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.
- 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

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.

> - 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

On Fri, 17 Apr 2026 12:09:03 -0400
Yury Norov <ynorov@nvidia.com> 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

On 4/18/2026 2:42 AM, David Laight wrote:
> On Fri, 17 Apr 2026 12:09:03 -0400
> Yury Norov <ynorov@nvidia.com> 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?

Thanks, David. I will follow your suggestion to introduce a
NEED_BYTE_REVERSE_TABLE Kconfig option.

This way, byte_rev_table is only compiled when !HAVE_ARCH_BITREVERSE or
when an architecture (like RISC-V) explicitly selects it as a fallback.
This avoids bloating the .data section for architectures that have full
hardware bit-reverse support and don't need the table."

> (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  
>