arch/riscv: Add bitrev.h file to support rev8 and brev8

PATCH Applied Jinjie Ruan 3 participants 10 messages Apr 21 – Apr 30

Subsystems (5)

Network drivers Library code RISC-V Generic include/asm header files +1 more subsystems

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lore

Jinjie Ruan

author

2026-04-21 13:04 UTC #0 lore raw

v5 cover arch/riscv: Add bitrev.h file to support rev8 and brev8

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 v5:
- Rename NEED_BYTE_REV_TABLE to GENERIC_BITREVERSE.
- Add help text and make GENERIC_BITREVERSE depends on BITREVERSE, and use def_bool.
- Split into three patches.
- Add suggested-by.

Changes in v4:
- Update the riscv implementation as David suggested.
- Add new config called NEED_BYTE_REV_TABLE as David suggested to avoid bloating the .data section for architectures that have full hardware bit-reverse support and don't need the table.

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 (3):
lib/bitrev: Introduce GENERIC_BITREVERSE and cleanup Kconfig
bitops: Define generic __bitrev8/16/32 for reuse arch/riscv: Add bitrev.h file to support rev8 and brev8

 arch/riscv/Kconfig                    |  2 ++
 arch/riscv/include/asm/bitrev.h       | 51 +++++++++++++++++++++++++++
 include/asm-generic/bitops/__bitrev.h | 25 +++++++++++++
 include/linux/bitrev.h                | 20 +++--------
 lib/Kconfig                           | 18 ++++++++++
 lib/bitrev.c                          |  4 +--
 6 files changed, 102 insertions(+), 18 deletions(-)
 create mode 100644 arch/riscv/include/asm/bitrev.h
 create mode 100644 include/asm-generic/bitops/__bitrev.h

Signature · 1 line

2.34.1

Jinjie Ruan

author

2026-04-21 13:04 UTC #1 lore raw

PATCH v5 1/3 lib/bitrev: Introduce GENERIC_BITREVERSE and cleanup Kconfig

Currently, the bit reversal lookup table is controlled by !HAVE_ARCH_BITREVERSE. This makes it difficult for architectures to provide a hardware-accelerated implementation while still falling back to the generic table for specific configurations.

Introduce CONFIG_GENERIC_BITREVERSE to explicitly manage the generic lookup table implementation. By using 'def_bool !HAVE_ARCH_BITREVERSE' with a dependency on 'BITREVERSE', we ensure that:

1. The table is only compiled when needed.
2. The .config is not polluted with useless options when BITREVERSE is disabled.
3. Avoids bloating the .data section for architectures that have full hardware bit-reverse support and don't need the table.

Update lib/bitrev.c to use CONFIG_GENERIC_BITREVERSE instead of checking the absence of HAVE_ARCH_BITREVERSE. This provides a cleaner interface for architectures like RISC-V that may want to selectively use the generic implementation as a fallback.

Signed-off-by Jinjie Ruan <[email protected]>

---
 lib/Kconfig  | 18 ++++++++++++++++++
 lib/bitrev.c |  4 ++--
 2 files changed, 20 insertions(+), 2 deletions(-)

lib/Kconfig +18 -0

index 0f2fb9610647..e3f9719a4d6f 100644
@@ -62,6 +62,24 @@ config HAVE_ARCH_BITREVERSE
 	  This option enables the use of hardware bit-reversal instructions on
 	  architectures which support such operations.
 
