[RFC v2 02/22] drm: Add Enhanced Gamma and color lut range attributes

Tue Nov 9 20:22:21 UTC 2021

On 2021-11-05 07:49, Ville Syrjälä wrote:
> On Thu, Nov 04, 2021 at 12:27:56PM -0400, Harry Wentland wrote:
>>
>>
>> On 2021-11-04 04:38, Pekka Paalanen wrote:
>>> On Wed, 3 Nov 2021 11:08:13 -0400
>>> Harry Wentland <harry.wentland at amd.com> wrote:
>>>
>>>> On 2021-09-06 17:38, Uma Shankar wrote:
>>>>> Existing LUT precision structure is having only 16 bit
>>>>> precision. This is not enough for upcoming enhanced hardwares
>>>>> and advance usecases like HDR processing. Hence added a new
>>>>> structure with 32 bit precision values.
>>>>>
>>>>> This also defines a new structure to define color lut ranges,
>>>>> along with related macro definitions and enums. This will help
>>>>> describe multi segmented lut ranges in the hardware.
>>>>>
>>>>> Signed-off-by: Uma Shankar <uma.shankar at intel.com>
>>>>> ---
>>>>>  include/uapi/drm/drm_mode.h | 58 +++++++++++++++++++++++++++++++++++++
>>>>>  1 file changed, 58 insertions(+)
>>>>>
>>>>> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
>>>>> index 90c55383f1ee..1079794c86c3 100644
>>>>> --- a/include/uapi/drm/drm_mode.h
>>>>> +++ b/include/uapi/drm/drm_mode.h
>>>>> @@ -903,6 +903,64 @@ struct hdr_output_metadata {
>>>>>  	};
>>>>>  };
>>>>>  
>>>>> +/*
>>>>> + * DRM_MODE_LUT_GAMMA|DRM_MODE_LUT_DEGAMMA is legal and means the LUT
>>>>> + * can be used for either purpose, but not simultaneously. To expose
>>>>> + * modes that support gamma and degamma simultaneously the gamma mode
>>>>> + * must declare distinct DRM_MODE_LUT_GAMMA and DRM_MODE_LUT_DEGAMMA
>>>>> + * ranges.
>>>>> + */
>>>>> +/* LUT is for gamma (after CTM) */
>>>>> +#define DRM_MODE_LUT_GAMMA BIT(0)
>>>>> +/* LUT is for degamma (before CTM) */
>>>>> +#define DRM_MODE_LUT_DEGAMMA BIT(1)
>>>>> +/* linearly interpolate between the points */
>>>>> +#define DRM_MODE_LUT_INTERPOLATE BIT(2)
>>>>> +/*
>>>>> + * the last value of the previous range is the
>>>>> + * first value of the current range.
>>>>> + */
>>>>> +#define DRM_MODE_LUT_REUSE_LAST BIT(3)
>>>>> +/* the curve must be non-decreasing */
>>>>> +#define DRM_MODE_LUT_NON_DECREASING BIT(4)
>>>>> +/* the curve is reflected across origin for negative inputs */
>>>>> +#define DRM_MODE_LUT_REFLECT_NEGATIVE BIT(5)
>>>>> +/* the same curve (red) is used for blue and green channels as well */
>>>>> +#define DRM_MODE_LUT_SINGLE_CHANNEL BIT(6)
>>>>> +
>>>>> +struct drm_color_lut_range {
>>>>> +	/* DRM_MODE_LUT_* */
>>>>> +	__u32 flags;
>>>>> +	/* number of points on the curve */
>>>>> +	__u16 count;
>>>>> +	/* input/output bits per component */
>>>>> +	__u8 input_bpc, output_bpc;
>>>>> +	/* input start/end values */
>>>>> +	__s32 start, end;
>>>>> +	/* output min/max values */
>>>>> +	__s32 min, max;
>>>>> +};
>>>>> +
>>>>> +enum lut_type {
>>>>> +	LUT_TYPE_DEGAMMA = 0,
>>>>> +	LUT_TYPE_GAMMA = 1,
>>>>> +};
>>>>> +
>>>>> +/*
>>>>> + * Creating 64 bit palette entries for better data
>>>>> + * precision. This will be required for HDR and
>>>>> + * similar color processing usecases.
>>>>> + */
>>>>> +struct drm_color_lut_ext {
>>>>> +	/*
>>>>> +	 * Data is U32.32 fixed point format.
>>>>> +	 */
>>>>> +	__u64 red;
>>>>> +	__u64 green;
>>>>> +	__u64 blue;
>>>>> +	__u64 reserved;
>>>>> +};  
>>>>
>>>> I've been drawing out examples of drm_color_lut_range defined PWLs
>>>> and understand a bit better what you and Ville are trying to accomplish
>>>> with it. It actually makes a lot of sense and would allow for a generic
>>>> way to populate different PWL definitions with a generic function.
>>>>
>>>> But I still have some key questions that either are not answered in these
>>>> patch sets or that I somehow overlooked.
>>>>
>>>> Can you explain how the U32.32 fixed point format relates to the input_bpc
>>>> and output_bpc in drm_color_lut_range, as we as to the pixel coming in from
>>>> the framebuffer.
>>>>
>>>> E.g. if we have ARGB2101010 what happens to a 0x3ff red value (assuming alpha
>>>> is non-multiplied)?
>>>>
>>>> If the drm_color_lut_range segments are defined with input_bpc of 24 bpc will
>>>> 0x3ff be zero-expanded to 24-bit? Is the 24 bpc an integer? I.e. would our 3xff
>>>> be interpreted as 0x3ff << (24-10)? 
>>>>
>>>> Assuming the output_bpc is 16 bpc and the programmed LUT makes this 1-to-1 would
>>>> the output value be 0x3ff << (16-10)?
>>>>
>>>> On AMD HW the pipe-internal format is a custom floating point format. We could
>>>> probably express that in terms of input/output_bpc and do the translation in our
>>>> driver between that and the internal floating point format, depending on the
>>>> framebuffer format, but there is the added complication of the magnitude of the
>>>> pixel data and correlating HDR with SDR planes.
>>>>
>>>> E.g. any SDR data would map from 0.0 to 1.0 floating point, while HDR content would
>>>> map from 0.0 to some value larger than 1.0. I don't (yet) have a clear picture how
>>>> to represent that with the drm_color_lut_range definition.
>>>
>>>
>>> Hi Harry,
>>>
>>> I think you just would not. Conceptually an SDR plane gets its very own
>>> LUT that converts the FB [0.0, 1.0] range to any appropriate [a >= 0.0,
>>> b <= 1.0] range in HDR. This is purely conceptual, in the terms of the
>>> abstract KMS color pipeline, where [0.0, 1.0] is always the full
>>> dynamic range at any point of the pipeline, meaning it is relative to
>>> its placement in the pipeline. If you want to use values >1.0 in hw,
>>> you can do so under the hood.
>>>
>>> At least that is how I would imagine things. With LUTs in general, I
>>> don't think I have ever seen LUT input domain being explicitly defined
>>> to something else than [0.0, 1.0] relative to the elements in the LUT
>>> where 0.0 maps exactly to the first element and 1.0 maps exactly to the
>>> last element.
>>>
>>> I'm of course open to other suggestions, but having values outside of
>>> [0.0, 1.0] range in the abstract pipeline will always raise the
>>> question: how do you feed those to the LUT next in the pipeline.
>>>
>>
>> AMD HW defines the LUT addressing in floating point space and allows
>> for addressing beyond 1.0. In fact on other OSes our driver uses
>> [0.0, 1.0] for SDR LUTs and [0.0, 128.0] for HDR LUTs.
>>
>> There are color spaces that extend beyond 1.0 and even into the negative
>> range: https://en.wikipedia.org/wiki/ScRGB>>>
>> I don't think we should define the LUT to be limited to [0.0, 1.0].
> 
> That is not the intention, or at least wasn't my intention when
> originally I proposed this gamma mode stuff. I specifically wanted
> support for extended values. So 0.0-1.0 is supposed to be just the
> range for the in gamut values.
> 

