coreboot-kgpe-d16/Documentation/gfx/libgfxinit.md

libgfxinit - Native Graphics Initialization

Introduction and Current State in coreboot

libgfxinit is a library of full-featured graphics initialization (aka. modesetting) drivers. It's implemented in SPARK (a subset of Ada with formal verification features). While not restricted to in any way, it currently only supports Intel's integrated graphics controllers (GMA).

Currently, it supports the Intel Core i3/i5/i7 processor line, HDMI and DP on the Apollo Lake processors and everything but SDVO on G45 and GM45 chipsets. At the time of writing, G45, GM45, everything from Arrandale to Coffee Lake, and Apollo Lake are verified to work within coreboot.

GMA: Framebuffer Configuration

coreboot supports two different framebuffer setups. The default enables the legacy VGA plane in textmode. Due to legacy hardware constraints, only the first found display is enabled in this mode. (cf. src/drivers/intel/gma/text_fb/gma.adb).

The second option sets up a high-resolution framebuffer with the native resolution of the display if only one is detected, or the smallest of all resolutions (per dimension) if multiple displays are detected. This option is selected by CONFIG_FRAMEBUFFER_KEEP_VESA_MODE. (cf. src/drivers/intel/gma/hires_fb/gma.adb).

In any case, a smaller framebuffer is up-scaled to each display's native resolution while keeping aspect ratio.

GMA: Hook-up in Chipset Initialization

Both configurations described above implement a procedure GMA.gfxinit():

procedure gfxinit (lightup_ok : out int);

This procedure is exported as the C function gma_gfxinit() as follows:

void gma_gfxinit(int *lightup_ok);

• lightup_ok: returns whether the initialization succeeded 1 or failed 0. Currently, only the case that no display could be found counts as failure. A failure at a later stage (e.g. failure to train a DP) is not propagated.

GMA: Per Board Configuration

In order to set up the display panel, see the display panel-specific documentation.

There are a few Kconfig symbols to consider. To indicate that a board can initialize graphics through libgfxinit:

select MAINBOARD_HAS_LIBGFXINIT

Internal ports share some hardware blocks (e.g. backlight, panel power sequencer). Therefore, each board has to select either eDP or LVDS as the internal port, if any:

select GFX_GMA_INTERNAL_IS_EDP	# the default, or
select GFX_GMA_INTERNAL_IS_LVDS

Boards with a DVI-I connector share the DDC (I2C) pins for both analog and digital displays. In this case, libgfxinit needs to know through which interface the EDID can be queried:

select GFX_GMA_ANALOG_I2C_HDMI_B	# or
select GFX_GMA_ANALOG_I2C_HDMI_C	# or
select GFX_GMA_ANALOG_I2C_HDMI_D

Beside Kconfig options, libgfxinit needs to know which ports are implemented on a board and should be probed for displays. The mapping between the physical ports and these entries depends on the hardware implementation and can be recovered by testing or studying the output of intelvbttool or intel_vbt_decode. Each board has to implement the package GMA.Mainboard with a list:

ports : HW.GFX.GMA.Display_Probing.Port_List;

or a function returning such a list:

function ports return HW.GFX.GMA.Display_Probing.Port_List;

You can select from the following Ports:

type Port_Type is
  (Disabled,	-- optionally terminates the list
   Internal,	-- either eDP or LVDS as selected in Kconfig
   DP1,
   DP2,
   DP3,
   HDMI1,		-- also DVI-D, or HDMI over DP++
   HDMI2,
   HDMI3,
   Analog);		-- legacy VGA port, or analog part of DVI-I

Each DPx and HDMIx pair share pins. If they are exposed as DP ports, they are usually DP++ (aka. dual-mode DP) ports that can also output HDMI signals through passive adapters. In this case, both DPx and HDMIx should be listed.

A good example is the mainboard Kontron/KTQM77, it features two DP++ ports (DP2/HDMI2, DP3/HDMI3), one DVI-I port (HDMI1/Analog), eDP and LVDS. Due to the constraints mentioned above, only one of eDP and LVDS can be enabled. It defines ports as follows:

ports : constant Port_List :=
  (DP2,
   DP3,
   HDMI1,
   HDMI2,
   HDMI3,
   Analog,
   Internal,
   others => Disabled);

The GMA.gfxinit() procedure probes for display EDIDs in the given order until all available pipes are taken. That's 1 pipe in VGA textmode, 2 pipes in high-resolution mode until Sandy Bridge, 3 pipes from Ivy Bridge on.