Re: [PATCH 01/10] drm/colorop: Add DRM_COLOROP_CSC_FF

Harry Wentland
2026-03-30T15:37:58Z

On 2026-03-16 13:59, Harry Wentland wrote:
>
>
> On 2026-03-16 12:03, Pekka Paalanen wrote:
>> On Mon, 16 Mar 2026 10:36:44 -0400
>> Harry Wentland <harry.wentland@amd.com> wrote:
>>
>>> On 2026-03-16 07:53, Pekka Paalanen wrote:
>>>> On Mon, 16 Mar 2026 16:04:32 +0530
>>>> "Borah, Chaitanya Kumar" <chaitanya.kumar.borah@intel.com> wrote:
>>>>
>>>>> On 3/16/2026 2:27 PM, Pekka Paalanen wrote:
>>>>>> On Mon, 16 Mar 2026 12:46:39 +0530
>>>>>> "Borah, Chaitanya Kumar" <chaitanya.kumar.borah@intel.com> wrote:
>>>>>>
>>>>>>> Hi Pekka,
>>>>>>>
>>>>>>> Thank you for looking into the patch.
>>>>>>
>>>>>> Hi Chaitanya!
>>>>>>
>>>>>> Replies inline below.
>>>>>>
>>>>>>>
>>>>>>> On 3/10/2026 8:02 PM, Pekka Paalanen wrote:
>>>>>>>> On Fri, 6 Mar 2026 22:22:58 +0530
>>>>>>>> Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com> wrote:
>>>>>>>>
>>>>>>>>> Introduce DRM_COLOROP_CSC_FF, a new colorop type representing a
>>>>>>>>> fixed-function Color Space Conversion (CSC) block.
>>>>>>>>>
>>>>>>>>> Unlike CTM-based colorops, this block does not expose programmable
>>>>>>>>> coefficients. Instead, userspace selects one of the predefined
>>>>>>>>> hardware modes via a new CSC_FF_TYPE enum property. Supported modes
>>>>>>>>> include common YUV->RGB and RGB709->RGB2020 conversions.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
>>>>
>>>> ...
>>>>
>>>>>>>>> diff --git a/drivers/gpu/drm/drm_colorop.c b/drivers/gpu/drm/drm_colorop.c
>>>>>>>>> index f421c623b3f0..49422c625f4d 100644
>>>>>>>>> --- a/drivers/gpu/drm/drm_colorop.c
>>>>>>>>> +++ b/drivers/gpu/drm/drm_colorop.c
>>>>>>>>> @@ -68,6 +68,7 @@ static const struct drm_prop_enum_list drm_colorop_type_enum_list[] = {
>>>>>>>>> { DRM_COLOROP_CTM_3X4, "3x4 Matrix"},
>>>>>>>>> { DRM_COLOROP_MULTIPLIER, "Multiplier"},
>>>>>>>>> { DRM_COLOROP_3D_LUT, "3D LUT"},
>>>>>>>>> + { DRM_COLOROP_CSC_FF, "CSC Fixed-Function"},
>>>>>>>>
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> the fundamental idea seems fine to me, but I have a lot to say about the
>>>>>>>> nomenclature.
>>>>>>>>
>>>>>>>> What would you think of a more readable name DRM_COLOROP_FIXED_MATRIX
>>>>>>>> "Fixed Matrix"?
>>>>>>>>
>>>>>>>> Alternatively DRM_COLOROP_ENUM_MATRIX "Enumerated Matrix".
>>>>>>>>
>>>>>>>
>>>>>>> I was intentionally staying away from the word matrix because there was
>>>>>>> no programmable matrix but it would make sense to name it something like
>>>>>>> DRM_COLOROP_FIXED_MATRIX (or *_PRESET_MATRIX for that matter).
>>>>>>>
>>>>>>>>> };
>>>>>>>>>
>>>>>>>>> static const char * const colorop_curve_1d_type_names[] = {
>>>>>>>>> @@ -90,6 +91,13 @@ static const struct drm_prop_enum_list drm_colorop_lut3d_interpolation_list[] =
>>>>>>>>> { DRM_COLOROP_LUT3D_INTERPOLATION_TETRAHEDRAL, "Tetrahedral" },
>>>>>>>>> };
>>>>>>>>>
>>>>>>>>> +static const char * const colorop_csc_ff_type_names[] = {
>>>>>>>>> + [DRM_COLOROP_CSC_FF_YUV601_RGB601] = "YUV601 to RGB601",
>>>>>>>>> + [DRM_COLOROP_CSC_FF_YUV709_RGB709] = "YUV709 to RGB709",
>>>>>>>>> + [DRM_COLOROP_CSC_FF_YUV2020_RGB2020] = "YUV2020 to RGB2020",
>>>>>>>>> + [DRM_COLOROP_CSC_FF_RGB709_RGB2020] = "RGB709 to RGB2020",
>>>>>>>>
>>>>>>>> I'd suggest names:
>>>>>>>>
>>>>>>>> "YCbCr 601 to RGB"
>>>>>>>> "YCbCr 709 to RGB"
>>>>>>>> "YCbCr 2020 NC to RGB"
>>>>>>>> "RGB709 to RGB2020"
>>>>>>>>
>>>>>>>> or something in that direction.
>>>>>>>>
>>>>>>>> The relevant ITU-R BT specifications use YCbCr nomenclature IIRC. Wrt.
>>>>>>>> YCbCr-to-RGB conversion, there is no RGB601, RGB709 or RGB2020. There
>>>>>>>> is only some RGB, and which primaries it uses is not always tied to
>>>>>>>> which YCbCr conversion was used.
>>>>>>>>
>>>>>>>
>>>>>>> What I understand from this is that the BT.709(et al.) only defines the
>>>>>>> matrix that is used for YCbCr->RGB, "what" RGB it is defined by the
>>>>>>> primaries (which comes with metadata?).
>>>>>>
>>>>>> Unfortunately, BT.601, BT.709 and BT.2020 define two separate things each:
>>>>>> - the YCbCr<->RGB conversion, and
>>>>>> - the colorspace primaries (and white point, but that is the same for
>>>>>> them all).
>>>>>>
>>>
>>> Would it make sense to treat these as two separate things in terms
>>> of colorops?
>>
>> Hi Harry,
>>
>> no, if your hardware not care. There are no semantics for the numbers
>> in the UAPI, it's just whatever numbers, and mathematical operations on
>> them.
>>
>> I suspect that your hardware does care, though, and actually has
>> separate hardware elements for the YCbCr conversion and the colorspace
>> conversion matrix. After all, one has to be able to put a LUT or a
>> curve between the two to make sense.
>>
>> IOW, two different colorops, yes. But different colorop types? Maybe
>> that depends on whether they would have the same colorop properties or
>> not.
>>
>
> If I understand you correctly you're saying we can have a single
> colorop type to represent either type. A client of the API needs
> to understand what it's doing with the colorop and can use it
> as either a YCbCr conversion matrix in the case of YCbCr-to-RGB
> conversion, or an NPM for conversion of linear, normalized data
> from one set of primaries to another.
>
> Is that understanding correct?
>
>>> I have done some work on a CSC colorop and intend to send out the
>>> patches in the next couple of days.
>>>
>>> https://gitlab.freedesktop.org/hwentland/linux/-/commits/csc-colorop
>>>
>>> It follows the drm_plane's COLOR_RANGE and COLOR_ENCODING semantic
>>> and is only intended for YCbCr-to-RGB conversion, like the original
>>> properties on the plane.
>>>
>>> For the colorspace conversion within RGB (e.g., BT709 to BT2020)
>>> it might make sense then to have its own colorop if HW works on
>>> pre-defined transformations, or use the CTM 3x3 or 3x4 matrix ops
>>> if HW provides a flexible matrix.
>>>
>>> We might need to think about naming, since colorspace conversion (CSC)
>>> right now seems to refer to both YCbCr conversion and primaries
>>> conversion.
>>
>> Indeed.
>>
>> YCbCr conversion is usually a matrix operation. H.273 calls it
>> MatrixCoefficients, but it also lists cases where you need the EOTF in
>> the mix. I could go with "YCbCr coefficients". The pure matrix forms
>> are used on electrical pixel values.
>>
>> The color space conversion matrices that are based on (Normalized)
>> Primary Matrices (NPM) must be used on optical pixel values. NPM is the
>> matrix that converts optical RGB values to CIE 1931 XYZ. For an
>> RGB-to-RGB conversion you need one NPM and another inverse NPM chained.
>>
>
> This could be expressed in a single matrix, right?
>
> I intend to send out my patches as an RFC either way, but I think I
> could just as well work with the CSC_FF colorop. I'll have a look at
> basing my work on this.
>

