[PATCH v3] drm/i915: restore backlight precision when converting from opregion

[Aaron Lu]

On 05/04/2014 03:22 PM, Chris Wilson wrote:
> 32b * 32b = 32b
> 
> n = (u64)level * freq; to avoid overflow as you claim.

Updated patch to fix this problem is here, thanks!


>From a0f41a92d949c814c203672ff7efe219a90ca6df Mon Sep 17 00:00:00 2001
From: Aaron Lu <aaron.lu at intel.com>
Date: Mon, 28 Apr 2014 11:02:52 +0800
Subject: [PATCH] drm/i915: restore backlight precision when converting from
 ACPI

When we set backlight on behalf of ACPI opregion, we will convert the
backlight value in the 0-255 range defined in opregion to the actual
hardware level. Commit 22505b82a2 (drm/i915: avoid brightness overflow
when doing scale) is meant to fix the overflow problem when doing the
conversion, but it also caused a problem that the converted hardware
level doesn't quite represent the intended value: say user wants maximum
backlight level(255 in opregion's range), then we will calculate the
actual hardware level to be: level = freq / max * level, where freq is
the hardware's max backlight level(937 on an user's box), and max and
level are all 255. The converted value should be 937 but the above
calculation will yield 765.

To fix this issue, just use 64 bits to do the calculation to keep the
precision and avoid overflow at the same time.

Buglink: https://bugzilla.kernel.org/show_bug.cgi?id=72491
Reported-by: Nico Schottelius <nico-bugzilla.kernel.org at schottelius.org>
Signed-off-by: Aaron Lu <aaron.lu at intel.com>
---
 drivers/gpu/drm/i915/intel_panel.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index 0eead16aeda7..cb8cfb7e0974 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -492,6 +492,7 @@ void intel_panel_set_backlight(struct intel_connector *connector, u32 level,
 	enum pipe pipe = intel_get_pipe_from_connector(connector);
 	u32 freq;
 	unsigned long flags;
+	u64 n;
 
 	if (!panel->backlight.present || pipe == INVALID_PIPE)
 		return;
@@ -502,10 +503,9 @@ void intel_panel_set_backlight(struct intel_connector *connector, u32 level,
 
 	/* scale to hardware max, but be careful to not overflow */
 	freq = panel->backlight.max;
-	if (freq < max)
-		level = level * freq / max;
-	else
-		level = freq / max * level;
+	n = (u64)level * freq;
+	do_div(n, max);
+	level = n;
 
 	panel->backlight.level = level;
 	if (panel->backlight.device)
-- 
1.9.0