[PATCH 11/11] bootm: Support string substitution in bootargs

[Rasmus Villemoes]

On 19/10/2020 15.56, Simon Glass wrote:
> In some cases it is necessary to pass parameters to Linux so that it will
> boot correctly. For example, the rootdev parameter is often used to
> specify the root device. However the root device may change depending on
> whence U-Boot loads the kernel. At present it is necessary to build up
> the command line by adding device strings to it one by one.
> 
> It is often more convenient to provide a template for bootargs, with
> U-Boot doing the substitution from other environment variables.
> 
> Add a way to substitute strings in the bootargs variable. This allows
> things like "rootdev=%U" to be used in bootargs, with the %U substitution
> providing the UUID of the root device.
> 
> For example, to substitute the GUID of the kernel partition:
> 
>   setenv bootargs "console=/dev/ttyS0 rootdev=%U/PARTNROFF=1 kern_guid=%U"
>   part uuid mmc 2:2 uuid
>   setenv bootargs_U $uuid
>   bootm
> 
> This is particularly useful when the command line from another place. For
> example, Chrome OS stores the command line next to the kernel itself. It
> depends on the kernel version being used as well as the hardware features,
> so it is extremely difficult to devise a U-Boot script that works on all
> boards and kernel versions. With this feature, the command line can be
> read from disk and used directly, with a few substitutions set up.
> 
> Signed-off-by: Simon Glass <sjg at chromium.org>
> ---
> 
>  README              |  16 +++++++
>  arch/Kconfig        |   1 +
>  common/Kconfig.boot |  20 ++++++++
>  common/bootm.c      |  72 +++++++++++++++++++++++++++--
>  include/bootm.h     |  14 ++++--
>  test/bootm.c        | 110 ++++++++++++++++++++++++++++++++++++++++++--
>  6 files changed, 221 insertions(+), 12 deletions(-)
> 
> diff --git a/README b/README
> index 91c5a1a8fa3..263e31ab7f6 100644
> --- a/README
> +++ b/README
> @@ -3229,6 +3229,22 @@ List of environment variables (most likely not complete):
>  
>    bootargs	- Boot arguments when booting an RTOS image
>  
> +  bootargs_subst - Substitution parameters for bootargs. These are applied after
> +		  the commandline has been built. The format is:
> +
> +		    <key>=<value>[!<key>=<value>...]
> +
> +		  where
> +		    <key> is a string to replace
> +		    <value> is the value to replace it with (either a simple
> +		       string or an environment variable starting with $
> +
> +		  One use for this is to insert the root-disk UUID into the
> +		  command line where bootargs contains "root=%U"
> +
> +		    part uuid mmc 2:2 uuid
> +		    setenv cmdline_repl %U=$uuid

cmdline_repl seems to be stale, it should be bootargs_subst. But the
whole paragraph seems stale, as you actually implement and test the
bootargs_X approach.

Anyway, this does seem useful, but I really question the choice of
leaving %A in there if bootargs_A does not exist. It's hard if not
impossible to create an environment variable whose value is empty, and I
can easily imagine it would be useful to allow a %A to expand to
nothing. So why not just use the usual semantics of requiring a double
%% to put a single % in the output? It's probably quite rare that one
would need that anyway.

Rasmus