Re: [PATCH] drm/i915: intel_backlight scale() math WA v2

[Jani Nikula <jani.nikula@linux.intel.com>]

On Mon, 10 Nov 2014, Jani Nikula <jani.nikula@linux.intel.com> wrote:
> On Sat, 08 Nov 2014, "Eoff, Ullysses A" <ullysses.a.eoff@intel.com> wrote:
>> On 09/24/2014 10:42 AM, Eoff, Ullysses A wrote:
>>>> -----Original Message-----
>>>> From: Intel-gfx [mailto:intel-gfx-bounces@lists.freedesktop.org] On Behalf Of Jani Nikula
>>>> Sent: Wednesday, September 24, 2014 10:08 AM
>>>> To: Hans de Goede; Joe Konno; intel-gfx@lists.freedesktop.org
>>>> Subject: Re: [Intel-gfx] [PATCH] drm/i915: intel_backlight scale() math WA v2
>>>>
>>>> On Wed, 24 Sep 2014, Hans de Goede <hdegoede@redhat.com> wrote:
>>>>> Hi,
>>>>>
>>>>> On 09/24/2014 05:54 PM, Joe Konno wrote:
>>>>>> From: Joe Konno <joe.konno@intel.com>
>>>>>>
>>>>>> Improper truncated integer division 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
>>>>>>
>>>>>> v2: (thanks to Chris Wilson) clarify commit message, use rounded division
>>>>>> macro
>>>>> I wonder why do scaling at all, why not simply shift hw_min - hw_max range
>>>>> to 0 - (hw_max - hw_min) range and set max_brightness as seen by userspace
>>>>> to (hw_max - hw_min) ?
>>>> Mostly in preparation for a future where we expose an arbitrary range,
>>>> say 0..100 or 0..255 to the userspace.
>>>>
>>> The problem with this scaling method is that scaling from user level to hw level and
>>> back to user level is ambiguous since there isn't a 1:1 mapping between the user
>>> range and the hw range.
>>>
>>> On the other hand, this patch does fix a bug in the currently used method (scaling).
>>> That, at least, is an improvement nonetheless.
>>>
>>> U. Artie
>> Apologies for resurrecting an old thread.  But I think we still need to
>> address
>> this issue about not having a 1:1 mapping between user and hw levels.
>>
>> Right now, the problem is that the user range is larger than the hw
>> range which
>> results in one or more user levels mapping to the same hw level.  And when
>> userspace requests one of those levels, the result that is reported back to
>> userspace might not be the same as what was requested.  Take for example, on
>> my system the hw range is [398, 7812] and the user range is [0, 7812]. 
>> Suppose
>> userspace requests level 7017.  This maps to hw level 7058.  And when
>> userspace requests the current level, 7018 is reported back (+1 from what
>> was originally requested).  In fact, with these particular ranges, there
>> are exactly
>> 398 values that this occurs.
>>
>> This problem will be compounded the larger the difference in length of the
>> discrete ranges; so long as user range > hw range.
>>
>> Hans' solution would fix this problem, giving 1:1 mapping from hw to user
>> levels.
>>
>> Jani's [future] solution would work too, since exposing a smaller range to
>> userspace than the hw range would isolate the non 1:1 mapping inside the
>> driver.
>
> I think we should just pick an arbitrary range, say 0..100, and be done
> with it. It's not like you'd be able to get much more than 100 distinct
> brightness levels out of the backlight anyway, no matter what the PWM
> settings.
>
> BR,
> Jani.

PS. This (totally untested) patch should do it:

diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index b001c90312e7..a6680081415b 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -1035,7 +1035,7 @@ static int intel_backlight_device_register(struct intel_connector *connector)
 	 * Note: Everything should work even if the backlight device max
 	 * presented to the userspace is arbitrarily chosen.
 	 */
-	props.max_brightness = panel->backlight.max;
+	props.max_brightness = 100;
 	props.brightness = scale_hw_to_user(connector,
 					    panel->backlight.level,
 					    props.max_brightness);

>
>
>
>
>>
>> U. Artie
>>>> BR,
>>>> Jani.
>>>>
>>>>
>>>>> Regards,
>>>>>
>>>>> Hans
>>>>>
>>>>>
>>>>>> Signed-off-by: Joe Konno <joe.konno@intel.com>
>>>>>> ---
>>>>>>  drivers/gpu/drm/i915/intel_panel.c | 2 +-
>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
>>>>>> index f17ada3..dcdfbb3 100644
>>>>>> --- a/drivers/gpu/drm/i915/intel_panel.c
>>>>>> +++ b/drivers/gpu/drm/i915/intel_panel.c
>>>>>> @@ -421,7 +421,7 @@ 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 = DIV_ROUND_CLOSEST(target_val, source_max - source_min);
>>>>>>  	target_val += target_min;
>>>>>>
>>>>>>  	return target_val;
>>>>>>
>>>>> _______________________________________________
>>>>> Intel-gfx mailing list
>>>>> Intel-gfx@lists.freedesktop.org
>>>>> http://lists.freedesktop.org/mailman/listinfo/intel-gfx
>>>> --
>>>> Jani Nikula, Intel Open Source Technology Center
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>>>
>>
>
> -- 
> Jani Nikula, Intel Open Source Technology Center
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Jani Nikula, Intel Open Source Technology Center
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