+config GENERIC_BITREVERSE
+	def_bool !HAVE_ARCH_BITREVERSE
+	depends on BITREVERSE
+	help
+	  This option provides the standard software-based bit reversal
+	  implementation using a lookup table.
+
+	  Architecture-specific implementations (HAVE_ARCH_BITREVERSE)
+	  and this generic version are not necessarily mutually exclusive
+	  at the configuration level, but selecting this ensures that
+	  the generic `bitrev8/16/32` functions are available when the
+	  CPU does not provide native instructions (like RISC-V's ZBKB
+	  extension).
+
+	  If you are an architecture maintainer and your CPU has native
+	  bit-reversal instructions, you should select HAVE_ARCH_BITREVERSE
+	  to skip this table-based implementation.
+
 config ARCH_HAS_STRNCPY_FROM_USER
 	bool

lib/bitrev.c +2 -2

index 81b56e0a7f32..3a53ff67aeba 100644
@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0-only
-#ifndef CONFIG_HAVE_ARCH_BITREVERSE
+#ifdef CONFIG_GENERIC_BITREVERSE
 #include <linux/types.h>
 #include <linux/module.h>
 #include <linux/bitrev.h>

@@ -44,4 +44,4 @@ const u8 byte_rev_table[256] = {
 };
 EXPORT_SYMBOL_GPL(byte_rev_table);
 
-#endif /* CONFIG_HAVE_ARCH_BITREVERSE */
+#endif /* CONFIG_GENERIC_BITREVERSE */

Signature · 1 line

2.34.1

Jinjie Ruan

author

2026-04-21 13:04 UTC #2 lore raw

PATCH v5 2/3 bitops: Define generic __bitrev8/16/32 for reuse

In reply to Jinjie Ruan

Define generic __bitrev8/16/32 using the implementation in <linux/bitrev.h>, so they can be reused in <asm/bitrev.h>, such as RISCV.

Reviewed-by Yury Norov <[email protected]>

Signed-off-by Jinjie Ruan <[email protected]>

---
 include/asm-generic/bitops/__bitrev.h | 25 +++++++++++++++++++++++++
 include/linux/bitrev.h                | 20 ++++----------------
 2 files changed, 29 insertions(+), 16 deletions(-)
 create mode 100644 include/asm-generic/bitops/__bitrev.h

include/asm-generic/bitops/__bitrev.h new file +25 -0

new file mode 100644
index 000000000000..f06af929678d
@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_BITOPS___BITREV_H_
+#define _ASM_GENERIC_BITOPS___BITREV_H_
+
+#ifdef CONFIG_GENERIC_BITREVERSE
+#include <asm/types.h>
+
+extern u8 const byte_rev_table[256];
+static __always_inline __attribute_const__ u8 generic___bitrev8(u8 byte)
+{
+	return byte_rev_table[byte];
+}
+
+static __always_inline __attribute_const__ u16 generic___bitrev16(u16 x)
+{
+	return (generic___bitrev8(x & 0xff) << 8) | generic___bitrev8(x >> 8);
+}
+
+static __always_inline __attribute_const__ u32 generic___bitrev32(u32 x)
+{
+	return (generic___bitrev16(x & 0xffff) << 16) | generic___bitrev16(x >> 16);
+}
+#endif /* CONFIG_GENERIC_BITREVERSE */
+
+#endif /* _ASM_GENERIC_BITOPS___BITREV_H_ */

include/linux/bitrev.h +4 ~~-16~~

index d35b8ec1c485..11620a70e776 100644
@@ -12,22 +12,10 @@
 #define __bitrev8 __arch_bitrev8
 
 #else
-extern u8 const byte_rev_table[256];
-static inline u8 __bitrev8(u8 byte)
-{
-	return byte_rev_table[byte];
-}
-
-static inline u16 __bitrev16(u16 x)
-{
-	return (__bitrev8(x & 0xff) << 8) | __bitrev8(x >> 8);
-}
-
-static inline u32 __bitrev32(u32 x)
-{
-	return (__bitrev16(x & 0xffff) << 16) | __bitrev16(x >> 16);
-}
-
+#include <asm-generic/bitops/__bitrev.h>
+#define __bitrev32 generic___bitrev32
+#define __bitrev16 generic___bitrev16
+#define __bitrev8 generic___bitrev8
 #endif /* CONFIG_HAVE_ARCH_BITREVERSE */
 
 #define __bitrev8x4(x)	(__bitrev32(swab32(x)))

