On 6/13/2023 4:23 AM, Pekka Paalanen wrote:
On Mon, 12 Jun 2023 12:56:57 -0400
Christopher Braga <quic_cbraga@xxxxxxxxxxx> wrote:
On 6/12/2023 5:21 AM, Pekka Paalanen wrote:
On Fri, 9 Jun 2023 19:11:25 -0400
Christopher Braga <quic_cbraga@xxxxxxxxxxx> wrote:
On 6/9/2023 12:30 PM, Simon Ser wrote:
Hi Christopher,
On Friday, June 9th, 2023 at 17:52, Christopher Braga <quic_cbraga@xxxxxxxxxxx> wrote:
The new COLOROP objects also expose a number of KMS properties. Each has a
type, a reference to the next COLOROP object in the linked list, and other
type-specific properties. Here is an example for a 1D LUT operation:
Color operation 42
├─ "type": enum {Bypass, 1D curve} = 1D curve
├─ "1d_curve_type": enum {LUT, sRGB, PQ, BT.709, HLG, …} = LUT
The options sRGB / PQ / BT.709 / HLG would select hard-coded 1D
curves? Will different hardware be allowed to expose a subset of these
enum values?
Yes. Only hardcoded LUTs supported by the HW are exposed as enum entries.
├─ "lut_size": immutable range = 4096
├─ "lut_data": blob
└─ "next": immutable color operation ID = 43
Some hardware has per channel 1D LUT values, while others use the same
LUT for all channels. We will definitely need to expose this in the
UAPI in some form.
Hm, I was assuming per-channel 1D LUTs here, just like the existing GAMMA_LUT/
DEGAMMA_LUT properties work. If some hardware can't support that, it'll need
to get exposed as another color operation block.
To configure this hardware block, user-space can fill a KMS blob with
4096 u32
entries, then set "lut_data" to the blob ID. Other color operation types
might
have different properties.
The bit-depth of the LUT is an important piece of information we should
include by default. Are we assuming that the DRM driver will always
reduce the input values to the resolution supported by the pipeline?
This could result in differences between the hardware behavior
and the shader behavior.
Additionally, some pipelines are floating point while others are fixed.
How would user space know if it needs to pack 32 bit integer values vs
32 bit float values?
Again, I'm deferring to the existing GAMMA_LUT/DEGAMMA_LUT. These use a common
definition of LUT blob (u16 elements) and it's up to the driver to convert.
Using a very precise format for the uAPI has the nice property of making the
uAPI much simpler to use. User-space sends high precision data and it's up to
drivers to map that to whatever the hardware accepts.
Conversion from a larger uint type to a smaller type sounds low effort,
however if a block works in a floating point space things are going to
get messy really quickly. If the block operates in FP16 space and the
interface is 16 bits we are good, but going from 32 bits to FP16 (such
as in the matrix case or 3DLUT) is less than ideal.
Hi Christopher,
are you thinking of precision loss, or the overhead of conversion?
Conversion from N-bit fixed point to N-bit floating-point is generally
lossy, too, and the other direction as well.
What exactly would be messy?
Overheard of conversion is the primary concern here. Having to extract
and / or calculate the significand + exponent components in the kernel
is burdensome and imo a task better suited for user space. This also has
to be done every blob set, meaning that if user space is re-using
pre-calculated blobs we would be repeating the same conversion
operations in kernel space unnecessarily.
What is burdensome in that calculation? I don't think you would need to
use any actual floating-point instructions. Logarithm for finding the
exponent is about finding the highest bit set in an integer and
everything is conveniently expressed in base-2. Finding significand is
just masking the integer based on the exponent.
Oh it definitely can be done, but I think this is just a difference of
opinion at this point. At the end of the day we will do it if we have
to, but it is just more optimal if a more agreeable common type is used.
Can you not cache the converted data, keyed by the DRM blob unique
identity vs. the KMS property it is attached to?
If the userspace compositor has N common transforms (ex: standard P3 ->
sRGB matrix), they would likely have N unique blobs. Obviously from the
kernel end we wouldn't want to cache the transform of every blob passed
down through the UAPI.
You can assume that userspace will not be re-creating DRM blobs without
a reason to believe the contents have changed. If the same blob is set
on the same property repeatedly, I would definitely not expect a driver
to convert the data again.
If the blob ID is unchanged there is no issue since caching the last
result is already common. As you say, blobs are immutable so no update
is needed. I'd question why the compositor keeps trying to send down the
same blob ID though.
If a driver does that, it seems like it
should be easy to avoid, though I'm no kernel dev. Even if the
conversion was just a memcpy, I would still posit it needs to be
avoided when the data has obviously not changed. Blobs are immutable.
