[PATCH v2 03/10] common: bootargs: drop legacy bootargs fallback with FLEXIBLE_BOOTARGS

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

linux_bootargs_get() falls back to the legacy bootargs environment variable
whenever the concatenation of all global.linux.bootargs.* variables comes
out empty. That contradicts the documentation, which describes bootargs as
the CONFIG_FLEXIBLE_BOOTARGS=n way of passing a command line, and it only
worked until the first boot entry had run: bootscript_boot() registers
global.linux.bootargs.dyn.ip and .dyn.root, and the separator between the
two empty variables made the result non-empty, so the kernel got a command
line consisting of spaces instead.

Drop the fallback and return NULL when there is nothing to pass. All
callers already handle a NULL command line, as the fallback could return
NULL as well.

Assisted-by: Claude:opus-5
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 .../migration-guides/migration-master.rst        | 16 ++++++++++++++++
 Documentation/user/booting-linux.rst             |  6 ++++--
 common/bootargs.c                                | 14 ++++++--------
 3 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/Documentation/migration-guides/migration-master.rst b/Documentation/migration-guides/migration-master.rst
index 43d0b27bb0fe..815d1732cee4 100644
--- a/Documentation/migration-guides/migration-master.rst
+++ b/Documentation/migration-guides/migration-master.rst
@@ -27,3 +27,19 @@ Deep probe is now the default on STM32MP13 and STM32MP15
 barebox now enables deep probe for every ``st,stm32mp1xx`` compatible, even
 over ``barebox,disable-deep-probe``. Report to the mailing list if that
 breaks your board.
+
+Legacy bootargs variable ignored with CONFIG_FLEXIBLE_BOOTARGS
+--------------------------------------------------------------
+
+With ``CONFIG_FLEXIBLE_BOOTARGS`` enabled, the kernel command line used to
+fall back to the legacy ``bootargs`` environment variable whenever the
+concatenation of all ``global.linux.bootargs.*`` variables came out empty.
+This fallback is gone, only the global variables are used now.
+
+The fallback was already mostly unreachable: once a boot entry had run,
+``global.linux.bootargs.dyn.ip`` and ``global.linux.bootargs.dyn.root`` were
+registered and the separator between the two empty variables made the
+concatenation non-empty.
+
+Set ``global.linux.bootargs.base`` instead of ``bootargs``, or disable
+``CONFIG_FLEXIBLE_BOOTARGS`` to keep using the legacy variable.
diff --git a/Documentation/user/booting-linux.rst b/Documentation/user/booting-linux.rst
index 529d4de5c5b8..1608fc1ea431 100644
--- a/Documentation/user/booting-linux.rst
+++ b/Documentation/user/booting-linux.rst
@@ -86,8 +86,10 @@ The simple method to pass bootargs to the kernel is with
 takes the bootargs from the :ref:`bootargs <magicvar_bootargs>` environment variable.
 
 With ``CONFIG_FLEXIBLE_BOOTARGS`` enabled, the bootargs are composed
-from different :ref:`global device<global_device>` variables. All variables beginning
-with ``global.linux.bootargs.`` will be concatenated to the bootargs:
+from different :ref:`global device<global_device>` variables and the
+:ref:`bootargs <magicvar_bootargs>` environment variable is ignored.
+All variables beginning with ``global.linux.bootargs.`` will be concatenated
+to the bootargs:
 
 .. code-block:: sh
 
diff --git a/common/bootargs.c b/common/bootargs.c
index 6373d9ccb1f7..91e92e594876 100644
--- a/common/bootargs.c
+++ b/common/bootargs.c
@@ -17,12 +17,10 @@ static int linux_bootargs_overwritten;
 /*
  * This returns the Linux bootargs
  *
- * There are two ways to handle bootargs. The old legacy way is to use the
- * 'bootargs' environment variable. The new and more flexible way is to use
- * global variables beginning with "global.linux.bootargs." and
- * "global.linux.mtdparts.". These variables will be concatenated together to
- * the resulting bootargs. If there are no "global.linux.bootargs." variables
- * we fall back to "bootargs"
+ * The bootargs are concatenated from the global variables beginning with
+ * "global.linux.bootargs.", "global.linux.mtdparts." and
+ * "global.linux.blkdevparts.". The legacy 'bootargs' environment variable
+ * is only used by the CONFIG_FLEXIBLE_BOOTARGS=n stub in <bootargs.h>.
  */
 const char *linux_bootargs_get(void)
 {
@@ -35,9 +33,9 @@ const char *linux_bootargs_get(void)
 	linux_bootargs = NULL;
 
 	bootargs = globalvar_get_match("linux.bootargs.", " ");
-	if (!strlen(bootargs)) {
+	if (!*bootargs) {
 		free(bootargs);
-		return getenv("bootargs");
+		return NULL;
 	}
 
 	linux_bootargs = bootargs;
-- 
2.47.3