[v8,2/6] bits: introduce fixed-type GENMASK_U*()

Commit Message

Vincent Mailhol March 25, 2025, 3:59 p.m. UTC

From: Vincent Mailhol <[e-mail]>

Add GENMASK_TYPE() which generalizes __GENMASK() to support different
types, and implement fixed-types versions of GENMASK() based on it.
The fixed-type version allows more strict checks to the min/max values
accepted, which is useful for defining registers like implemented by
i915 and xe drivers with their REG_GENMASK*() macros.

The strict checks rely on shift-count-overflow compiler check to fail
the build if a number outside of the range allowed is passed.
Example:

  #define FOO_MASK GENMASK_U32(33, 4)

will generate a warning like:

  include/linux/bits.h:51:27: error: right shift count >= width of type [-Werror=shift-count-overflow]
     51 |               type_max(t) >> (BITS_PER_TYPE(t) - 1 - (h)))))
        |                           ^~

The result is casted to the corresponding fixed width type. For
example, GENMASK_U8() returns an u8. Note that because of the C
promotion rules, GENMASK_U8() and GENMASK_U16() will immediately be
promoted to int if used in an expression. Regardless, the main goal is
not to get the correct type, but rather to enforce more checks at
compile time.

While GENMASK_TYPE() is crafted to cover all variants, including the
already existing GENMASK(), GENMASK_ULL() and GENMASK_U128(), for the
moment, only use it for the newly introduced GENMASK_U*(). The
consolidation will be done in a separate change.

Co-developed-by: Yury Norov <[e-mail]>
Signed-off-by: Yury Norov <[e-mail]>
Signed-off-by: Lucas De Marchi <[e-mail]>
Acked-by: Jani Nikula <[e-mail]>
Signed-off-by: Vincent Mailhol <[e-mail]>
---
Changelog:

  v7 -> v8:

    - s/shift-count-overflow/-Wshift-count-overflow/g

    - add an explanation about the return value cast in the patch
      description.

  v6 -> v7:

    - Fix grammar in comment: 'GENMASK_U*() depends' -> 'GENMASK_U*()
      depend'.

    - Fix typo in comment: 'Nethertheless' -> 'Nevertheless'

    - Do an artificial early line wrap in comment so that the next
      patch only has a one line diff change.

    - Re-wrap the comments to the 80th column.

    - The patch changed a lot since Yury first version: put myself as
      main author and Yury as Co-developer.

    - Add a new paragraph to the patch description to explain that
      consolidation will be done later.

  v5 -> v6:

    - No changes.

  v4 -> v5:

    - Rename GENMASK_t() to GENMASK_TYPE().

    - Fix typo in patch description.

    - Use tab indentations instead of single space to separate the
      macro name from its body.

    - s/__GENMASK_U*()/GENMASK_U*()/g in the comment.

    - Add a tag to credit myself as Co-developer. Keep Yury as the
      main author.

    - Modify GENMASK_TYPE() to match the changes made to __GENMASK()
      in: https://github.com/norov/linux/commit/1e7933a575ed

    - Replace (t)~_ULL(0) with type_max(t). This is OK because
      GENMASK_TYPE() is not available in asm.

    - linux/const.h and asm/bitsperlong.h are not used anymore. Remove
      them.

    - Apply GENMASK_TYPE() to GENMASK_U128().

    - Remove the unsigned int cast for the U8 and U16 variants. Cast
      to the target type instead. Do that cast directly in
      GENMASK_TYPE().

  v3 -> v4:

    - The v3 is one year old. Meanwhile people started using
      __GENMASK() directly. So instead of generalizing __GENMASK() to
      support different types, add a new GENMASK_t().

    - replace ~0ULL by ~_ULL(0). Otherwise, GENMASK_t() would fail in
      asm code.

    - Make GENMASK_U8() and GENMASK_U16() return an unsigned int. In
      v3, due to the integer promotion rules, these were returning a
      signed integer. By casting these to unsigned int, at least the
      signedness is kept.
---
 include/linux/bitops.h |  1 -
 include/linux/bits.h   | 30 ++++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

Comments

Andy Shevchenko March 25, 2025, 4:24 p.m. UTC | #1

On Wed, Mar 26, 2025 at 12:59:57AM +0900, Vincent Mailhol via B4 Relay wrote:
> From: Vincent Mailhol <[e-mail]>
> 
> Add GENMASK_TYPE() which generalizes __GENMASK() to support different
> types, and implement fixed-types versions of GENMASK() based on it.
> The fixed-type version allows more strict checks to the min/max values
> accepted, which is useful for defining registers like implemented by
> i915 and xe drivers with their REG_GENMASK*() macros.
> 
> The strict checks rely on shift-count-overflow compiler check to fail