I sent out a new version of my series, reworked to use the CSC_FF
colorop.

https://lore.kernel.org/dri-devel/20260330153451.99472-1-harry.wentland@amd.com/

Harry

> Harry
>
>> I guess using CSC for the latter is not obvious enough because it has
>> been used for the former (color model conversion) as well?
>>
>> How about "color-primary conversion"?
>>
>> I stole it from
>> https://onlinelibrary.wiley.com/doi/pdf/10.1002/9780470994375.app8
>>
>>
>> Thanks,
>> pq
>>
>>>>>> BT.601 actually has two different sets of primaries. Bt.2020 defines
>>>>>> two different YCbCr conversions. BT.709 uses the same primaries as
>>>>>> sRGB, but is different from sRGB on all other aspects.
>>>>>>
>>>>>> Therefore, when you refer to any one of these, you also need to be
>>>>>> clear whether you are referring to the YCbCr conversion or to the
>>>>>> primaries.
>>>>>>
>>>>>
>>>>> In that case, if the HW block says that it does YCbCr to RGB conversion
>>>>> using rec BT.709, the resultant RGB follows the primaries as described
>>>>> by BT.709 or mathematically it does not really matter?
>>>>
>>>> Hi,
>>>>
>>>> the resultant RGB may or may not follow BT.709 primaries, and knowing
>>>> the primaries is important for further processing in general.
>>>>
>>>> YCbCr<->RGB conversion does not change the primaries. What went in,
>>>> will come out.
>>>>
>>>>>>> I will read up on why our HW names these bits as such.
>>>>>>
>>>>>> Sure, but keep in mind that your hardware naming is irrelevant for the
>>>>>> UAPI design.
>>>>>
>>>>> Understood, I just want to make sure that the HW does exactly what we
>>>>> will advertise through the UAPI.
>>>>
>>>> Precisely.
>>>>
>>>>
>>>> Thanks,
>>>> pq
>>>>
>>>>>>>> For YCbCr 2020 I feel it's nice to remember, that there are two
>>>>>>>> different conversions in the specification: the simple matrix one
>>>>>>>> called "non-constant luminance", and the complex one called "constant
>>>>>>>> luminance". Hence "NC".
>>>>>>>>
>>>>>>>> It's also good to recall that YCbCr-RGB conversions are done in an
>>>>>>>> electrical space, while RGB709-to-RGB2020 conversion must be done in the
>>>>>>>> optical space. It is up to the userspace to arrange the neighbouring
>>>>>>>> colorops to use the fixed matrix right.
>>>>>>>>
>>>>>>>
>>>>>>> Ack on the above.
>>>
>>
>