[PATCH v1 0/2] RISC-V: Fixes for riscv_has_extension[un]likely()'s alternative dependency

[Conor Dooley]

Here's my attempt at fixing both the use of an FPU on XIP kernels and
the issue that Jason ran into where CONFIG_FPU, which needs the
alternatives frame work for has_fpu() checks, could be enabled without
the alternatives actually being present.

For the former, a "slow" fallback that does not use alternatives is
added to riscv_has_extension_[un]likely() that can be used with XIP.
Obviously, we want to make use of Jisheng's alternatives based approach
where possible, so any users of riscv_has_extension_[un]likely() will
want to make sure that they select RISCV_ALTERNATIVE.
If they don't however, they'll hit the fallback path which (should,
sparing a silly mistake from me!) behave in the same way, thus
succeeding silently. Sounds like a

To prevent "depends on !XIP_KERNEL; select RISCV_ALTERNATIVE" spreading
like the plague through the various places that want to check for the
presence of extensions, and sidestep the potential silent "success"
mentioned above, all users RISCV_ALTERNATIVE are converted from selects
to dependencies, with the option being selected for all !XIP_KERNEL
builds.

I know that the VDSO was a key place that Jisheng wanted to use the new
helper rather than static branches, and I think the fallback path
should not cause issues there.

See the thread at [1] for the prior discussion.

Cheers,
Conor.

1 - https://lore.kernel.org/linux-riscv/20230128172856.3814-1-jszhang@xxxxxxxxxx/T/#m21390d570997145d31dd8bb95002fd61f99c6573

CC: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
CC: Palmer Dabbelt <palmer@xxxxxxxxxxx>
CC: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
CC: Heiko Stuebner <heiko.stuebner@xxxxxxxx>
CC: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
CC: Anup Patel <apatel@xxxxxxxxxxxxxxxx>
CC: Jisheng Zhang <jszhang@xxxxxxxxxx>
CC: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
CC: Jason A. Donenfeld <Jason@xxxxxxxxx>
CC: linux-riscv@xxxxxxxxxxxxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx

Conor Dooley (2):
  RISC-V: add non-alternative fallback for
    riscv_has_extension_[un]likely()
  RISC-V: always select RISCV_ALTERNATIVE for non-xip kernels

 arch/riscv/Kconfig             | 12 ++++----
 arch/riscv/Kconfig.erratas     |  6 ++--
 arch/riscv/include/asm/hwcap.h | 50 ++++++++++++++++++++--------------
 3 files changed, 38 insertions(+), 30 deletions(-)

-- 
2.39.2