-Wshift-count-overflow

but it's not so important here. It's good that it's fixed in the comment.
In any case if Yury wants/can do the change, it might be done when applying.
(I.o.w. no need to resend)

> the build if a number outside of the range allowed is passed.
> Example:
> 
>   #define FOO_MASK GENMASK_U32(33, 4)
> 
> will generate a warning like:
> 
>   include/linux/bits.h:51:27: error: right shift count >= width of type [-Werror=shift-count-overflow]
>      51 |               type_max(t) >> (BITS_PER_TYPE(t) - 1 - (h)))))
>         |                           ^~
> 
> The result is casted to the corresponding fixed width type. For
> example, GENMASK_U8() returns an u8. Note that because of the C
> promotion rules, GENMASK_U8() and GENMASK_U16() will immediately be
> promoted to int if used in an expression. Regardless, the main goal is
> not to get the correct type, but rather to enforce more checks at
> compile time.
> 
> While GENMASK_TYPE() is crafted to cover all variants, including the
> already existing GENMASK(), GENMASK_ULL() and GENMASK_U128(), for the
> moment, only use it for the newly introduced GENMASK_U*(). The
> consolidation will be done in a separate change.

The change LGTM, FWIW,
Reviewed-by: Andy Shevchenko <[e-mail]>

Patch

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index c1cb53cf2f0f8662ed3e324578f74330e63f935d..9be2d50da09a417966b3d11c84092bb2f4cd0bef 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -8,7 +8,6 @@ 
 
 #include <uapi/linux/kernel.h>
 
-#define BITS_PER_TYPE(type)	(sizeof(type) * BITS_PER_BYTE)
 #define BITS_TO_LONGS(nr)	__KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
 #define BITS_TO_U64(nr)		__KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u64))
 #define BITS_TO_U32(nr)		__KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u32))
diff --git a/include/linux/bits.h b/include/linux/bits.h
index e1e5177691406919ba4f5bbc875bd230414fd117..9718c5ae5fc356e66958cf48667bb1edda4f9673 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -12,6 +12,7 @@ 
 #define BIT_ULL_MASK(nr)	(ULL(1) << ((nr) % BITS_PER_LONG_LONG))
 #define BIT_ULL_WORD(nr)	((nr) / BITS_PER_LONG_LONG)
 #define BITS_PER_BYTE		8
+#define BITS_PER_TYPE(type)	(sizeof(type) * BITS_PER_BYTE)
 
 /*
  * Create a contiguous bitmask starting at bit position @l and ending at
@@ -20,11 +21,40 @@ 
  */
 #if !defined(__ASSEMBLY__)
 
+/*
+ * Missing asm support
+ *
+ * GENMASK_U*() depend on BITS_PER_TYPE() which relies on sizeof(),
+ * something not available in asm. Nevertheless, fixed width integers is a C
+ * concept. Assembly code can rely on the long and long long versions instead.
+ */
+
 #include <linux/build_bug.h>
 #include <linux/compiler.h>
+#include <linux/overflow.h>
 
 #define GENMASK_INPUT_CHECK(h, l) BUILD_BUG_ON_ZERO(const_true((l) > (h)))
 
+/*
+ * Generate a mask for the specified type @t. Additional checks are made to
+ * guarantee the value returned fits in that type, relying on
+ * -Wshift-count-overflow compiler check to detect incompatible arguments.
+ * For example, all these create build errors or warnings:
+ *
+ * - GENMASK(15, 20): wrong argument order
+ * - GENMASK(72, 15): doesn't fit unsigned long
+ * - GENMASK_U32(33, 15): doesn't fit in a u32
+ */
+#define GENMASK_TYPE(t, h, l)					\
+	((t)(GENMASK_INPUT_CHECK(h, l) +			\
+	     (type_max(t) << (l) &				\
+	      type_max(t) >> (BITS_PER_TYPE(t) - 1 - (h)))))
+
+#define GENMASK_U8(h, l)	GENMASK_TYPE(u8, h, l)
+#define GENMASK_U16(h, l)	GENMASK_TYPE(u16, h, l)
+#define GENMASK_U32(h, l)	GENMASK_TYPE(u32, h, l)
+#define GENMASK_U64(h, l)	GENMASK_TYPE(u64, h, l)
+
 #else /* defined(__ASSEMBLY__) */
 
 /*