[PATCH] initramfs: Fix disabling of initramfs (and its compression)

[Florian Fainelli]

Commit db2aa7fd15e8 ("initramfs: allow again choice of the embedded
initram compression algorithm") introduced the possibility to select the
initramfs compression algorithm from Kconfig and while this is a nice
feature it broke the use case described below.

Here is what my build system does:

- kernel is initially configured not to have an initramfs included
- build the user space root file system
- re-configure the kernel to have an initramfs included
(CONFIG_INITRAMFS_SOURCE="/path/to/romfs") and set relevant
CONFIG_INITRAMFS options, in my case, no compression option
(CONFIG_INITRAMFS_COMPRESSION_NONE)
- kernel is re-built with these options -> kernel+initramfs image is
  copied
- kernel is re-built again without these options -> kernel image is
  copied

Building a kernel without an initramfs means setting this option:

CONFIG_INITRAMFS_SOURCE="" (and this one only)

whereas building a kernel with an initramfs means setting these options:

CONFIG_INITRAMFS_SOURCE="/home/fainelli/work/uclinux-rootfs/romfs
/home/fainelli/work/uclinux-rootfs/misc/initramfs.dev"
CONFIG_INITRAMFS_ROOT_UID=1000
CONFIG_INITRAMFS_ROOT_GID=1000
CONFIG_INITRAMFS_COMPRESSION_NONE=y
CONFIG_INITRAMFS_COMPRESSION=""

Commit db2aa7fd15e857891cefbada8348c8d938c7a2bc ("initramfs: allow again
choice of the embedded initram compression algorithm") is problematic
because CONFIG_INITRAMFS_COMPRESSION which is used to determine the
initramfs_data.cpio extension/compression is a string, and due to how
Kconfig works it will evaluate in order, how to assign it.

Setting CONFIG_INITRAMFS_COMPRESSION_NONE with
CONFIG_INITRAMFS_SOURCE="" cannot possibly work (because of the depends
on INITRAMFS_SOURCE!="" imposed on CONFIG_INITRAMFS_COMPRESSION ) yet we
still get CONFIG_INITRAMFS_COMPRESSION assigned to ".gz" because
CONFIG_RD_GZIP=y is set in my kernel, even when there is no initramfs
being built.

So we basically end-up generating two initramfs_data.cpio* files, one
without extension, and one with .gz. This causes usr/Makefile to track
usr/initramfs_data.cpio.gz, and not usr/initramfs_data.cpio anymore,
that is also largely problematic after
9e3596b0c6539e28546ff7c72a06576627068353 ("kbuild: initramfs cleanup,
set target from Kconfig") because we used to track all possible
initramfs_data files in the $(targets) variable before that commit.

The end result is that the kernel with an initramfs clearly does not
contain what we expect it to, it has a stale initramfs_data.cpio file
built into it, and we keep re-generating an initramfs_data.cpio.gz file
which is not the one that we want to include in the kernel image proper.

The fix consists in hiding CONFIG_INITRAMFS_COMPRESSION when
CONFIG_INITRAMFS_SOURCE="". This puts us back in a state to the pre-4.10
behavior where we can properly disable and re-enable initramfs within
the same kernel .config file, and be in control of what
CONFIG_INITRAMFS_COMPRESSION is set to.

Fixes: db2aa7fd15e8 ("initramfs: allow again choice of the embedded initram compression algorithm")
Fixes: 9e3596b0c653 ("kbuild: initramfs cleanup, set target from Kconfig")
Signed-off-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
---
 usr/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/usr/Kconfig b/usr/Kconfig
index c0c48507e44e..ad0543e21760 100644
--- a/usr/Kconfig
+++ b/usr/Kconfig
@@ -220,6 +220,7 @@ config INITRAMFS_COMPRESSION_LZ4
 endchoice
 
 config INITRAMFS_COMPRESSION
+	depends on INITRAMFS_SOURCE!=""
 	string
 	default ""      if INITRAMFS_COMPRESSION_NONE
 	default ".gz"   if INITRAMFS_COMPRESSION_GZIP
-- 
2.9.3