Re: [PATCH v3 0/2] arch/riscv: Add bitrev.h file to support rev8 and brev8

[David Laight]

On Fri, 17 Apr 2026 12:09:03 -0400
Yury Norov <ynorov@xxxxxxxxxx> wrote:

> On Fri, Apr 17, 2026 at 05:31:00PM +0800, Jinjie Ruan wrote:
> > Add bitrev.h file to support rev8 and brev8 for riscv.
> > 
> > Tested functionally on riscv64 QEMU with:
> > 	"-M virt,acpi=on,zbkb=true,zbb=true"
> > 
> > Changes in v3:
> > - Fix the build issue by remving the CONFIG_HAVE_ARCH_BITREVERSE macro
> >   for byte_rev_table.  
> 
> No arch needs byte_rev_table, except risc-v under a very certain
> configuration. Please find a better approach that wouldn't bloat
> random victims' .data section.

Eh?
x86 doesn't have a bit-reverse instruction.
The only arch that 'select HAVE_ARCH_BITREVERSE' are arm64, arm32 (some cpu),
loongarch and mips (for CPU_MIPSR6).

I think you mean that no arch that sets CONFIG_HAVE_ARCH_BITREVERSE needs it
except riscv.

Could you globally have:
	select NEED_BYTE_REV_TABLE if !HAVE_ARCH_BITREVERSE
and then riscv could also select it?
(And isn't there a method of including files in the build based on
kconfig options rather than unconditionally compiling it and getting cpp
to throw the contents away?)

	David

> 
> > - Update the riscv implementation as David suggested.
> > - Add Reviwed-by.
> > 
> > Changes in v2:
> > - Define generic __bitrev8/16/32 for reuse in riscv.
> > 
> > Jinjie Ruan (2):
> >   bitops: Define generic __bitrev8/16/32 for reuse
> >   arch/riscv: Add bitrev.h file to support rev8 and brev8
> > 
> >  arch/riscv/Kconfig                    |  1 +
> >  arch/riscv/include/asm/bitrev.h       | 55 +++++++++++++++++++++++++++
> >  include/asm-generic/bitops/__bitrev.h | 22 +++++++++++
> >  include/linux/bitrev.h                | 20 ++--------
> >  lib/bitrev.c                          |  3 --
> >  5 files changed, 82 insertions(+), 19 deletions(-)
> >  create mode 100644 arch/riscv/include/asm/bitrev.h
> >  create mode 100644 include/asm-generic/bitops/__bitrev.h
> > 
> > -- 
> > 2.34.1