[PATCH 07/21] bits: Add BITS() macro

["Yury Norov (NVIDIA)" <yury.norov@gmail.com>]

The BITS(low, high) macro is preferable over a similar GENMASK(high, low)
because (low, high) parameters order is more natural. The (high, low)
order is confusing and has a record of misuse.

To enforce unintuitive parameters order, GENMASK() is enforced with
compile time checks. In addition, fixed-width versions of GENMASK() had
been developed. They make sense in describing hardware registers.

In generic code, using standard ordering (low to high) is more preferable,
and fixed-width features are not that useful.

In non-driver code, BITS() must be taken over GENMASK(). In drivers code,
BITS() is preferable over GENMASK().

The following pattern of using GENMASK() is highly unfavorable:

  /* Status register (SR) */
  #define I2C_SR_OP               GENMASK(1, 0)   /* Operation */
  #define I2C_SR_STATUS           GENMASK(3, 2)   /* controller status */
  #define I2C_SR_CAUSE            GENMASK(6, 4)   /* Abort cause */
  #define I2C_SR_TYPE             GENMASK(8, 7)   /* Receive type */
  #define I2C_SR_LENGTH           GENMASK(19, 9)  /* Transfer length */

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/all/CAHk-=whoOUsqPKb7OQwhQf9H_3=5sXGPJrDbfQfwLB3Bi13tcQ@mail.gmail.com/
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
---
 include/linux/bits.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/bits.h b/include/linux/bits.h
index a40cc861b3a7..c7c587e90e2d 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -57,6 +57,9 @@
 #define GENMASK_U64(h, l)	GENMASK_TYPE(u64, h, l)
 #define GENMASK_U128(h, l)	GENMASK_TYPE(u128, h, l)
 
+#define BITS(l, h)		GENMASK(h, l)
+#define BITS_ULL(l, h)		GENMASK_ULL(h, l)
+
 /*
  * Fixed-type variants of BIT(), with additional checks like GENMASK_TYPE(). The
  * following examples generate compiler warnings due to -Wshift-count-overflow:
-- 
2.43.0