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

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

[Aaron Lu]

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 <nico-bugzilla.kernel.org@schottelius.org>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
---
 drivers/gpu/drm/i915/intel_panel.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index a953b081ee38..bdd2f24b7a6b 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -502,10 +502,7 @@ 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;
+	level = (u64)level * freq / max;
 
 	panel->backlight.level = level;
 	if (panel->backlight.device)
-- 
1.9.0

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

[Daniel Vetter]

On Mon, Apr 28, 2014 at 11:19:29AM +0800, Aaron Lu wrote:
> 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 <nico-bugzilla.kernel.org@schottelius.org>
> Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_panel.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index a953b081ee38..bdd2f24b7a6b 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -502,10 +502,7 @@ 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;
> +	level = (u64)level * freq / max;

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 ;-)
-Daniel

>  
>  	panel->backlight.level = level;
>  	if (panel->backlight.device)
> -- 
> 1.9.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

[PATCH v2] drm/i915: restore backlight precision when converting from opregion

[Aaron Lu]

On 04/28/2014 09:41 PM, Daniel Vetter wrote:
> 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 <aaron.lu@intel.com>
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 <nico-bugzilla.kernel.org@schottelius.org>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
---
v2: use do_div as reminded by Daniel.

 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 a953b081ee38..8725917a3d0d 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 = level * freq;
+	do_div(n, max);
+	level = n;
 
 	panel->backlight.level = level;
 	if (panel->backlight.device)
-- 
1.9.0

Re: [PATCH v2] drm/i915: restore backlight precision when converting from opregion

[Chris Wilson]

On Sun, May 04, 2014 at 03:16:05PM +0800, Aaron Lu wrote:
> On 04/28/2014 09:41 PM, Daniel Vetter wrote:
> > 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 <aaron.lu@intel.com>
> 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 <nico-bugzilla.kernel.org@schottelius.org>
> Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> ---
> v2: use do_div as reminded by Daniel.
> 
>  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 a953b081ee38..8725917a3d0d 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 = 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.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

Re: [PATCH v2] drm/i915: restore backlight precision when converting from opregion

[Aaron Lu]

On 05/04/2014 03:22 PM, Chris Wilson wrote:
> On Sun, May 04, 2014 at 03:16:05PM +0800, Aaron Lu wrote:
>> On 04/28/2014 09:41 PM, Daniel Vetter wrote:
>>> 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 <aaron.lu@intel.com>
>> 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 <nico-bugzilla.kernel.org@schottelius.org>
>> Signed-off-by: Aaron Lu <aaron.lu@intel.com>
>> ---
>> v2: use do_div as reminded by Daniel.
>>
>>  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 a953b081ee38..8725917a3d0d 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 = level * freq;
> 
> 32b * 32b = 32b
> 
> n = (u64)level * freq; to avoid overflow as you claim.

Ah...yes, my fault, thanks.

> 
> Also this still has the same rounding error as before.

I didn't get this, care to explain?

Re: [PATCH v2] drm/i915: restore backlight precision when converting from opregion

[Chris Wilson]

On Sun, May 04, 2014 at 03:31:01PM +0800, Aaron Lu wrote:
> On 05/04/2014 03:22 PM, Chris Wilson wrote:
> > 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.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

Re: [Intel-gfx] [PATCH v2] drm/i915: restore backlight precision when converting from opregion

[Aaron Lu]

On 05/04/2014 06:41 PM, Chris Wilson wrote:
> On Sun, May 04, 2014 at 03:31:01PM +0800, Aaron Lu wrote:
>> On 05/04/2014 03:22 PM, Chris Wilson wrote:
>>> 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.

This patch is meant to fix the problem that when user specify the max
backlight level in acpi_video interface, we will get the hardware max
level too in i915. With this patch, the hardware level we will get is:
level * freq / max = 255 * 937 / 255 = 937, which is correct.

For other values, the truncates is OK:
254 * 937 / 255 = 933
253 * 937 / 255 = 929
... ...

[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@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@schottelius.org>
Signed-off-by: Aaron Lu <aaron.lu@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

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

[Jani Nikula]

On Mon, 12 May 2014, Aaron Lu <aaron.lu@intel.com> wrote:
> 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!

Pushed to -fixes with Chris' IRC r-b, thanks for the patch and review.

BR,
Jani.



>
>
> From a0f41a92d949c814c203672ff7efe219a90ca6df Mon Sep 17 00:00:00 2001
> From: Aaron Lu <aaron.lu@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@schottelius.org>
> Signed-off-by: Aaron Lu <aaron.lu@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
>
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Jani Nikula, Intel Open Source Technology Center