On 11/9/2022 1:18 AM, Pekka Paalanen wrote:
On Tue, 8 Nov 2022 23:01:47 +0100
Sebastian Wick <sebastian.wick@xxxxxxxxxx> wrote:
On Tue, Nov 8, 2022 at 7:51 PM Simon Ser <contact@xxxxxxxxxxx> wrote:
cc'ing Pekka and wayland-devel for userspace devs feedback on the new uAPI.
Hi all,
thanks! Comments below.
Thanks for the feedback!
On Saturday, October 29th, 2022 at 14:08, Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx> wrote:
On 29/10/2022 01:59, Jessica Zhang wrote:
Add support for COLOR_FILL and COLOR_FILL_FORMAT properties for
drm_plane. In addition, add support for setting and getting the values
of these properties.
COLOR_FILL represents the color fill of a plane while COLOR_FILL_FORMAT
represents the format of the color fill. Userspace can set enable solid
fill on a plane by assigning COLOR_FILL to a uint64_t value, assigning
the COLOR_FILL_FORMAT property to a uint32_t value, and setting the
framebuffer to NULL.
Signed-off-by: Jessica Zhang <quic_jesszhan@xxxxxxxxxxx>
Planes report supported formats using the drm_mode_getplane(). You'd
also need to tell userspace, which formats are supported for color fill.
I don't think one supports e.g. YV12.
A bit of generic comment for the discussion (this is an RFC anyway).
Using color_fill/color_fill_format properties sounds simple, but this
might be not generic enough. Limiting color_fill to 32 bits would
prevent anybody from using floating point formats (e.g.
DRM_FORMAT_XRGB16161616F, 64-bit value). Yes, this can be solved with
e.g. using 64-bit for the color_fill value, but then this doesn't sound
extensible too much.
So, a question for other hardware maintainers. Do we have hardware that
supports such 'color filled' planes? Do we want to support format
modifiers for filling color/data? Because what I have in mind is closer
to the blob structure, which can then be used for filling the plane:
struct color_fill_blob {
u32 pixel_format;
u64 modifiers4];
u32 pixel_data_size; // fixme: is this necessary?
u8 pixel_data[];
};
And then... This sounds a lot like a custom framebuffer.
So, maybe what should we do instead is to add new DRM_MODE_FB_COLOR_FILL
flag to the framebuffers, which would e.g. mean that the FB gets stamped
all over the plane. This would also save us from changing if (!fb)
checks all over the drm core.
Another approach might be using a format modifier instead of the FB flag.
What do you think?
First off, we only need to represent the value of a single pixel here. So I'm
not quite following why we need format modifiers. Format modifiers describe how
pixels are laid out in memory. Since there's a single pixel described, this
is non-sensical to me, the format modifier is always LINEAR.
Agreed.
Then, I can understand why putting the pixel_format in there is tempting to
guarantee future extensibility, but it also adds complexity. For instance, how
does user-space figure out which formats can be used for COLOR_FILL? Can
user-space use any format supported by the plane? What does it mean for
multi-planar formats? Do we really want the kernel to have conversion logic for
all existing formats? Do we need to also add a new read-only blob prop to
indicate supported COLOR_FILL formats?
FWIW the formats supported by solid_fill wouldn't necessarily be all the
formats supported by the plane (ex. for msm/dpu, solid_fill only
supports all RGB color variants, though planes can normally support YUV
formats too).
That being said, I'm ok with having the solid_fill take in only
RGBA32323232 format based on the comments below.
Right. This does not seem to require pixel formats at all.
The point of pixel formats is to be able to feed large amounts of data
as-is into hardware and avoid the CPU ever touching it. You do that
with DRM FBs pointing to suitably allocated hardware buffers. But here
we have exactly one pixel, which I imagine will always be read by the
CPU so the driver will convert it into a hardware-specific format and
program it; probably the driver will not create an internal DRM FB for
it. >
The above might also be a reason to not model this as a special-case
DRM FB in UAPI. Or, at least you need a whole new ioctl to create such
DRM FB to avoid the need to allocate e.g. a dumb buffer or a
GPU-specific buffer. >
What one does need is what Sebastian brought up: does it support alpha
or not?
Hmm, the drm_plane struct already supports an alpha property so it seems
a bit redundant to also have a separate alpha value in the solid fill color.