Signature · 1 line

2.34.1

Nathan Chancellor

2026-04-29 20:29 UTC #3 lore raw

Re: [PATCH v5 2/3] bitops: Define generic __bitrev8/16/32 for reuse

Hi Jinjie,

Quoted 18 lines · Jinjie Ruan

Define generic __bitrev8/16/32 using the implementation in <linux/bitrev.h>, so they can be reused in <asm/bitrev.h>, such as RISCV.
Reviewed-by Yury Norov <[email protected]>
Signed-off-by Jinjie Ruan <[email protected]>
---
 include/asm-generic/bitops/__bitrev.h | 25 +++++++++++++++++++++++++
 include/linux/bitrev.h                | 20 ++++----------------
 2 files changed, 29 insertions(+), 16 deletions(-)
 create mode 100644 include/asm-generic/bitops/__bitrev.h
include/asm-generic/bitops/__bitrev.h new file +0 -0
new file mode 100644
index 000000000000..f06af929678d

include/asm-generic/bitops/__bitrev.h line 5

Quoted 5 lines

include/asm-generic/bitops/__bitrev.h new file +5 -0

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_BITOPS___BITREV_H_
+#define _ASM_GENERIC_BITOPS___BITREV_H_
+
+#ifdef CONFIG_GENERIC_BITREVERSE

The dependencies of this symbol seem insufficient, as I can trigger a build failure on next-20260429 like so:

  $ make -skj"$(nproc)" ARCH=s390 CROSS_COMPILE=s390-linux- mrproper tinyconfig fs/select.o
  In file included from include/linux/crc32.h:6,
                   from include/linux/etherdevice.h:23,
                   from include/linux/if_vlan.h:11,
                   from include/linux/filter.h:21,
                   from include/net/xdp.h:10,
                   from include/net/busy_poll.h:19,
                   from fs/select.c:33:
  include/linux/etherdevice.h: In function 'eth_hw_addr_crc':
  include/linux/bitrev.h:16:20: error: implicit declaration of function 'generic___bitrev32' [-Wimplicit-function-declaration]
     16 | #define __bitrev32 generic___bitrev32
        |                    ^~~~~~~~~~~~~~~~~~
  include/linux/bitrev.h:67:9: note: in expansion of macro '__bitrev32'
     67 |         __bitrev32(__x);                                \
        |         ^~~~~~~~~~
  include/linux/crc32.h:107:36: note: in expansion of macro 'bitrev32'
    107 | #define ether_crc(length, data)    bitrev32(crc32_le(~0, data, length))
        |                                    ^~~~~~~~
  include/linux/etherdevice.h:292:16: note: in expansion of macro 'ether_crc'
    292 |         return ether_crc(ETH_ALEN, ha->addr);
        |                ^~~~~~~~~
  make[5]: *** [scripts/Makefile.build:289: fs/select.o] Error 1
  ...

  $ scripts/config -s BITREVERSE
  undef

  $ rg BITREVERSE .config

Quoted 54 lines

include/asm-generic/bitops/__bitrev.h new file +20 -0

+#include <asm/types.h>
+
+extern u8 const byte_rev_table[256];
+static __always_inline __attribute_const__ u8 generic___bitrev8(u8 byte)
+{
+	return byte_rev_table[byte];
+}
+
+static __always_inline __attribute_const__ u16 generic___bitrev16(u16 x)
+{
+	return (generic___bitrev8(x & 0xff) << 8) | generic___bitrev8(x >> 8);
+}
+
+static __always_inline __attribute_const__ u32 generic___bitrev32(u32 x)
+{
+	return (generic___bitrev16(x & 0xffff) << 16) | generic___bitrev16(x >> 16);
+}
+#endif /* CONFIG_GENERIC_BITREVERSE */
+
+#endif /* _ASM_GENERIC_BITOPS___BITREV_H_ */

include/linux/bitrev.h +4 ~~-16~~

index d35b8ec1c485..11620a70e776 100644
@@ -12,22 +12,10 @@
 #define __bitrev8 __arch_bitrev8
 