> Userspace having to use hardware-specific number formats would probably
not be well received.
To be clear, I am not asking user space to use custom value packing made
purely for the hardware's benefit (this sounds like a problem just
waiting to happen). Just support in the color pipeline UAPI for common
numerical data types such as 16-bit floats. That said...
I agree normalization of the value causing precision loss and rounding
we can't avoid.
We should also consider the fact that float pipelines have been known to
use the scrgb definition for floating point values
(https://registry.khronos.org/EGL/extensions/EXT/EGL_EXT_gl_colorspace_scrgb_linear.txt).
scRGB is as good a definition of color encoding as "floating-point" is
for numbers. What I mean is that it carries very little usable meaning,
and without further information it is practically boundless
- infinite - in both color gamut and dynamic range. Just like any
floating-point quantity.
However, what we want from KMS color pipeline is zero implied or
defined meaning. That means scRGB carries too much meaning, because the
primaries are fixed and (1.0, 1.0, 1.0) should match sRGB/SDR white.
> Btw. if one brings in nit units, you assume a specific viewing
environment which is rarely true in reality. I'll leave that rabbit
hole for another time. I just want to mention that nit (cd/m²) is a
unit that is relative to the chosen viewing environment when your goal
is a specific perception of brightness.
In cases like this where there may be a expected value range in the
pipeline, how to normalize a larger input becomes a little confusing. Ex
- Does U32 MAX become FP16 MAX or value MAX (i.e 127).
UAPI simply needs to specify the number encoding used in the UAPI, how
bit patterns map to real numbers. Real numbers are then what the color
pipeline operates on.
If we plan to have the color pipeline UAPI expose these details then I
am satisfied.
However, intermediate value representation used between two KMS colorop
blocks is never observable to userspace. All userspace needs to know is
the usable value range and precision behaviour. I think that is best
defined for the input and output of each block rather than what flows
in between, because an optional (e.g. LUT) block when bypassed does not
impose its limitations.
Sure. Everything in between can be inferred from the pipeline.
What does 1.0 actually mean, that is left for userspace to use however
it wishes. There are only pipeline boundary conditions to that: the
input to a pipeline comes from a DRM FB, so it has a number encoding
specified mostly by pixel format, and an arbitrary colorimetric
encoding that only userspace knows. The output of the pipeline has to
be standardised so that drivers can number-encode the pipeline output
correctly to wire format on e.g. HDMI. Userspace alone is responsible
for making sure the colorimetry matches what the sink expects.
Individual KMS color pipeline colorop blocks need to define their own
acceptable input and output ranges. E.g. a look-up table may assume
that it's input is in [0.0, 1.0] and anything outside is clamped to
that range. That poses restrictions on how userspace can use the block.
Exposing the actual hardware precision is something we've talked about during
the hackfest. It'll probably be useful to some extent, but will require some
discussion to figure out how to design the uAPI. Maybe a simple property is
enough, maybe not (e.g. fully describing the precision of segmented LUTs would
probably be trickier).
I'd rather keep things simple for the first pass, we can always add more
properties for bit depth etc later on.
Indicating if a block operates on / with fixed vs float values is
significant enough that I think we should account for this in initial
design. It will have a affect on both the user space value packing +
expected value ranges in the hardware.
What do you mean by "value packing"? Memory layout of the bits forming
a value? Or possible exact values of a specific type? >
Both really. If the kernel is provided a U32 value, we need to know if
this is a U32 value, or a float packed into a U32 container. Likewise as
mentioned with the scRGB above, float could even adjust the value range
expectations.
Right. The UAPI will simply define that.
Great!
Thanks,
Christopher
I don't think fixed vs. float is the most important thing. Even fixed
point formats can have different numbers of bits for whole numbers,
which changes the usable value range and not only precision. Userspace
at the very least needs to know the usable value range for the block's
inputs, outputs, and parameters.
When defining the precision for inputs, outputs and parameters, then
fixed- vs. floating-point becomes meaningful in explaining what "N bits
of precision" means.
Then there is the question of variable precision that depends on the
actual block input and parameter values, how to represent that. Worst
case precision might be too pessimistic alone.
Agreed. More information probably is needed to full define the interface
expectations.
Here is another example with a 3D LUT:
Color operation 42
├─ "type": enum {Bypass, 3D LUT} = 3D LUT
├─ "lut_size": immutable range = 33
├─ "lut_data": blob
└─ "next": immutable color operation ID = 43
We are going to need to expose the packing order here to avoid any
programming uncertainty. I don't think we can safely assume all hardware
is equivalent.
