[PATCH RESEND 18/62] doc: modernize Documentation/driver-api/early-userspace/early_userspace_support.rst

Askar Safin
2025-09-13T00:59:41Z

Update it to reflect initrd removal

Signed-off-by: Askar Safin <safinaskar@gmail.com>
---
.../early_userspace_support.rst | 18 ++++++------------
1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/Documentation/driver-api/early-userspace/early_userspace_support.rst b/Documentation/driver-api/early-userspace/early_userspace_support.rst
index 61bdeac1bae5..0ca923c1007b 100644
--- a/Documentation/driver-api/early-userspace/early_userspace_support.rst
+++ b/Documentation/driver-api/early-userspace/early_userspace_support.rst
@@ -127,28 +127,22 @@ mailing list at https://www.zytor.com/mailman/listinfo/klibc
How does it work?
=================

-The kernel has currently 3 ways to mount the root filesystem:
+The kernel has currently 2 ways to mount the root filesystem:

a) all required device and filesystem drivers compiled into the kernel, no
- initrd. init/main.c:init() will call prepare_namespace() to mount the
+ initramfs. init/main.c:kernel_init_freeable() will call prepare_namespace() to mount the
final root filesystem, based on the root= option and optional init= to run
- some other init binary than listed at the end of init/main.c:init().
+ some other init binary than listed at the end of init/main.c:kernel_init().

-b) some device and filesystem drivers built as modules and stored in an
- initrd. The initrd must contain a binary '/linuxrc' which is supposed to
- load these driver modules. It is also possible to mount the final root
- filesystem via linuxrc and use the pivot_root syscall. The initrd is
- mounted and executed via prepare_namespace().
-
-c) using initramfs. The call to prepare_namespace() must be skipped.
+b) using initramfs. The call to prepare_namespace() must be skipped.
This means that a binary must do all the work. Said binary can be stored
into initramfs either via modifying usr/gen_init_cpio.c or via the new
- initrd format, an cpio archive. It must be called "/init". This binary
+ initramfs format, an cpio archive. It must be called "/init". This binary
is responsible to do all the things prepare_namespace() would do.

To maintain backwards compatibility, the /init binary will only run if it
comes via an initramfs cpio archive. If this is not the case,
- init/main.c:init() will run prepare_namespace() to mount the final root
+ init/main.c:kernel_init_freeable() will run prepare_namespace() to mount the final root
and exec one of the predefined init binaries.

Bryan O'Sullivan <bos@serpentine.com>
--
2.47.2