 #else
-extern u8 const byte_rev_table[256];
-static inline u8 __bitrev8(u8 byte)
-{
-	return byte_rev_table[byte];
-}
-
-static inline u16 __bitrev16(u16 x)
-{
-	return (__bitrev8(x & 0xff) << 8) | __bitrev8(x >> 8);
-}
-
-static inline u32 __bitrev32(u32 x)
-{
-	return (__bitrev16(x & 0xffff) << 16) | __bitrev16(x >> 16);
-}
-
+#include <asm-generic/bitops/__bitrev.h>
+#define __bitrev32 generic___bitrev32
+#define __bitrev16 generic___bitrev16
+#define __bitrev8 generic___bitrev8
 #endif /* CONFIG_HAVE_ARCH_BITREVERSE */
 
 #define __bitrev8x4(x)	(__bitrev32(swab32(x)))

Signature · 1 line

2.34.1

Signature · 2 lines

Cheers,
Nathan

Yury Norov

2026-04-30 01:47 UTC #4 lore raw

Re: [PATCH v5 2/3] bitops: Define generic __bitrev8/16/32 for reuse

+ networking maintainers

Quoted 54 lines · Nathan Chancellor

Hi Jinjie,

Quoted 18 lines · Jinjie Ruan

Define generic __bitrev8/16/32 using the implementation in <linux/bitrev.h>, so they can be reused in <asm/bitrev.h>, such as RISCV.
Reviewed-by Yury Norov <[email protected]>
Signed-off-by Jinjie Ruan <[email protected]>
---
 include/asm-generic/bitops/__bitrev.h | 25 +++++++++++++++++++++++++
 include/linux/bitrev.h                | 20 ++++----------------
 2 files changed, 29 insertions(+), 16 deletions(-)
 create mode 100644 include/asm-generic/bitops/__bitrev.h
include/asm-generic/bitops/__bitrev.h new file +0 -0
new file mode 100644
index 000000000000..f06af929678d

include/asm-generic/bitops/__bitrev.h line 5

Quoted 5 lines

include/asm-generic/bitops/__bitrev.h new file +5 -0

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_BITOPS___BITREV_H_
+#define _ASM_GENERIC_BITOPS___BITREV_H_
+
+#ifdef CONFIG_GENERIC_BITREVERSE

The dependencies of this symbol seem insufficient, as I can trigger a build failure on next-20260429 like so:

  $ make -skj"$(nproc)" ARCH=s390 CROSS_COMPILE=s390-linux- mrproper tinyconfig fs/select.o
  In file included from include/linux/crc32.h:6,
                   from include/linux/etherdevice.h:23,
                   from include/linux/if_vlan.h:11,
                   from include/linux/filter.h:21,
                   from include/net/xdp.h:10,
                   from include/net/busy_poll.h:19,
                   from fs/select.c:33:
  include/linux/etherdevice.h: In function 'eth_hw_addr_crc':
  include/linux/bitrev.h:16:20: error: implicit declaration of function 'generic___bitrev32' [-Wimplicit-function-declaration]
     16 | #define __bitrev32 generic___bitrev32
        |                    ^~~~~~~~~~~~~~~~~~
  include/linux/bitrev.h:67:9: note: in expansion of macro '__bitrev32'
     67 |         __bitrev32(__x);                                \
        |         ^~~~~~~~~~
  include/linux/crc32.h:107:36: note: in expansion of macro 'bitrev32'
    107 | #define ether_crc(length, data)    bitrev32(crc32_le(~0, data, length))
        |                                    ^~~~~~~~
  include/linux/etherdevice.h:292:16: note: in expansion of macro 'ether_crc'
    292 |         return ether_crc(ETH_ALEN, ha->addr);
        |                ^~~~~~~~~
  make[5]: *** [scripts/Makefile.build:289: fs/select.o] Error 1
  ...

Quoted 4 lines

  $ scripts/config -s BITREVERSE
  undef

  $ rg BITREVERSE .config

Confirm the same for x86 tinyconfig.