That being said, we could have it so that setting the alpha for the
solid_fill property will also change the value of the plane's alpha
property too.
Userspace would also be interested in the supported precision of the
values, but the hardware pixel component order is irrelevant because the
driver will always convert the one pixel with CPU anyway.
YUV vs. RGB is a another question. The KMS color pipeline is defined in
terms of RGBA as far as I know, and alpha-blending YUV values makes no
sense. So will there ever be any need to set an YUV fill? I have a hard
time imagining it.
If you do set an YUV fill, the KMS color pipeline most likely needs to
convert it to RGBA for blending, and then you need the plane properties
COLOR_ENCODING and COLOR_RANGE.
But why bother when userspace can convert that one pixel to RGBA itself
anyway?
Noted, I think this is reasonable.
We've recently-ish standardized a new Wayland protocol [1] which has the same
purpose as this new kernel uAPI. The conclusion there was that using 32-bit
values for each channel (R, G, B, A) would be enough for almost all use-cases.
The driver can convert these high-precision values to what the hardware expects.
The only concern was about sending values outside of the [0.0, 1.0] range,
which may have HDR use-cases.
This is what I would suggest, yes. This representation has enough
precision to be future-proof, and the driver will be converting the
value anyway.
The question about values outside of the unit range is a good one, too.
With Wayland, we can simply add another request to set a value in
floating-point if that turns up necessary.
Whether that will ever be necessary is connected to how the DRM KMS
abstract color pipeline is modelled, and that you must define from the
beginning:
If DRM KMS gets color processing related plane properties like CTM,
GAMMA or DEGAMMA (they already exist for CRTC, and these have been
proposed for planes quite some time ago), does the fill color go
through all these operations, or will the fill color skip all these
operations and go straight to plane blending?
The fill color would still go through color processing operations,
though FWIW the MSM driver doesn't support GAMMA/DEGAMMA.
Whether values outside of the unit range will ever be needed depends
also on the userspace design. Userspace could choose the value range
freely if the KMS color pipeline elements happen to support that range.
However things like LUTs are naturally limited to unit range input, so
using values outside of the unit range might be difficult if not
impossible. Therefore I'm not concerned about this question much.
So, there are multiple ways to go about this. I can think of:
- Put "RGBA32" in the name of the prop, and if we ever need a different
color format, pick a different name.
I could see problems with that if the same device supports more than
one kind. How to turn off all but one color fill property?
What if userspace understands an old color fill property but not the
new one, and the new one happens to be set for some reason? Userspace
will attempt to use the old property without setting the new property
to nil, and fail.
- Define a struct with an enum of possible fill kinds:
#define FILL_COLOR_RGBA32 1
#define FILL_COLOR_F32 2
struct color_fill_blob { u32 kind; u8 data[]; };
This could work.
Btw. maybe call it RGBA32323232 to make it follow the drm_fourcc naming
convention. Some userspace libraries already use RGBA32 to mean 32 bit
per pixel instead of per channel.
How will userspace know what kinds are supported?
- Define a struct with a version and RGBA values:
struct color_fill_blob { u32 version; u32 rgba[4]; };
If we need to add more formats later, or new metadata:
struct color_fill_blob2 { u32 version; /* new fields */ };
where version must be set to 2.
This could work.
Leaning towards this option.
Thanks,
Jessica Zhang
- Define a struct with a "pixel_format" prop, but force user-space to use a
fixed format for now. Later, if we need another format, add a new prop to
advertise supported formats.
- More complicated solutions, e.g. advertise the list of supported formats from
the start.
Feels more complicated than necessary.
Anyway, the idea of creating a blob and then setting that into some KMS
plane property sounds a very good mechanism.
[1]: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/104
Agreeing with most of what you said here. However, what's the idea
behind a format anyway? The 4 values provided here are fed directly
into the color pipeline which seems to define the color channels it's
working on as RGBA (or doesn't define anything at all). The only
reason I can think of is that hardware might support only ingesting
values either in a format with high bit depth color channels and no
alpha or a format with low bit depth color but with alpha, so choosing
between the formats provides a real trade-off. Is that actually
something hardware might be restricted to or do they all just support
ingesting the color data with enough precision on every channel? >
Right.
Thanks,
pq
