness Never trust (your interpretation of) the VBT. Regression from commit 6dda730e55f412a6dfb181cae6784822ba463847 Author: Jani Nikula Date: Tue Jun 24 18:27:40 2014 +0300 drm/i915: respect the VBT minimum backlight brightness causing div by zero if VBT minimum brightness equals maximum brightness. Despite my attempts I've failed in my detective work to figure out what the root cause is. This is not the real fix, but we have to do something. Reported-by: Mike Auty Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=86551 Cc: stable@vger.kernel.org (v3.17+) Reviewed-by: Daniel Vetter Signed-off-by: Jani Nikula --- drivers/gpu/drm/i915/intel_panel.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-) diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c index 0e018cb49147..41b3be217493 100644 --- a/drivers/gpu/drm/i915/intel_panel.c +++ b/drivers/gpu/drm/i915/intel_panel.c @@ -1098,12 +1098,25 @@ static u32 get_backlight_min_vbt(struct intel_connector *connector) struct drm_device *dev = connector->base.dev; struct drm_i915_private *dev_priv = dev->dev_private; struct intel_panel *panel = &connector->panel; + int min; WARN_ON(panel->backlight.max == 0); + /* + * XXX: If the vbt value is 255, it makes min equal to max, which leads + * to problems. There are such machines out there. Either our + * interpretation is wrong or the vbt has bogus data. Or both. Safeguard + * against this by letting the minimum be at most (arbitrarily chosen) + * 25% of the max. + */ + min = clamp_t(int, dev_priv->vbt.backlight.min_brightness, 0, 64); + if (min != dev_priv->vbt.backlight.min_brightness) { + DRM_DEBUG_KMS("clamping VBT min backlight %d/255 to %d/255\n", + dev_priv->vbt.backlight.min_brightness, min); + } + /* vbt value is a coefficient in range [0..255] */ - return scale(dev_priv->vbt.backlight.min_brightness, 0, 255, - 0, panel->backlight.max); + return scale(min, 0, 255, 0, panel->backlight.max); } static int bdw_setup_backlight(struct intel_connector *connector) -- 2.20.1