The problem is that the patch makes generic bitrevXX() conditional on CONFIG_GENERIC_BITREVERSE, while before they were conditional on !CONFIG_HAVE_ARCH_BITREVERSE. So if you don't have arch bitreverse(), and dont' enable BITREVERSE, the generic implementation is not defined now.

Luckily, the only user of bitrev() in unconditionally compiled objects is CRC32, and it's not needed if CRC32 is disabled.

This is the minimal working fix for me. Please let me know what do you think. I can prepend Jinjie's pathes in my tree with it if it's OK for you.

Thanks,
Yury

include/linux/etherdevice.h +4 -0

index df8f88f63a70..245b206dd38b 100644
@@ -20,7 +20,9 @@
 #include <linux/if_ether.h>
 #include <linux/netdevice.h>
 #include <linux/random.h>
+#ifdef CONFIG_CRCC32
 #include <linux/crc32.h>
+#endif
 #include <linux/unaligned.h>
 #include <asm/bitsperlong.h>

@@ -281,6 +283,7 @@ static inline void eth_hw_addr_random(struct net_device *dev)
 	dev->addr_assign_type = NET_ADDR_RANDOM;
 }
 
+#ifdef CONFIG_CRCC32
 /**
  * eth_hw_addr_crc - Calculate CRC from netdev_hw_addr
  * @ha: pointer to hardware address

@@ -291,6 +294,7 @@ static inline u32 eth_hw_addr_crc(struct netdev_hw_addr *ha)
 {
 	return ether_crc(ETH_ALEN, ha->addr);
 }
+#endif
 
 /**
  * ether_addr_copy - Copy an Ethernet address

Nathan Chancellor

2026-04-30 03:59 UTC #5 lore raw

Re: [PATCH v5 2/3] bitops: Define generic __bitrev8/16/32 for reuse

Quoted 70 lines · Yury Norov

+ networking maintainers
Quoted 54 lines · Nathan Chancellor
Hi Jinjie,
Quoted 18 lines · Jinjie Ruan
Define generic __bitrev8/16/32 using the implementation in <linux/bitrev.h>, so they can be reused in <asm/bitrev.h>, such as RISCV.
Reviewed-by Yury Norov <[email protected]>
Signed-off-by Jinjie Ruan <[email protected]>
---
 include/asm-generic/bitops/__bitrev.h | 25 +++++++++++++++++++++++++
 include/linux/bitrev.h                | 20 ++++----------------
 2 files changed, 29 insertions(+), 16 deletions(-)
 create mode 100644 include/asm-generic/bitops/__bitrev.h
include/asm-generic/bitops/__bitrev.h new file +0 -0
new file mode 100644
index 000000000000..f06af929678d
include/asm-generic/bitops/__bitrev.h line 5
Quoted 5 lines
include/asm-generic/bitops/__bitrev.h new file +5 -0
@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_BITOPS___BITREV_H_
+#define _ASM_GENERIC_BITOPS___BITREV_H_
+
+#ifdef CONFIG_GENERIC_BITREVERSE
The dependencies of this symbol seem insufficient, as I can trigger a build failure on next-20260429 like so:
  $ make -skj"$(nproc)" ARCH=s390 CROSS_COMPILE=s390-linux- mrproper tinyconfig fs/select.o
  In file included from include/linux/crc32.h:6,
                   from include/linux/etherdevice.h:23,
                   from include/linux/if_vlan.h:11,
                   from include/linux/filter.h:21,
                   from include/net/xdp.h:10,
                   from include/net/busy_poll.h:19,
                   from fs/select.c:33:
  include/linux/etherdevice.h: In function 'eth_hw_addr_crc':
  include/linux/bitrev.h:16:20: error: implicit declaration of function 'generic___bitrev32' [-Wimplicit-function-declaration]
     16 | #define __bitrev32 generic___bitrev32
        |                    ^~~~~~~~~~~~~~~~~~
  include/linux/bitrev.h:67:9: note: in expansion of macro '__bitrev32'
     67 |         __bitrev32(__x);                                \
        |         ^~~~~~~~~~
  include/linux/crc32.h:107:36: note: in expansion of macro 'bitrev32'
    107 | #define ether_crc(length, data)    bitrev32(crc32_le(~0, data, length))
        |                                    ^~~~~~~~
  include/linux/etherdevice.h:292:16: note: in expansion of macro 'ether_crc'
    292 |         return ether_crc(ETH_ALEN, ha->addr);
        |                ^~~~~~~~~
  make[5]: *** [scripts/Makefile.build:289: fs/select.o] Error 1
  ...
Quoted 4 lines
  $ scripts/config -s BITREVERSE
  undef
  $ rg BITREVERSE .config
Confirm the same for x86 tinyconfig.
The problem is that the patch makes generic bitrevXX() conditional on CONFIG_GENERIC_BITREVERSE, while before they were conditional on !CONFIG_HAVE_ARCH_BITREVERSE. So if you don't have arch bitreverse(), and dont' enable BITREVERSE, the generic implementation is not defined now.

