[PATCH 0/2] Fix CMDLINE_EXTEND handling for FDT "bootargs"

[Will Deacon]

On Mon, Mar 01, 2021 at 08:19:32AM -0600, Rob Herring wrote:
> On Thu, Feb 25, 2021 at 6:59 AM Will Deacon <will at kernel.org> wrote:
> > We recently [1] enabled support for CMDLINE_EXTEND on arm64, however
> > when I started looking at replacing Android's out-of-tree implementation [2]
> 
> Did anyone go read the common, reworked version of all this I
> referenced that supports prepend and append. Here it is again[1].
> Maybe I should have been more assertive there and said 'extend' is
> ambiguous.

I tried reading that, but (a) most of the series is not in the mailing list
archives and (b) the patch that _is_ doesn't touch CMDLINE_EXTEND at all.
Right now the code in mainline does the opposite of what it's documented to
do.

> > with the upstream version, I noticed that the two behave significantly
> > differently: Android follows the Kconfig help text of appending the
> > bootloader arguments to the kernel command line, whereas upstream appends
> > the kernel command line to the bootloader arguments. That is, except for
> > the EFI stub, which follows the documented behaviour.
> >
> > I think the documented behaviour is more useful, so this patch series
> > reworks the FDT code to follow that and updates the very recently merged
> > arm64 idreg early command-line parsing as well.
> 
> I can just as easily argue that the kernel having the last say makes
> sense.

Dunno, I'd say that's what CMDLINE_FORCE is for. Plus you'd be arguing
against both the documentation and the EFI stub implementation.

> Regardless, I'm pretty sure there's someone out there relying on current
> behavior. What is the impact of this change to other arches?

On arm64, I doubt it, as Android is the main user of this (where it's been
supported for 9 years with the documented behaviour).

The other option, then, is reverting CMDLINE_EXTEND from arm64 until this is
figured out. I think that's preferable to having divergent behaviour.

As for other architectures, I think the ATAGs-based solution on arch/arm/
gets it right:

  static int __init parse_tag_cmdline(const struct tag *tag)
  {
  #if defined(CONFIG_CMDLINE_EXTEND)
          strlcat(default_command_line, " ", COMMAND_LINE_SIZE);
          strlcat(default_command_line, tag->u.cmdline.cmdline,
                  COMMAND_LINE_SIZE);

For now I think we have two options for arm64: either fix the fdt code,
or revert CMDLINE_EXTEND until the PREPEND/APPEND series is merged. Which
do you prefer?

Will