Make sense and good to hear.

>>
>> If the framebuffer is not in FP16 the question then becomes how
>> the integer pixel values relate to LUT addressing.
> 
> The idea again is that 0.0-1.0 is the range for the in gamut
> values. IIRC our hw does have the possibility of scaling all
> the fp16 input values by some programmable constant factor,
> but exposing that would require yet another uapi addition.
> 
>>
>> As well, LUT entries are defined to be U32.32 fixed point, also
>> allowing for entries much greater than 1.0. If those are programmed
>> as entries in the input (degamma) LUT how will they be used to address
>> entries in the output (gamma) LUT?
> 
> The u32.32 is a mistake I think. IMO we should be going for signed
> values everywhere. Though our hw does not actually let us directly
> program negative values for the LUT, rather the hw just reflects
> the whole curve across the origin so the lookup is basically just
> something like:
> output = input < 0 ? -lut[abs(input)] : lut[input];
> 
> That is why there is that proposed DRM_MODE_LUT_REFLECT_NEGATIVE flag.
> 

That's pretty much how I expect AMD HW to operate as well.

Harry

> I think nouveau had something funny in its lut programming that
> made me think that it might actually have straight up programmable
> LUT entries for negative inputs as well. But I'm not sure if anyone
> has actually tested that.
> 
>>
>> Maybe we want to say the actual input values are being used to
>> address the LUT entries? But if we look at segment 1 of Uma's
>> d13_degamma_hdr definition we see that the range of 0 to
>> (1 << 24) -1 is covered by 128 (1 << 7) entries, so the range
>> of 0 to 256 (for RGB with 8 bpc) would only be covered by 2
>> LUT entries. So this interpretation doesn't make sense.
>>
>> You can see, I'm still confused by these definitions. An IGT
>> test would help explain the API intentions a bit better, though
>> I would like to see more precise documentation.
>>
>> Despite my confusion I think the segmented PWL definitions are
>> a neat way to concisely describe the HW capabilities to
>> userspace and a concise way for userspace to provide the LUT
>> more precisely than with a uniformly spaced LUT.
>>
>> Harry
>>
>>> Yeah, I have no idea what it should mean if an FB has a format that
>>> allows values beyond [0.0, 1.0].
>>>
>>>
>>> Thanks,
>>> pq
>>>
>>>
>>>> If some of these questions should be obvious I apologize for being a bit dense,
>>>> though it helps to make this accessible to the lowest common denominator
>>>> to ensure not only the smartest devs can work with this.
>>>>
>>>> Harry
>>>>
>>>>> +
>>>>>  #define DRM_MODE_PAGE_FLIP_EVENT 0x01
>>>>>  #define DRM_MODE_PAGE_FLIP_ASYNC 0x02
>>>>>  #define DRM_MODE_PAGE_FLIP_TARGET_ABSOLUTE 0x4
>>>>>   
>>>>
>>>
> 