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Date: Tue, 26 Mar 2024 16:57:00 +0100
From: Louis Chauvet <louis.chauvet@bootlin.com>
To: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>,
	Melissa Wen <melissa.srw@gmail.com>,
	=?iso-8859-1?Q?Ma=EDra?= Canal <mairacanal@riseup.net>,
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	Daniel Vetter <daniel@ffwll.ch>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Maxime Ripard <mripard@kernel.org>,
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	miquel.raynal@bootlin.com, thomas.petazzoni@bootlin.com,
	seanpaul@google.com, marcheu@google.com, nicolejadeyee@google.com
Subject: Re: [PATCH v5 09/16] drm/vkms: Introduce pixel_read_direction enum
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 <20240313-yuv-v5-9-e610cbd03f52@bootlin.com>
 <20240325151103.0a5f7112.pekka.paalanen@collabora.com>
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Le 25/03/24 - 15:11, Pekka Paalanen a ?crit :
> On Wed, 13 Mar 2024 18:45:03 +0100
> Louis Chauvet <louis.chauvet@bootlin.com> wrote:
> 
> > The pixel_read_direction enum is useful to describe the reading direction
> > in a plane. It avoids using the rotation property of DRM, which not
> > practical to know the direction of reading.
> > This patch also introduce two helpers, one to compute the
> > pixel_read_direction from the DRM rotation property, and one to compute
> > the step, in byte, between two successive pixel in a specific direction.
> > 
> > Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
> > ---
> >  drivers/gpu/drm/vkms/vkms_composer.c | 36 ++++++++++++++++++++++++++++++++++++
> >  drivers/gpu/drm/vkms/vkms_drv.h      | 11 +++++++++++
> >  drivers/gpu/drm/vkms/vkms_formats.c  | 30 ++++++++++++++++++++++++++++++
> >  3 files changed, 77 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> > index 9254086f23ff..989bcf59f375 100644
> > --- a/drivers/gpu/drm/vkms/vkms_composer.c
> > +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> > @@ -159,6 +159,42 @@ static void apply_lut(const struct vkms_crtc_state *crtc_state, struct line_buff
> >  	}
> >  }
> >  
> > +/**
> > + * direction_for_rotation() - Get the correct reading direction for a given rotation
> > + *
> > + * This function will use the @rotation setting of a source plane to compute the reading
> > + * direction in this plane which correspond to a "left to right writing" in the CRTC.
> > + * For example, if the buffer is reflected on X axis, the pixel must be read from right to left
> > + * to be written from left to right on the CRTC.
> 
> That is a well written description.

Thanks
 
> > + *
> > + * @rotation: Rotation to analyze. It correspond the field @frame_info.rotation.
> > + */
> > +static enum pixel_read_direction direction_for_rotation(unsigned int rotation)
> > +{
> > +	if (rotation & DRM_MODE_ROTATE_0) {
> > +		if (rotation & DRM_MODE_REFLECT_X)
> > +			return READ_RIGHT_TO_LEFT;
> > +		else
> > +			return READ_LEFT_TO_RIGHT;
> > +	} else if (rotation & DRM_MODE_ROTATE_90) {
> > +		if (rotation & DRM_MODE_REFLECT_Y)
> > +			return READ_BOTTOM_TO_TOP;
> > +		else
> > +			return READ_TOP_TO_BOTTOM;
> > +	} else if (rotation & DRM_MODE_ROTATE_180) {
> > +		if (rotation & DRM_MODE_REFLECT_X)
> > +			return READ_LEFT_TO_RIGHT;
> > +		else
> > +			return READ_RIGHT_TO_LEFT;
> > +	} else if (rotation & DRM_MODE_ROTATE_270) {
> > +		if (rotation & DRM_MODE_REFLECT_Y)
> > +			return READ_TOP_TO_BOTTOM;
> > +		else
> > +			return READ_BOTTOM_TO_TOP;
> > +	}
> > +	return READ_LEFT_TO_RIGHT;
> 
> I'm a little worried seeing REFLECT_X is supported only for some
> rotations, and REFLECT_Y for other rotations. Why is an analysis of all
> combinations not necessary?

I don't need to manage all the combination because this is only about 
the "horizontal writing".

So, if you want to write a line in the CRTC, with:
- ROT_0 || REF_X => You need to read the source line from right to left
- ROT_0 => You need to read source buffer from left to right
- ROT_0 || REF_Y => You need to read the source line from left to right

In this case, REF_Y only have an effect on the "column reading". It is not 
needed here because the new version of the blend function will use the 
drm_rect_* helpers to compute the correct y coordinate.

If you think it's clearer, I can create a big switch(rotation) like this:

	switch (rotation) {
	case ROT_0:
	case ROT_0 || REF_X:
		return L2R;
	case ROT_0 || REF_Y:
		return R2L;
	case ROT_90:
	case ROT_90 || REF_X:
		return T2B;
	[...]
	}

So all cases are clearly covered?