Quoted 6 lines

Luckily, the only user of bitrev() in unconditionally compiled objects is CRC32, and it's not needed if CRC32 is disabled.
This is the minimal working fix for me. Please let me know what do you think. I can prepend Jinjie's pathes in my tree with it if it's OK for you.

I will run it through my full build matrix overnight and report back with any problems noticed.

include/linux/etherdevice.h line 23

Quoted 9 lines

include/linux/etherdevice.h +1 -0

index df8f88f63a70..245b206dd38b 100644
@@ -20,7 +20,9 @@
 #include <linux/if_ether.h>
 #include <linux/netdevice.h>
 #include <linux/random.h>
+#ifdef CONFIG_CRCC32

Initial testing shows that this should be

  #ifdef CONFIG_CRC32

here and below though.

Quoted 21 lines

include/linux/etherdevice.h +3 -0

 #include <linux/crc32.h>
+#endif
 #include <linux/unaligned.h>
 #include <asm/bitsperlong.h>

@@ -281,6 +283,7 @@ static inline void eth_hw_addr_random(struct net_device *dev)
 	dev->addr_assign_type = NET_ADDR_RANDOM;
 }
 
+#ifdef CONFIG_CRCC32
 /**
  * eth_hw_addr_crc - Calculate CRC from netdev_hw_addr
  * @ha: pointer to hardware address

@@ -291,6 +294,7 @@ static inline u32 eth_hw_addr_crc(struct netdev_hw_addr *ha)
 {
 	return ether_crc(ETH_ALEN, ha->addr);
 }
+#endif
 
