[RFC PATCH v3 10/13] Documentation: Add bootsplash main documentation

[Max Staudt]

Signed-off-by: Max Staudt <mstaudt@xxxxxxx>
---
 .../ABI/testing/sysfs-platform-bootsplash          |  11 +
 Documentation/bootsplash.rst                       | 285 +++++++++++++++++++++
 MAINTAINERS                                        |   2 +
 3 files changed, 298 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-platform-bootsplash
 create mode 100644 Documentation/bootsplash.rst

diff --git a/Documentation/ABI/testing/sysfs-platform-bootsplash b/Documentation/ABI/testing/sysfs-platform-bootsplash
new file mode 100644
index 000000000000..742c7b035ded
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-platform-bootsplash
@@ -0,0 +1,11 @@
+What:		/sys/devices/platform/bootsplash.0/enabled
+Date:		Oct 2017
+KernelVersion:	4.14
+Contact:	Max Staudt <mstaudt@xxxxxxx>
+Description:
+		Can be set and read.
+
+		0: Splash is disabled.
+		1: Splash is shown whenever fbcon would show a text console
+		   (i.e. no graphical application is running), and a splash
+		   file is loaded.
diff --git a/Documentation/bootsplash.rst b/Documentation/bootsplash.rst
new file mode 100644
index 000000000000..611f0c558925
--- /dev/null
+++ b/Documentation/bootsplash.rst
@@ -0,0 +1,285 @@
+====================
+The Linux bootsplash
+====================
+
+:Date: November, 2017
+:Author: Max Staudt <mstaudt@xxxxxxx>
+
+
+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 initramfs to load.
+
+  The splash theme is loaded via request_firmware(), thus to load
+  ``/lib/firmware/bootsplash/mytheme`` pass the command line:
+
+  ``bootsplash.bootfile=bootsplash/mytheme``
+
+  Note: The splash file *has to be* in the initramfs, as it needs to be
+  available when the splash is initialized early on.
+
+  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.
+
+
+
+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.
+
+
+
+FAQ: Frequently Asked Questions
+===============================
+
+I want to see the log! How do I show the log?
+---------------------------------------------
+
+Press ESC while the splash is shown, or remove the ``bootsplash.bootfile``
+parameter from the kernel cmdline. Without that parameter, the bootsplash
+will boot disabled.
+
+
+Why use FB instead of modern DRM/KMS?
+-------------------------------------
+
+This is a semantic problem:
+ - What memory to draw the splash to?
+ - And what mode will the screen be set to?
+
+Using the fbdev emulation solves these issues.
+
+Let's start from a bare KMS system, without fbcon, and without fbdev
+emulation. In this case, as long as userspace doesn't open the KMS
+device, the state of the screen is undefined. No framebuffer is
+allocated in video RAM, and no particular mode is set.
+
+In this case, we'd have to allocate a framebuffer to show the splash,
+and set our mode ourselves. This either wastes a screenful of video RAM
+if the splash is to co-exist with the userspace program's own allocated
+framebuffer, or there is a flicker as we deactivate and delete the
+bootsplash's framebuffer and hand control over to userspace. Since we
+may set a different mode than userspace, we'd also have flicker due
+to mode switching.
+
+This logic is already contained in every KMS driver that performs fbdev
+emulation. So we might as well use that. And the correct API to do so is
+fbdev. Plus, we get compatibility with old, pure fbdev drivers for free.
+With the fbdev emulation, there is *always* a well-defined framebuffer
+to draw on. And the selection of mode has already been done by the
+graphics driver, so we don't need to reinvent that wheel, either.
+Finally, if userspace decides to use /dev/fbX, we don't have to worry
+about wasting video RAM, either.
+
+
+Why is the bootsplash integrated in fbcon?
+------------------------------------------
+
+Right now, the bootsplash is drawn from within fbcon, as this allows us
+to easily know *when* to draw - i.e. when we're safe from fbcon and
+userspace drawing all over our beautiful splash logo.
+
+Separating them is not easy - see the to-do list below.
+
+
+
+TO DO list for future development
+=================================
+
+Second enable/disable switch for the system
+-------------------------------------------
+
+It may be helpful to differentiate between the system and the user
+switching off the bootsplash. Thus, the system may make it disappear and
+reappear e.g. for a password prompt, yet once the user has pressed ESC,
+it could stay gone.
+
+
+Fix buggy DRM/KMS drivers
+-------------------------
+
+Currently, the splash code manually checks for fbdev emulation provided by
+the ast, cirrus, and mgag200 DRM/KMS drivers.
+These drivers use a manual mechanism similar to deferred I/O for their FB
+emulation, and thus need to be manually flushed onto the screen in the same
+way.
+
+This may be improved upon in several ways:
+
+1. Changing these drivers to expose the fbdev BO's memory directly, like
+   bochsdrmfb does.
+2. Creating a new fb_ops->fb_flush() API to allow the kernel to flush the
+   framebuffer once the bootsplash has been drawn into it.
+
+
+Separating from fbcon
+---------------------
+
+Separating these two components would yield independence from fbcon being
+compiled into the kernel, and thus lowering code size in embedded
+applications.
+
+To do this cleanly will involve a clean separation of users of an FB device
+within the kernel, i.e. fbcon, bootsplash, and userspace. Right now, the
+legacy fbcon code and VT code co-operate to switch between fbcon and
+userspace (by setting the VT into KD_GRAPHICS mode). Installing a muxer
+between these components ensues refactoring of old code and checking for
+correct locking.
diff --git a/MAINTAINERS b/MAINTAINERS
index 5c237445761e..7ffac272434e 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2709,6 +2709,8 @@ BOOTSPLASH
 M:	Max Staudt <mstaudt@xxxxxxx>
 L:	linux-fbdev@xxxxxxxxxxxxxxx
 S:	Maintained
+F:	Documentation/ABI/testing/sysfs-platform-bootsplash
+F:	Documentation/bootsplash.rst
 F:	drivers/video/fbdev/core/bootsplash*.*
 F:	drivers/video/fbdev/core/dummycon.c
 F:	include/linux/bootsplash.h
-- 
2.12.3