drm/vblank: Add intro to documentation

/**

 * 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 * For historical reference, the vertical blanking period was designed to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