Re: [PATCH v2 1/2] arm64: cpufeatures: Fix handling of CONFIG_CMDLINE for idreg overrides

On Wed, Mar 3, 2021 at 7:50 AM Will Deacon [off-list ref] wrote:

The built-in kernel commandline (CONFIG_CMDLINE) can be configured in
three different ways:

  1. CMDLINE_FORCE: Use CONFIG_CMDLINE instead of any bootloader args
  2. CMDLINE_EXTEND: Append the bootloader args to CONFIG_CMDLINE
  3. CMDLINE_FROM_BOOTLOADER: Only use CONFIG_CMDLINE if there aren't
     any bootloader args.

The early cmdline parsing to detect idreg overrides gets (2) and (3)
slightly wrong: in the case of (2) the bootloader args are parsed first
and in the case of (3) the CMDLINE is always parsed.

Fix these issues by moving the bootargs parsing out into a helper
function and following the same logic as that used by the EFI stub.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Fixes: 33200303553d ("arm64: cpufeature: Add an early command-line cpufeature override facility")
Signed-off-by: Will Deacon <will@kernel.org>
---
 arch/arm64/kernel/idreg-override.c | 44 +++++++++++++++++-------------
 1 file changed, 25 insertions(+), 19 deletions(-)

It bothers me a bit having command line handling here. It means how
the command line is assembled in 2 places. I guess if we get rid of
ambiguous 'extend' then it's better, but perhaps a better
implementation would be an api get a specific command line parameter.
The main downside would be searching the DT again for each parameter
if we can't store any data in between calls, but there's ways around
that. PowerPC also needs similar functionality in
disabled_on_cmdline().

Anyways, that's all beyond the scope of this.

Rob