Documentation: Add display panel specifics

Change-Id: If1a393578556d51499c700b68187034830d19215
Signed-off-by: Daniel Maslowski <daniel.maslowski@img.ly>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33265
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Nico Huber <nico.h@gmx.de>

Documentation/gfx/display-panel.md

@@ -0,0 +1,64 @@
+Display Panel Specifics
+=======================
+
+Timing Parameters
+-----------------
+
+From the binary file `edid` in the sys filesystem on Linux, the panel can be
+identified. The exact path may differ slightly. Here is an example:
+
+```sh
+$ strings /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/edid
+@0 5
+LG Display
+LP140WF3-SPD1
+```
+
+To figure out the timing parameters, refer to the [Intel Programmer's Reference
+Manuals](https://01.org/linuxgraphics/documentation/hardware-specification-prms)
+and try to find the datasheet of the panel using the information from `edid`.
+In the example above, you would search for `LP140WF3-SPD1`. Find a table listing
+the power sequence timing parameters, which are usually named T[N] and also
+referenced in Intel's respective registers listing. You need the values for
+`PP_ON_DELAYS`, `PP_OFF_DELAYS` and `PP_DIVISOR` for your `devicetree.cb`:
+
+```eval_rst
++-----------------------------+---------------------------------------+-----+
+| Intel docs                  | devicetree.cb                         | eDP |
++-----------------------------+---------------------------------------+-----+
+| Power up delay              | `gpu_panel_power_up_delay`            | T3  |
++-----------------------------+---------------------------------------+-----+
+| Power on to backlight on    | `gpu_panel_power_backlight_on_delay`  | T7  |
++-----------------------------+---------------------------------------+-----+
+| Power Down delay            | `gpu_panel_power_down_delay`          | T10 |
++-----------------------------+---------------------------------------+-----+
+| Backlight off to power down | `gpu_panel_power_backlight_off_delay` | T9  |
++-----------------------------+---------------------------------------+-----+
+| Power Cycle Delay           | `gpu_panel_power_cycle_delay`         | T12 |
++-----------------------------+---------------------------------------+-----+
+```
+
+Intel GPU Tools and VBT
+-----------------------
+
+The Intel GPU tools are in a package called either `intel-gpu-tools` or
+`igt-gpu-tools` in most distributions of Linux-based operating systems.
+In the coreboot `util/` directory, you can find `intelvbttool`.
+
+From a running system, you can dump the register values directly:
+```sh
+$ intel_reg dump --all | grep PCH_PP
+                      PCH_PP_STATUS (0x000c7200): 0x80000008
+                     PCH_PP_CONTROL (0x000c7204): 0x00000007
+                   PCH_PP_ON_DELAYS (0x000c7208): 0x07d00001
+                  PCH_PP_OFF_DELAYS (0x000c720c): 0x01f40001
+                     PCH_PP_DIVISOR (0x000c7210): 0x0004af06
+```
+
+You can obtain the timing values from a VBT (Video BIOS Table), which you can
+dump from a vendor UEFI image:
+```sh
+$ intel_vbt_decode data.vbt | grep T3
+                Power Sequence: T3 2000 T7 10 T9 2000 T10 500 T12 5000
+                T3 optimization: no
+```

Documentation/gfx/libgfxinit.md

@@ -55,6 +55,9 @@ follows:
 GMA: Per Board Configuration
 ----------------------------
 
+In order to set up the display panel, see the
+[display panel-specific documentation](/gfx/display-panel.md).
+
 There are a few Kconfig symbols to consider. To indicate that a
 board can initialize graphics through *libgfxinit*:
 

Documentation/index.md

@@ -173,6 +173,7 @@ Contents:
 * [Dealing with Untrusted Input in SMM](technotes/2017-02-dealing-with-untrusted-input-in-smm.md)
 * [GPIO toggling in ACPI AML](acpi/gpio.md)
 * [Native Graphics Initialization with libgfxinit](gfx/libgfxinit.md)
+* [Display panel-specific documentation](gfx/display-panel.md)
 * [Architecture-specific documentation](arch/index.md)
 * [Northbridge-specific documentation](northbridge/index.md)
 * [System on Chip-specific documentation](soc/index.md)