 /**
  * ether_addr_copy - Copy an Ethernet address

Signature · 2 lines

Cheers,
Nathan

Yury Norov

2026-04-30 16:41 UTC #6 lore raw

Re: [PATCH v5 2/3] bitops: Define generic __bitrev8/16/32 for reuse

Quoted 93 lines · Nathan Chancellor

Quoted 70 lines · Yury Norov
+ networking maintainers
Quoted 54 lines · Nathan Chancellor
Hi Jinjie,
Quoted 18 lines · Jinjie Ruan
Define generic __bitrev8/16/32 using the implementation in <linux/bitrev.h>, so they can be reused in <asm/bitrev.h>, such as RISCV.
Reviewed-by Yury Norov <[email protected]>
Signed-off-by Jinjie Ruan <[email protected]>
---
 include/asm-generic/bitops/__bitrev.h | 25 +++++++++++++++++++++++++
 include/linux/bitrev.h                | 20 ++++----------------
 2 files changed, 29 insertions(+), 16 deletions(-)
 create mode 100644 include/asm-generic/bitops/__bitrev.h
include/asm-generic/bitops/__bitrev.h new file +0 -0
new file mode 100644
index 000000000000..f06af929678d
include/asm-generic/bitops/__bitrev.h line 5
Quoted 5 lines
include/asm-generic/bitops/__bitrev.h new file +5 -0
@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_BITOPS___BITREV_H_
+#define _ASM_GENERIC_BITOPS___BITREV_H_
+
+#ifdef CONFIG_GENERIC_BITREVERSE
The dependencies of this symbol seem insufficient, as I can trigger a build failure on next-20260429 like so:
  $ make -skj"$(nproc)" ARCH=s390 CROSS_COMPILE=s390-linux- mrproper tinyconfig fs/select.o
  In file included from include/linux/crc32.h:6,
                   from include/linux/etherdevice.h:23,
                   from include/linux/if_vlan.h:11,
                   from include/linux/filter.h:21,
                   from include/net/xdp.h:10,
                   from include/net/busy_poll.h:19,
                   from fs/select.c:33:
  include/linux/etherdevice.h: In function 'eth_hw_addr_crc':
  include/linux/bitrev.h:16:20: error: implicit declaration of function 'generic___bitrev32' [-Wimplicit-function-declaration]
     16 | #define __bitrev32 generic___bitrev32
        |                    ^~~~~~~~~~~~~~~~~~
  include/linux/bitrev.h:67:9: note: in expansion of macro '__bitrev32'
     67 |         __bitrev32(__x);                                \
        |         ^~~~~~~~~~
  include/linux/crc32.h:107:36: note: in expansion of macro 'bitrev32'
    107 | #define ether_crc(length, data)    bitrev32(crc32_le(~0, data, length))
        |                                    ^~~~~~~~
  include/linux/etherdevice.h:292:16: note: in expansion of macro 'ether_crc'
    292 |         return ether_crc(ETH_ALEN, ha->addr);
        |                ^~~~~~~~~
  make[5]: *** [scripts/Makefile.build:289: fs/select.o] Error 1
  ...
Quoted 4 lines
  $ scripts/config -s BITREVERSE
  undef
  $ rg BITREVERSE .config
Confirm the same for x86 tinyconfig.
The problem is that the patch makes generic bitrevXX() conditional on CONFIG_GENERIC_BITREVERSE, while before they were conditional on !CONFIG_HAVE_ARCH_BITREVERSE. So if you don't have arch bitreverse(), and dont' enable BITREVERSE, the generic implementation is not defined now.
Quoted 6 lines
Luckily, the only user of bitrev() in unconditionally compiled objects is CRC32, and it's not needed if CRC32 is disabled.
This is the minimal working fix for me. Please let me know what do you think. I can prepend Jinjie's pathes in my tree with it if it's OK for you.
I will run it through my full build matrix overnight and report back with any problems noticed.
include/linux/etherdevice.h line 23
Quoted 9 lines
include/linux/etherdevice.h +1 -0
index df8f88f63a70..245b206dd38b 100644
@@ -20,7 +20,9 @@
 #include <linux/if_ether.h>
 #include <linux/netdevice.h>
 #include <linux/random.h>
+#ifdef CONFIG_CRCC32
Initial testing shows that this should be

Quoted 3 lines

  #ifdef CONFIG_CRC32
here and below though.

Sure. Please let me know if this works for you. If it does, I'll make it a regular patch, combine with Jinjie's work and my follow-up and resend. Meanwhile, removed all bitreverse material from bitmap-for-next.

Thanks,
Yury

Nathan Chancellor

2026-04-30 19:02 UTC #7 lore raw

Re: [PATCH v5 2/3] bitops: Define generic __bitrev8/16/32 for reuse

Quoted 4 lines · Yury Norov

Sure. Please let me know if this works for you. If it does, I'll make it a regular patch, combine with Jinjie's work and my follow-up and resend. Meanwhile, removed all bitreverse material from bitmap-for-next.

My builds saw no issues overnight with the CONFIG_CRC32 version, so your plan seems good to me.

