lib/bitrev: Introduce GENERIC_BITREVERSE

The generic bit reversal implementation is controlled by
!HAVE_ARCH_BITREVERSE. This makes it difficult for architectures to
provide a hardware-accelerated implementation while being able to
fall back to the generic version if needed.

This patch adds GENERIC_BITREVERSE, so bitreverse API is controlled by
BITREVERSE, GENERIC_BITREVERSE and HAVE_ARCH_BITREVERSE options. The
relationship between them is described as follows:

 - BITREVERSE is selected by user code; it's required to generate the API;
 - Architectures may select HAVE_ARCH_BITREVERSE and provide an arch
   implementation in arch/$(ARCH)/include/asm/bitrev.h.
 - if HAVE_ARCH_BITREVERSE isn't set, BITREVERSE selects GENERIC_BITREVERSE;
 - if GENERIC_BITREVERSE is set and HAVE_ARCH_BITREVERSE is not, the kernel
   provides generic implementation only, and wires bitrevXX() to it.
 - if HAVE_ARCH_BITREVERSE is set and GENERIC_BITREVERSE is not, the arch
   code provides __arch_bitrevXX(), and it is wired to bitrevXX();
 - if both GENERIC_BITREVERSE and HAVE_ARCH_BITREVERSE are selected, the kernel
   generates generic___bitrev(), but wires bitrev() to the __arch_bitrev().

The last option allows architectures to use generic___bitrev() as a
fallback option.

Drivers and core code should never select GENERIC_BITREVERSE or
HAVE_ARCH_BITREVERSE explicitly.

Architectures that require generic bitreverse API as a fallback should
explicitly enable GENERIC_BITREVERSE together with HAVE_ARCH_BITREVERSE.

Signed-off-by: Yury Norov <ynorov@nvidia.com>

lib/Kconfig

@@ -54,6 +54,7 @@ config PACKING_KUNIT_TEST
 
 config BITREVERSE
 	tristate
+	select GENERIC_BITREVERSE if !HAVE_ARCH_BITREVERSE
 
 config HAVE_ARCH_BITREVERSE
 	bool
@@ -63,6 +64,17 @@ config HAVE_ARCH_BITREVERSE
 	  This option enables the use of hardware bit-reversal instructions on
 	  architectures which support such operations.
 
+config GENERIC_BITREVERSE
+	tristate
+	depends on BITREVERSE
+	help
+	  Generic bit reversal implementation. Drivers should never enable
+	  it explicitly. Instead, enable BITREVERSE.
+
+	  Architectures may want to select it as a fall-back option for
+	  HAVE_ARCH_BITREVERSE, when the hardware-accelerated bit reverse
+	  instruction set is optional, like RISC-V ZBKB extension.
+
 config ARCH_HAS_STRNCPY_FROM_USER
 	bool
 

lib/Makefile

@@ -145,7 +145,7 @@ obj-$(CONFIG_DEBUG_PREEMPT) += smp_processor_id.o
 obj-$(CONFIG_LIST_HARDENED) += list_debug.o
 obj-$(CONFIG_DEBUG_OBJECTS) += debugobjects.o
 
-obj-$(CONFIG_BITREVERSE) += bitrev.o
+obj-$(CONFIG_GENERIC_BITREVERSE) += bitrev.o
 obj-$(CONFIG_LINEAR_RANGES) += linear_ranges.o
 obj-$(CONFIG_PACKING)	+= packing.o
 obj-$(CONFIG_PACKING_KUNIT_TEST) += packing_test.o

lib/bitrev.c

@@ -1,5 +1,4 @@
 // SPDX-License-Identifier: GPL-2.0-only
-#ifndef CONFIG_HAVE_ARCH_BITREVERSE
 #include <linux/types.h>
 #include <linux/module.h>
 #include <linux/bitrev.h>
@@ -43,5 +42,3 @@ const u8 byte_rev_table[256] = {
 	0x1f, 0x9f, 0x5f, 0xdf, 0x3f, 0xbf, 0x7f, 0xff,
 };
 EXPORT_SYMBOL_GPL(byte_rev_table);
-
-#endif /* CONFIG_HAVE_ARCH_BITREVERSE */