[PATCH] drm/i915: intel_backlight scale() math WA

[PATCH] drm/i915: intel_backlight scale() math WA

[Joe Konno]

From: Joe Konno <joe.konno@intel.com>

Improper integer division-- truncated rather than rounded-- in the
scale() function causes actual_brightness != brightness. This (partial)
work-around should be sufficient for a majority of use-cases, but it is
by no means a complete solution.

TODO: Determine how best to scale "user" values to "hw" values, and
vice-versa, when the ranges are of different sizes. That would be a
buggy scenario even with this work-around.

The issue was introduced in the following (v3.17-rc1) commit:

    6dda730 drm/i915: respect the VBT minimum backlight brightness

Signed-off-by: Joe Konno <joe.konno@intel.com>
---
 drivers/gpu/drm/i915/intel_panel.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index f17ada3..c40f837 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -421,7 +421,8 @@ static uint32_t scale(uint32_t source_val,
 	/* avoid overflows */
 	target_val = (uint64_t)(source_val - source_min) *
 		(target_max - target_min);
-	do_div(target_val, source_max - source_min);
+	target_val = (target_val + ((source_max - source_min) / 2)) /
+		(source_max - source_min);
 	target_val += target_min;
 
 	return target_val;
-- 
2.1.0

Re: [PATCH] drm/i915: intel_backlight scale() math WA

[Eoff, Ullysses A]

On Tue, 2014-09-23 at 15:50 -0700, Joe Konno wrote:
> From: Joe Konno <joe.konno@intel.com>
> 
> Improper integer division-- truncated rather than rounded-- in the
> scale() function causes actual_brightness != brightness. This (partial)
> work-around should be sufficient for a majority of use-cases, but it is
> by no means a complete solution.
> 
> TODO: Determine how best to scale "user" values to "hw" values, and
> vice-versa, when the ranges are of different sizes. That would be a
> buggy scenario even with this work-around.
> 
> The issue was introduced in the following (v3.17-rc1) commit:
> 
>     6dda730 drm/i915: respect the VBT minimum backlight brightness
> 

Reviewed-by: U. Artie Eoff <ullysses.a.eoff@intel.com>

> Signed-off-by: Joe Konno <joe.konno@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_panel.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index f17ada3..c40f837 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -421,7 +421,8 @@ static uint32_t scale(uint32_t source_val,
>  	/* avoid overflows */
>  	target_val = (uint64_t)(source_val - source_min) *
>  		(target_max - target_min);
> -	do_div(target_val, source_max - source_min);
> +	target_val = (target_val + ((source_max - source_min) / 2)) /
> +		(source_max - source_min);
>  	target_val += target_min;
>  
>  	return target_val;

Re: [PATCH] drm/i915: intel_backlight scale() math WA

[Chris Wilson]

On Tue, Sep 23, 2014 at 03:50:25PM -0700, Joe Konno wrote:
> From: Joe Konno <joe.konno@intel.com>
> 
> Improper integer division-- truncated rather than rounded-- in the

That's confusing. It reads that you are converting to using truncated,
rather than to rounding.

> scale() function causes actual_brightness != brightness. This (partial)
> work-around should be sufficient for a majority of use-cases, but it is
> by no means a complete solution.
> 
> TODO: Determine how best to scale "user" values to "hw" values, and
> vice-versa, when the ranges are of different sizes. That would be a
> buggy scenario even with this work-around.
> 
> The issue was introduced in the following (v3.17-rc1) commit:
> 
>     6dda730 drm/i915: respect the VBT minimum backlight brightness
> 
> Signed-off-by: Joe Konno <joe.konno@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_panel.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index f17ada3..c40f837 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -421,7 +421,8 @@ static uint32_t scale(uint32_t source_val,
>  	/* avoid overflows */
>  	target_val = (uint64_t)(source_val - source_min) *
>  		(target_max - target_min);
> -	do_div(target_val, source_max - source_min);
> +	target_val = (target_val + ((source_max - source_min) / 2)) /
> +		(source_max - source_min);

target_val = DIV_ROUND_CLOSEST(target_val, source_max - source_min);

>  	target_val += target_min;
>  
>  	return target_val;

-- 
Chris Wilson, Intel Open Source Technology Centre

Re: [PATCH] drm/i915: intel_backlight scale() math WA

[Joe Konno]

Many thanks, Chris. I'll incorporate your feedback into v2.

On 09/23/2014 11:04 PM, Chris Wilson wrote:
> On Tue, Sep 23, 2014 at 03:50:25PM -0700, Joe Konno wrote:
>> From: Joe Konno <joe.konno@intel.com>
>>
>> Improper integer division-- truncated rather than rounded-- in the
> That's confusing. It reads that you are converting to using truncated,
> rather than to rounding.
>
>> scale() function causes actual_brightness != brightness. This (partial)
>> work-around should be sufficient for a majority of use-cases, but it is
>> by no means a complete solution.
>>
>> TODO: Determine how best to scale "user" values to "hw" values, and
>> vice-versa, when the ranges are of different sizes. That would be a
>> buggy scenario even with this work-around.
>>
>> The issue was introduced in the following (v3.17-rc1) commit:
>>
>>     6dda730 drm/i915: respect the VBT minimum backlight brightness
>>
>> Signed-off-by: Joe Konno <joe.konno@intel.com>
>> ---
>>  drivers/gpu/drm/i915/intel_panel.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
>> index f17ada3..c40f837 100644
>> --- a/drivers/gpu/drm/i915/intel_panel.c
>> +++ b/drivers/gpu/drm/i915/intel_panel.c
>> @@ -421,7 +421,8 @@ static uint32_t scale(uint32_t source_val,
>>  	/* avoid overflows */
>>  	target_val = (uint64_t)(source_val - source_min) *
>>  		(target_max - target_min);
>> -	do_div(target_val, source_max - source_min);
>> +	target_val = (target_val + ((source_max - source_min) / 2)) /
>> +		(source_max - source_min);
> target_val = DIV_ROUND_CLOSEST(target_val, source_max - source_min);
>
>>  	target_val += target_min;
>>  
>>  	return target_val;