Signature · 2 lines

Cheers,
Nathan

Jinjie Ruan

author

2026-04-21 13:04 UTC #8 lore raw

PATCH v5 3/3 arch/riscv: Add bitrev.h file to support rev8 and brev8

In reply to Jinjie Ruan

The RISC-V Bit-manipulation Extension for Cryptography (Zbkb) provides the 'brev8' instruction, which reverses the bits within each byte. Combined with the 'rev8' instruction (from Zbb or Zbkb), which reverses the byte order of a register, we can efficiently implement 16-bit, 32-bit, and (on RV64) 64-bit bit reversal.

This is significantly faster than the default software table-lookup implementation in lib/bitrev.c, as it replaces memory accesses and multiple arithmetic operations with just two or three hardware instructions.

Select HAVE_ARCH_BITREVERSE as well as GENERIC_BITREVERSE, and provide <asm/bitrev.h> to utilize these instructions when the Zbkb extension is available at runtime via the alternatives mechanism.

Link https://docs.riscv.org/reference/isa/unpriv/b-st-ext.html

Signed-off-by Jinjie Ruan <[email protected]>

---
 arch/riscv/Kconfig              |  2 ++
 arch/riscv/include/asm/bitrev.h | 51 +++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+)
 create mode 100644 arch/riscv/include/asm/bitrev.h

arch/riscv/Kconfig +2 -0

index 90c531e6abf5..4f9e1caee5f9 100644
@@ -105,6 +105,7 @@ config RISCV
 	select FUNCTION_ALIGNMENT_8B if DYNAMIC_FTRACE_WITH_CALL_OPS
 	select GENERIC_ARCH_TOPOLOGY
 	select GENERIC_ATOMIC64 if !64BIT
+	select GENERIC_BITREVERSE
 	select GENERIC_CLOCKEVENTS_BROADCAST if SMP
 	select GENERIC_CPU_DEVICES
 	select GENERIC_CPU_VULNERABILITIES

@@ -128,6 +129,7 @@ config RISCV
 	select HAS_IOPORT if MMU
 	select HAVE_ALIGNED_STRUCT_PAGE
 	select HAVE_ARCH_AUDITSYSCALL
+	select HAVE_ARCH_BITREVERSE if RISCV_ISA_ZBKB
 	select HAVE_ARCH_HUGE_VMALLOC if HAVE_ARCH_HUGE_VMAP
 	select HAVE_ARCH_HUGE_VMAP if MMU && 64BIT
 	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL

arch/riscv/include/asm/bitrev.h new file +51 -0

51 lines not on this page · raw message

Signature · 1 line

2.34.1

Yury Norov

Applied

2026-04-27 20:18 UTC #9 lore raw

In reply to Jinjie Ruan

Quoted 4 lines · Jinjie Ruan

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"

Applied in bitmap-for-next. Thanks!

Quoted 39 lines

Changes in v5:
- Rename NEED_BYTE_REV_TABLE to GENERIC_BITREVERSE.
- Add help text and make GENERIC_BITREVERSE depends on BITREVERSE, and use def_bool.
- Split into three patches.
- Add suggested-by.
Changes in v4:
- Update the riscv implementation as David suggested.
- Add new config called NEED_BYTE_REV_TABLE as David suggested to avoid bloating the .data section for architectures that have full hardware bit-reverse support and don't need the table.
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 (3):
lib/bitrev: Introduce GENERIC_BITREVERSE and cleanup Kconfig
bitops: Define generic __bitrev8/16/32 for reuse arch/riscv: Add bitrev.h file to support rev8 and brev8
 arch/riscv/Kconfig                    |  2 ++
 arch/riscv/include/asm/bitrev.h       | 51 +++++++++++++++++++++++++++
 include/asm-generic/bitops/__bitrev.h | 25 +++++++++++++
 include/linux/bitrev.h                | 20 +++--------
 lib/Kconfig                           | 18 ++++++++++
 lib/bitrev.c                          |  4 +--
 6 files changed, 102 insertions(+), 18 deletions(-)
 create mode 100644 arch/riscv/include/asm/bitrev.h
 create mode 100644 include/asm-generic/bitops/__bitrev.h
Signature · 1 line
2.34.1