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Date: Thu, 29 Feb 2024 14:41:37 +0200
From: Pekka Paalanen <pekka.paalanen@collabora.com>
To: Arthur Grillo <arthurgrillo@riseup.net>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>, Melissa Wen
 <melissa.srw@gmail.com>, =?UTF-8?B?TWHDrXJh?= Canal
 <mairacanal@riseup.net>, Haneen Mohammed <hamohammed.sa@gmail.com>, Daniel
 Vetter <daniel@ffwll.ch>, Maarten Lankhorst
 <maarten.lankhorst@linux.intel.com>, Maxime Ripard <mripard@kernel.org>,
 Thomas Zimmermann <tzimmermann@suse.de>, David Airlie <airlied@gmail.com>,
 Jonathan Corbet <corbet@lwn.net>, dri-devel@lists.freedesktop.org,
 linux-kernel@vger.kernel.org, jeremie.dautheribes@bootlin.com,
 miquel.raynal@bootlin.com, thomas.petazzoni@bootlin.com,
 seanpaul@google.com, marcheu@google.com, nicolejadeyee@google.com
Subject: Re: [PATCH v3 3/9] drm/vkms: write/update the documentation for
 pixel conversion and pixel write functions
Message-ID: <20240229144137.75ae1cde.pekka.paalanen@collabora.com>
In-Reply-To: <592e5da7-7aac-4735-ae8f-625402e381ae@riseup.net>
References: <20240226-yuv-v3-0-ff662f0994db@bootlin.com>
	<20240226-yuv-v3-3-ff662f0994db@bootlin.com>
	<406988be-48a4-4762-9c03-7a27c8e7b91e@riseup.net>
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	<592e5da7-7aac-4735-ae8f-625402e381ae@riseup.net>
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On Tue, 27 Feb 2024 15:47:08 -0300
Arthur Grillo <arthurgrillo@riseup.net> wrote:

> On 27/02/24 12:02, Louis Chauvet wrote:
> > Le 26/02/24 - 10:07, Arthur Grillo a =C3=A9crit : =20
> >>
> >>
> >> On 26/02/24 05:46, Louis Chauvet wrote: =20
> >>> Add some documentation on pixel conversion functions.
> >>> Update of outdated comments for pixel_write functions.
> >>>
> >>> Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
> >>> ---
> >>>  drivers/gpu/drm/vkms/vkms_composer.c |  4 +++
> >>>  drivers/gpu/drm/vkms/vkms_drv.h      | 13 ++++++++
> >>>  drivers/gpu/drm/vkms/vkms_formats.c  | 58 ++++++++++++++++++++++++++=
++++------
> >>>  3 files changed, 66 insertions(+), 9 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/v=
kms/vkms_composer.c
> >>> index c6d9b4a65809..5b341222d239 100644
> >>> --- a/drivers/gpu/drm/vkms/vkms_composer.c
> >>> +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> >>> @@ -189,6 +189,10 @@ static void blend(struct vkms_writeback_job *wb,
> >>> =20
> >>>  	size_t crtc_y_limit =3D crtc_state->base.crtc->mode.vdisplay;
> >>> =20
> >>> +	/*
> >>> +	 * The planes are composed line-by-line. It is a necessary complexi=
ty to avoid poor
> >>> +	 * blending performance. =20
> >>
> >> At this moment in the series, you have not yet reintroduced the
> >> line-by-line algorithm yet. Maybe it's better to add this comment when
> >> you do. =20
> >=20
> > Is it better with this:
> >=20
> > 	/*
> > 	 * The planes are composed line-by-line to avoid heavy memory usage. I=
t is a necessary
> > 	 * complexity to avoid poor blending performance.
> > 	 *
> > 	 * The function vkms_compose_row is used to read a line, pixel-by-pixe=
l, into the staging
> > 	 * buffer.
> > 	 */
> >   =20
> >> Also, I think it's good to give more context, like:
> >> "The planes are composed line-by-line, instead of pixel-by-pixel" =20
> >=20
> > And after PATCHv3 5/9:
> >=20
> > 	/*
> > 	 * The planes are composed line-by-line to avoid heavy memory usage. I=
t is a necessary
> > 	 * complexity to avoid poor blending performance.
> > 	 *
> > 	 * The function pixel_read_line callback is used to read a line, using=
 an efficient=20
> > 	 * algorithm for a specific format, into the staging buffer.
> > 	 */
> >  =20

Hi,

there are a few reasons for the line-by-line algorithm, and the
optimizations at large:

VKMS uses temporary stage and output buffers so that blending functions
can operate on just one high-precision pixel format, struct
pixel_argb_u16. We can make pixel-format-specific read and write
functions completely orthogonal from the blending operations and FB
format combinations. This avoids a combinatorial explosion of needed
functions for { input pixel formats =C3=97 blending operations =C3=97 outpu=
t pixel
formats }.

We can use a temporary stage and output buffer whose size is one line
and not whole FB or CRTC framebuffer. This is the memory savings.

Using a temporary output buffer also avoids repeated
read-decode-blend-encode-write cycles into the final destination
buffer, as we don't need to decode/encode the pixel format.

Finally, doing elementary operations (read, blend, write) line-by-line
is much more efficient than pixel-by-pixel, because it allows making
the inner-most loop very tight. It avoids repeatedly computing a result
that does not change, like which function to call for a specific pixel
format or blending equation.


Thanks,
pq

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