> I hope IGT uses FB patterns instead of solid color in its tests of
> rotation to be able to detect the difference.

They use solid colors, and even my new rotation test [3] use solid colors. 
It is mainly for yuv formats with subsampling: if you have formats with 
subsampling, a "software rotated buffer" and a "hardware rotated buffer" 
will not apply the same subsampling, so the colors will be slightly 
different.

> The return values do seem correct to me, assuming I have guessed
> correctly what "X" and "Y" refer to when combined with rotation. I did
> not find good documentation about that.

Yes, it is difficult to understand how rotation and reflexion should 
works in drm. I spend half a day testing all the combination in drm_rect_* 
helpers to understand how this works. According to the code:
- If only rotation or only reflexion, easy as expected
- If reflexion and rotation are mixed, the source buffer is first 
  reflected and then rotated.
 
> Btw. if there are already functions that are able to transform
> coordinates based on the rotation bitfield, you could alternatively use
> them. Transform CRTC point (0, 0) to A, and (1, 0) to B. Now A and B
> are in plane coordinate system, and vector B - A gives you the
> direction. The reason I'm mentioning this is that then you don't have
> to implement yet another copy of the rotation bitfield semantics from
> scratch.

You are totaly right. I will try this elegant method. Yes, there are some 
helpers (drm_rect_rotate_inv), so I will try to do something.

> 
> > +}
> > +
> >  /**
> >   * blend - blend the pixels from all planes and compute crc
> >   * @wb: The writeback frame buffer metadata
> > diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> > index 3ead8b39af4a..985e7a92b7bc 100644
> > --- a/drivers/gpu/drm/vkms/vkms_drv.h
> > +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> > @@ -69,6 +69,17 @@ struct vkms_writeback_job {
> >  	pixel_write_t pixel_write;
> >  };
> >  
> > +/**
> > + * enum pixel_read_direction - Enum used internaly by VKMS to represent a reading direction in a
> > + * plane.
> > + */
> > +enum pixel_read_direction {
> > +	READ_BOTTOM_TO_TOP,
> > +	READ_TOP_TO_BOTTOM,
> > +	READ_RIGHT_TO_LEFT,
> > +	READ_LEFT_TO_RIGHT
> > +};
> > +
> >  /**
> >   * typedef pixel_read_t - These functions are used to read a pixel in the source frame,
> >   * convert it to `struct pixel_argb_u16` and write it to @out_pixel.
> > diff --git a/drivers/gpu/drm/vkms/vkms_formats.c b/drivers/gpu/drm/vkms/vkms_formats.c
> > index 649d75d05b1f..743b6fd06db5 100644
> > --- a/drivers/gpu/drm/vkms/vkms_formats.c
> > +++ b/drivers/gpu/drm/vkms/vkms_formats.c
> > @@ -75,6 +75,36 @@ static void packed_pixels_addr(const struct vkms_frame_info *frame_info,
> >  	*addr = (u8 *)frame_info->map[0].vaddr + offset;
> >  }
> >  
> > +/**
> > + * get_step_next_block() - Common helper to compute the correct step value between each pixel block
> > + * to read in a certain direction.
> > + *
> > + * As the returned offset is the number of bytes between two consecutive blocks in a direction,
> > + * the caller may have to read multiple pixel before using the next one (for example, to read from
> > + * left to right in a DRM_FORMAT_R1 plane, each block contains 8 pixels, so the step must be used
> > + * only every 8 pixels.
> > + *
> > + * @fb: Framebuffer to iter on
> > + * @direction: Direction of the reading
> > + * @plane_index: Plane to get the step from
> > + */
> > +static int get_step_next_block(struct drm_framebuffer *fb, enum pixel_read_direction direction,
> > +			       int plane_index)
> > +{
> 
> I would have called this something like get_block_step_bytes() for
> example. That makes it clear it returns bytes (not e.g. pixels). "next"
> implies to me that I tell the function the current block, and then it
> gets me the next one. It does not do that, so I'd not use "next".

Nice name, I will took it for the v6.

Thanks,
Louis Chauvet

> > +	switch (direction) {
> > +	case READ_LEFT_TO_RIGHT:
> > +		return fb->format->char_per_block[plane_index];
> > +	case READ_RIGHT_TO_LEFT:
> > +		return -fb->format->char_per_block[plane_index];
> > +	case READ_TOP_TO_BOTTOM:
> > +		return (int)fb->pitches[plane_index];
> > +	case READ_BOTTOM_TO_TOP:
> > +		return -(int)fb->pitches[plane_index];
> > +	}
> > +
> > +	return 0;
> > +}
> 
> Looks good.
> 
> 
> Thanks,
> pq
> 
> > +
> >  static void *get_packed_src_addr(const struct vkms_frame_info *frame_info, int y,
> >  				 int plane_index)
> >  {
> > 
> 


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