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drm/vblank: Add intro to documentation 

Lyude Paul wrote a very good intro to vblank here:
https://lore.kernel.org/dri-devel/faf63d8a9ed23c16af69762f59d0dca6b2bf085f.camel@redhat.com/T/#mce6480be738160e9d07c5d023e88fd78d7a06d27

Add this to the intro chapter in drm_vblank.c so others
can benefit from it too.

v2:
  - Reworded to improve readability (Thomas)

v3:
  - Added nice ascii drawing from Lyude (Lyude)
  - Added referende to high-precision timestamp (Daniel)
  - Improved grammar (Thomas)
  - Combined it all and made kernel-doc happy
  - Dropped any a-b, r-b do to the amount of changes

v4:
  - Add intro to vblank interrupt (Liviu)
  - Add historical reference for blanking (Alex)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Co-developed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20200406194746.26433-2-sam@ravnborg.org
This commit is contained in:
Sam Ravnborg 2020-04-06 21:47:44 +02:00
parent e2d7fc20b3
commit 56b165feaf
1 changed files with 63 additions and 0 deletions
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63
drivers/gpu/drm/drm_vblank.c
View File

@ -41,6 +41,69 @@
/**
* DOC: vblank handling
*
* From the computer's perspective, every time the monitor displays
* a new frame the scanout engine has "scanned out" the display image
* from top to bottom, one row of pixels at a time. The current row
* of pixels is referred to as the current scanline.
*
* In addition to the display's visible area, there's usually a couple of
* extra scanlines which aren't actually displayed on the screen.
* These extra scanlines don't contain image data and are occasionally used
* for features like audio and infoframes. The region made up of these
* scanlines is referred to as the vertical blanking region, or vblank for
* short.
*
* For historical reference, the vertical blanking period was designed to
* give the electron gun (on CRTs) enough time to move back to the top of
* the screen to start scanning out the next frame. Similar for horizontal
* blanking periods. They were designed to give the electron gun enough
* time to move back to the other side of the screen to start scanning the
* next scanline.
*
* ::
*
*
* physical → ⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽
* top of | |
* display | |
* | New frame |
* | |
* |↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓|
* |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| ← Scanline,
* |↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓| updates the
* | | frame as it
* | | travels down
* | | ("sacn out")
* | Old frame |
* | |
* | |
* | |
* | | physical
* | | bottom of
* vertical |⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽| ← display
* blanking ┆xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx┆
* region → ┆xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx┆
* ┆xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx┆
* start of → ⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽
* new frame
*
* "Physical top of display" is the reference point for the high-precision/
* corrected timestamp.
*
* On a lot of display hardware, programming needs to take effect during the
* vertical blanking period so that settings like gamma, the image buffer
* buffer to be scanned out, etc. can safely be changed without showing
* any visual artifacts on the screen. In some unforgiving hardware, some of
* this programming has to both start and end in the same vblank. To help
* with the timing of the hardware programming, an interrupt is usually
* available to notify the driver when it can start the updating of registers.
* The interrupt is in this context named the vblank interrupt.
*
* The vblank interrupt may be fired at different points depending on the
* hardware. Some hardware implementations will fire the interrupt when the
* new frame start, other implementations will fire the interrupt at different
* points in time.
*
* Vertical blanking plays a major role in graphics rendering. To achieve
* tear-free display, users must synchronize page flips and/or rendering to
* vertical blanking. The DRM API offers ioctls to perform page flips