[PATCH v2 1/3] drm/vblank: Add intro to documentation

[Sam Ravnborg <sam@ravnborg.org>]

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

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Co-developed-by: Lyude Paul <lyude@redhat.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: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
---
 drivers/gpu/drm/drm_vblank.c | 53 ++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index bcf346b3e486..9633092c9ad5 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -41,6 +41,59 @@
 /**
  * DOC: vblank handling
  *
+ * From the computer's perspective, every time the monitor displays
+ * a new frame the scanout engine have "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.
+ *
+ * ::
+ *
+ *
+ *    physical →   ⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽
+ *    top of      |                                        |
+ *    display     |                                        |
+ *                |               New frame                |
+ *                |                                        |
+ *                |↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓|
+ *                |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| ← Scanline, updates
+ *                |↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓|   the frame as it
+ *                |                                        |   travels down
+ *                |                                        |   ("scan 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.
+ *
+ * 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
-- 
2.20.1

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