The driver can easily change the layout of the matrix and do any conversion
necessary when programming the hardware. We do need to document what layout is
used in the uAPI for sure.
And one last example with a matrix:
Color operation 42
├─ "type": enum {Bypass, Matrix} = Matrix
├─ "matrix_data": blob
└─ "next": immutable color operation ID = 43
It is unclear to me what the default sizing of this matrix is. Any
objections to exposing these details with an additional property?
The existing CTM property uses 9 uint64 (S31.32) values. Is there a case where
that wouldn't be enough?
Larger cases do exist, but as you mention this can be resolved with a
different type then. I don't have any issues with the default 'Matrix'
type being 9 entries.
Please, tell us more. How big, and what are they used for?
IIRC ICC has 3x3 matrix + offset vector. Do you have even more?
Offset is one. Range adjustment 'vector' is another. But ultimately this
proposal is flexible enough that this can probably just be another color
block in the pipeline. No complaints from me here.
What is a range adjustment vector? A vector of a multiplier per color
channel? Does it include offset?
Yes, sounds like just another block.
Dithering logic exists in some pipelines. I think we need a plan to
expose that here as well.
Hm, I'm not too familiar with dithering. Do you think it would make sense to
expose as an additional colorop block? Do you think it would have more
consequences on the design?
I think it would be an additional block, and no other consequences, be
it temporal and/or spatial dithering, as long as it does not look at
neighbouring pixels to determine the output for current pixel.
I want to re-iterate that we don't need to ship all features from day 1. We
just need to come up with a uAPI design on which new features can be built on.
Agreed. I don't think this will affect the proposed design so this can
be figured out once we have a DRM driver impl that declares this block.
[Simon note: an alternative would be to split the color pipeline into
two, by
having two plane properties ("color_pipeline_pre_scale" and
"color_pipeline_post_scale") instead of a single one. This would be
similar to
the way we want to split pre-blending and post-blending. This could be less
expressive for drivers, there may be hardware where there are dependencies
between the pre- and post-scaling pipeline?]
As others have noted, breaking up the pipeline with immutable blocks
makes the most sense to me here. This way we don't have to predict ahead
of time every type of block that maybe affected by pipeline ordering.
Splitting the pipeline into two properties now means future
logical splits would require introduction of further plane properties.
Right, if there are more "breaking points", then we'll need immutable blocks
anyways.
Then, Alex from NVIDIA described how their hardware works. NVIDIA hardware
contains some fixed-function blocks which convert from LMS to ICtCp and
cannot
be disabled/bypassed. NVIDIA hardware has been designed for descriptive
APIs
where user-space provides a high-level description of the colorspace
conversions it needs to perform, and this is at odds with our KMS uAPI
proposal. To address this issue, we suggest adding a special block type
which
describes a fixed conversion from one colorspace to another and cannot be
configured by user-space. Then user-space will need to accomodate its
pipeline
for these special blocks. Such fixed hardware blocks need to be well enough
documented so that they can be implemented via shaders.
A few questions here. What is the current plan for documenting the
mathematical model for each exposed block? Will each defined 'type' enum
value be locked to a definition in the kernel documents? As an example,
when we say '3D LUT' in this proposal does this mean the block will
expose a tri-linear interpolated 3D LUT interface? Is '1D Curve' a
direct in to out LUT mapping?
I think we'll want to document these things, yes. We do want to give _some_
slack to drivers, so that they can e.g. implement the "1D LUT" colorop via
hardware segmented LUTs with a different number of elements per LUT segment.
But being mathematically precise (probably with formulae in the docs) is
definitely a goal, and absolutely necessary to implement a shader-based
fallback.
I agree some driver slack is necessary, however ideally this will be
locked down enough that from the compositor side they see "1D LUT" and
know exactly what to expect independent of the hardware. This way
regardless of if I am running on a NVIDIA / AMD / QCOM / etc... chip,
common color pipeline strategies can be used. Assuming a perfect world
where there is a workable overlap between chips of course.
Yes, of course, at least for a start.
However, the long term plan includes a shared userspace library with
driver- and hardware-specific knowledge to use hardware- and
driver-specific blocks. All blocks still need to be explicitly
specified in the kernel UAPI documentation, the idea is that it should
not be a problem for many vendors to have blocks no-one else does. The
library would offer a much more generic API, and use snowflake blocks
to their fullest. The library would also spit out OpenGL shaders and
whatnot for the fallback.
The future in the long term could be either way: evolving towards
generic KMS UAPI blocks with no need for a userspace library
abstraction, or evolving towards hardware-specific KMS UAPI blocks with
a userspace library to abstract them like Mesa does for GPUs.
Sounds good to me!
Awesome!
Thanks,
pq
