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Date: Mon, 8 Apr 2024 09:50:19 +0200
From: Louis Chauvet <louis.chauvet@bootlin.com>
To: Pekka Paalanen <pekka.paalanen@collabora.com>
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	Daniel Vetter <daniel@ffwll.ch>,
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Subject: Re: [PATCH v5 11/16] drm/vkms: Add YUV support
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Le 27/03/24 - 14:59, Pekka Paalanen a ?crit :
> On Tue, 26 Mar 2024 16:57:03 +0100
> Louis Chauvet <louis.chauvet@bootlin.com> wrote:
> 
> > Le 25/03/24 - 11:26, Ma?ra Canal a ?crit :
> > > On 3/13/24 14:45, Louis Chauvet wrote:  
> > > > From: Arthur Grillo <arthurgrillo@riseup.net>
> > > > 
> > > > Add support to the YUV formats bellow:
> > > > 
> > > > - NV12/NV16/NV24
> > > > - NV21/NV61/NV42
> > > > - YUV420/YUV422/YUV444
> > > > - YVU420/YVU422/YVU444
> > > > 
> > > > The conversion from yuv to rgb is done with fixed-point arithmetic, using
> > > > 32.32 floats and the drm_fixed helpers.
> > > > 
> > > > To do the conversion, a specific matrix must be used for each color range
> > > > (DRM_COLOR_*_RANGE) and encoding (DRM_COLOR_*). This matrix is stored in
> > > > the `conversion_matrix` struct, along with the specific y_offset needed.
> > > > This matrix is queried only once, in `vkms_plane_atomic_update` and
> > > > stored in a `vkms_plane_state`. Those conversion matrices of each
> > > > encoding and range were obtained by rounding the values of the original
> > > > conversion matrices multiplied by 2^32. This is done to avoid the use of
> > > > floating point operations.
> > > > 
> > > > The same reading function is used for YUV and YVU formats. As the only
> > > > difference between those two category of formats is the order of field, a
> > > > simple swap in conversion matrix columns allows using the same function.
> > > > 
> > > > Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
> > > > [Louis Chauvet:
> > > > - Adapted Arthur's work
> > > > - Implemented the read_line_t callbacks for yuv
> > > > - add struct conversion_matrix
> > > > - remove struct pixel_yuv_u8
> > > > - update the commit message
> > > > - Merge the modifications from Arthur]  
> > > 
> > > A Co-developed-by tag would be more appropriate.  
> > 
> > I am not the main author of this part, I only applied a few simple 
> > suggestions, the complex part was done by Arthur.
> > 
> > I will wait for Arthur's confirmation to change it to Co-developed by if
> > he agrees.
> 
> Co-developed-by is an additional tag, and does not replace S-o-b. To my
> understanding, the kernel rules and Developers' Certificate of Origin
> require S-o-b to be added by anyone who has taken a patch and
> re-submitted it, regardless of who the original author is, and
> especially if the patch was modified.
> 
> Personally I also like to keep the list of changes like Louis added, to
> credit people better.

I will keep everything, don't worry!
 
> > > > Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
> > > > ---
> > > >   drivers/gpu/drm/vkms/vkms_drv.h     |  22 ++
> > > >   drivers/gpu/drm/vkms/vkms_formats.c | 431 ++++++++++++++++++++++++++++++++++++
> > > >   drivers/gpu/drm/vkms/vkms_formats.h |   4 +
> > > >   drivers/gpu/drm/vkms/vkms_plane.c   |  17 +-
> > > >   4 files changed, 473 insertions(+), 1 deletion(-)
> > > > 
> 
> ...
> 
> > > >   };
> > > >   
> > > >   static struct drm_plane_state *
> > > > @@ -117,12 +129,15 @@ static void vkms_plane_atomic_update(struct drm_plane *plane,
> > > >   	drm_framebuffer_get(frame_info->fb);
> > > >   	frame_info->rotation = drm_rotation_simplify(new_state->rotation, DRM_MODE_ROTATE_0 |
> > > >   									  DRM_MODE_ROTATE_90 |
> > > > +									  DRM_MODE_ROTATE_180 |  
> > > 
> > > Why do we need to add DRM_MODE_ROTATE_180 here? Isn't the same as
> > > reflecting both along the X and Y axis?  
> > 
> > Oops, I had no intention of putting that change here. I will move it to 
> > another patch.
> > 
> > I don't understand why DRM_MODE_ROTATE_180 isn't in this list. If I read 
> > the drm_rotation_simplify documentation, it explains that this argument 
> > should contain all supported rotations and reflections, and ROT_180 is 
> > supported by VKMS. Perhaps this call is unnecessary because all 
> > combinations are supported by vkms?
> 
> If you truly handle all bit patterns that the rotation bitfield can
> have, then yes, the call seems unnecessary.
> 
> However, as documented, the bitfield contains redundancy: the same
> orientation can be expressed in more than one bit pattern. One example
> is that ROTATE_180 is equivalent to REFLECT_X | REFLECT_Y.
> 
> Since it's a bitmask, userspace can give you funny values like
> ROTATE_0 | ROTATE_90 | ROTATE_180. That is a valid orientation of
> 270-degree rotation (according to UAPI doc), but it is very awkwardly
> expressed, hence the need to normalise it into a minimal bit pattern.

The userspace can't set multiple bit, if you look at [1]:

	if (!is_power_of_2(val & DRM_MODE_ROTATE_MASK)) {
		drm_dbg_atomic(plane->dev,
			       "[PLANE:%d:%s] bad rotation bitmask: 0x%llx\n",
			       plane->base.id, plane->name, val);
		return -EINVAL;
	}

I have a series ready for improving the drm documentation, you will be in 
copy.

[1]: https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_atomic_uapi.c#L527
 
> It does not look like drm_rotation_simplify() actually does this
> minimisation!

drm_rotation_simplify() apply an additionnal 
REFLECT_X|REFLECT_Y|ROTATE_180, it is a "no-op" operation, but it 
transform ROT_90|REF_X into ROT_270|REF_Y, so you have eliminated REF_X 
and ROT_90. I will create a patch to document a bit more this function.

In the current vkms code, it will just remove the 180? rotation. I 
think we can just delete this call as everything is supported. I will add 
the patch in the v6 (I don't know why it was there at the first place, 
and I don't want to introduce regression).

> I was not able to tell if DRM common code actually stops userspace from
> combining multiple ROTATE bits in the same value. I suspect it must
> stop them, or perhaps all code dealing with rotation is actually broken.

See [1], and yes, drm helpers are broken with multiple bit sets, they 
expect only one rotation bit.

> drm_rotation_simplify() is useful for cases where your hardware does
> not have exactly the same flexibility. Maybe it cannot do REFLECT_Y but
> it can do everything else? Then drm_rotation_simplify() gives you a bit
> pattern that you can use directly, or fails if the orientation is not
> representable with what your hardware can do.
> 
> At least, that's my understanding of quickly glancing over it.
> 
> IOW, if you wanted to never have to deal with REFLECT_Y bit, you could
> leave it out here. Or, if you never want to deal with ROTATE_180, leave
> that out - you will get REFLECT_X | REFLECT_Y instead. In theory.
> 
> 
> Thanks,
> pq


-- 
Louis Chauvet, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com