drm/i915: restore backlight precision when converting from opregion

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-and-tested-by Nico Schottelius <[email protected]>

Signed-off-by Aaron Lu <[email protected]>

---
 drivers/gpu/drm/i915/intel_panel.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

drivers/gpu/drm/i915/intel_panel.c +0 -0

index a953b081ee38..bdd2f24b7a6b 100644

drivers/gpu/drm/i915/intel_panel.c +0 -0

 	panel->backlight.level = level;
 	if (panel->backlight.device)

Signature · 1 line

1.9.0

64bit divisions won't compile on 32bit. You need one of the DO_DIV macros, or whatever they're called again. I pain, I know ;-)

Quoted 2 lines · Daniel Vetter

64bit divisions won't compile on 32bit. You need one of the DO_DIV macros, or whatever they're called again. I pain, I know ;-)

Thanks for the correction, here is an updated patch :-)

From: Aaron Lu <[email protected]>
Date: Mon, 28 Apr 2014 11:02:52 +0800
Subject: [PATCH v2] 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 <[email protected]>

Signed-off-by Aaron Lu <[email protected]>

---
v2: use do_div as reminded by Daniel.

 drivers/gpu/drm/i915/intel_panel.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

drivers/gpu/drm/i915/intel_panel.c +1 -0

index a953b081ee38..8725917a3d0d 100644
@@ -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;

Quoted 48 lines · Aaron Lu

Quoted 2 lines · Daniel Vetter

64bit divisions won't compile on 32bit. You need one of the DO_DIV macros, or whatever they're called again. I pain, I know ;-)

Thanks for the correction, here is an updated patch :-)

From: Aaron Lu <[email protected]>
Date: Mon, 28 Apr 2014 11:02:52 +0800
Subject: [PATCH v2] 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 <[email protected]>

Signed-off-by Aaron Lu <[email protected]>

---
v2: use do_div as reminded by Daniel.

 drivers/gpu/drm/i915/intel_panel.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

drivers/gpu/drm/i915/intel_panel.c +1 -0

index a953b081ee38..8725917a3d0d 100644
@@ -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;

drivers/gpu/drm/i915/intel_panel.c void intel_panel_set_backlight(struct intel_connector *connector, u32 level, · line 506

Quoted 8 lines

drivers/gpu/drm/i915/intel_panel.c +1 -4

@@ -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 = level * freq;

32b * 32b = 32b

n = (u64)level * freq; to avoid overflow as you claim.

Also this still has the same rounding error as before.

Quoted 1 line · Chris Wilson

Also this still has the same rounding error as before.

I didn't get this, care to explain?

Quoted 4 lines · Aaron Lu

Quoted 1 line · Chris Wilson

Also this still has the same rounding error as before.

I didn't get this, care to explain?

The calculation you use, truncates, rather than say round to nearest, would is the same discrepancy in your changelog.

32b * 32b = 32b

n = (u64)level * freq; to avoid overflow as you claim.

From a0f41a92d949c814c203672ff7efe219a90ca6df Mon Sep 17 00:00:00 2001

Quoted 3 lines · Chris Wilson

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 <[email protected]>
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 <[email protected]>

Signed-off-by Aaron Lu <[email protected]>

---
 drivers/gpu/drm/i915/intel_panel.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

drivers/gpu/drm/i915/intel_panel.c +4 -4

index 0eead16aeda7..cb8cfb7e0974 100644
@@ -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)

Signature · 6 lines

1.9.0

_______________________________________________
Intel-gfx mailing list
[email protected]
http://lists.freedesktop.org/mailman/listinfo/intel-gfx