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The Linux bootsplash

Date: November, 2017
Author: Max Staudt <mstaudt@suse.de>

The Linux bootsplash is a graphical replacement for the 'quiet' boot option, typically showing a logo and a spinner animation as the system starts.

Currently, it is a part of the Framebuffer Console support, and can be found as CONFIG_BOOTSPLASH in the kernel configuration. This means that as long as it is enabled, it hijacks fbcon's output and draws a splash screen instead.

Purely compiling in the bootsplash will not render it functional - to actually render a splash, you will also need a splash theme file. See the example utility and script in tools/bootsplash for a live demo.

Motivation

  • The 'quiet' boot option only suppresses most messages during boot, but errors are still shown.

  • A user space implementation can only show a logo once user space has been initialized far enough to allow this. A kernel splash can display a splash immediately as soon as fbcon can be displayed.

  • Implementing a splash screen in user space (e.g. Plymouth) is problematic due to resource conflicts.

    For example, if Plymouth is keeping /dev/fb0 (provided via vesafb/efifb) open, then most DRM drivers can't replace it because the address space is still busy - thus leading to a VRAM reservation error.

    See: https://bugzilla.opensuse.org/show_bug.cgi?id=980750

Command line arguments

bootsplash.bootfile
Which file in the initrd to load. Default: none, i.e. a non-functional splash, falling back to showing text.

sysfs run-time configuration

/sys/devices/platform/bootsplash.0/enabled
Enable/disable the bootsplash. The system boots with this set to 1, but will not show a splash unless a splash theme file is also loaded.
/sys/devices/platform/bootsplash.0/drop_splash
Unload splash data and free memory.
/sys/devices/platform/bootsplash.0/load_file
Load a splash file from /lib/firmware/. Note that trailing newlines will be interpreted as part of the file name.

Kconfig

BOOTSPLASH
Whether to compile in bootsplash support (depends on fbcon compiled in, i.e. FRAMEBUFFER_CONSOLE=y)

Bootsplash file format

A file specified in the kernel configuration as CONFIG_BOOTSPLASH_FILE or specified on the command line as bootsplash.bootfile will be loaded and displayed as soon as fbcon is initialized.

Main blocks

There are 3 main blocks in each file:

  • one File header
  • n Picture headers
  • m (Blob header + payload) blocks

Structures

The on-disk structures are defined in drivers/video/fbdev/core/bootsplash_file.h and represent these blocks:

  • struct splash_file_header

    Represents the file header, with splash-wide information including:

    • The magic string "Linux bootsplash" on big-endian platforms (the reverse on little endian)
    • The file format version (for incompatible updates, hopefully never)
    • The background color
    • Number of picture and blob blocks
    • Animation speed (we only allow one delay for all animations)

    The file header is followed by the first picture header.

  • struct splash_picture_header

    Represents an object (picture) drawn on screen, including its immutable properties:

    • Width, height
    • Positioning relative to screen corners or in the center
    • Animation, if any
    • Animation type
    • Number of blobs

    The picture header is followed by another picture header, up until n picture headers (as defined in the file header) have been read. Then, the (blob header, payload) pairs follow.

  • struct splash_blob_header (followed by payload)

    Represents one raw data stream. So far, only picture data is defined.

    The blob header is followed by a payload, then padding to n*16 bytes, then (if further blobs are defined in the file header) a further blob header.

Alignment

The bootsplash file is designed to be loaded into memory as-is.

All structures are a multiple of 16 bytes long, all elements therein are aligned to multiples of their length, and the payloads are always padded up to multiples of 16 bytes. This is to allow aligned accesses in all cases while still simply mapping the structures over an in-memory copy of the bootsplash file.

Further information

Please see drivers/video/fbdev/core/bootsplash_file.h for further details and possible values in the file.

Hooks - how the bootsplash is integrated

drivers/video/fbdev/core/fbcon.c

fbcon_init() calls bootsplash_init(), which loads the default bootsplash file or the one specified on the kernel command line.

fbcon_switch() draws the bootsplash when it's active, and is also one of the callers of set_blitting_type().

set_blitting_type() calls fbcon_set_dummyops() when the bootsplash is active, overriding the text rendering functions.

fbcon_cursor() will call bootsplash_disable() when an oops is being printed in order to make a kernel panic visible.

drivers/video/fbdev/core/dummyblit.c
This contains the dummy text rendering functions used to suppress text output while the bootsplash is shown.
drivers/tty/vt/keyboard.c
kbd_keycode() can call bootsplash_disable() when the user presses ESC or F1-F12 (changing VT). This is to provide a built-in way of disabling the splash manually at any time.