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 system with an integrated panel should set GFX_GMA_PANEL_1_PORT to the respective port, e.g.:

config GFX_GMA_PANEL_1_PORT
        default "DP3"

For the most common cases, LVDS and eDP, exists a shorthand, one can select either:

select GFX_GMA_PANEL_1_ON_EDP	# the default, or
select GFX_GMA_PANEL_1_ON_LVDS

Some newer chips feature a second block of panel control logic. For this, GFX_GMA_PANEL_2_PORT can be set.

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

libgfxinit needs to know which ports are implemented on a board and should be probed for displays. There are two mechanisms to constrain the list of ports to probe, 1. port presence straps on the mainboard, and 2. a list of ports provided by coreboot (see below).

Presence straps are configured via the state of certains pins of the chipset at reset time. They are documented in the chipset’s datasheets. By default, libgfxinit honors these straps for safety. However, some boards don’t implement the straps correctly. If ports are not strapped as implemented by error, one can select an option to ignore the straps:

select GFX_GMA_IGNORE_PRESENCE_STRAPS

In the opposite case, that ports are strapped as implemented, but are actually unconnected, one has to make sure that the list of ports in coreboot omits them.

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
   LVDS,
   eDP,
   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. It defines ports as follows:

ports : constant Port_List :=
  (DP2,
   DP3,
   HDMI1,
   HDMI2,
   HDMI3,
   Analog,
   LVDS,
   eDP,
   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.