[PATCH v3 01/20] bitmap: introduce hardware-specific bitfield operations

Nicolas Frattaroli
2025-08-25T08:30:23Z

Hardware of various vendors, but very notably Rockchip, often uses
32-bit registers where the upper 16-bit half of the register is a
write-enable mask for the lower half.

This type of hardware setup allows for more granular concurrent register
write access.

Over the years, many drivers have hand-rolled their own version of this
macro, usually without any checks, often called something like
HIWORD_UPDATE or FIELD_PREP_HIWORD, commonly with slightly different
semantics between them.

Clearly there is a demand for such a macro, and thus the demand should
be satisfied in a common header file. As this is a convention that spans
across multiple vendors, and similar conventions may also have
cross-vendor adoption, it's best if it lives in a vendor-agnostic header
file that can be expanded over time.

Add hw_bitfield.h with two macros: FIELD_PREP_WM16, and
FIELD_PREP_WM16_CONST. The latter is a version that can be used in
initializers, like FIELD_PREP_CONST.

Suggested-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
---
MAINTAINERS | 1 +
include/linux/hw_bitfield.h | 62 +++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 63 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index d4bddc462c079f85ee35e685527c93dd1620f890..75a266a78a13721372a92b9ae45b9ae56e301347 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4281,6 +4281,7 @@ F: include/linux/bits.h
F: include/linux/cpumask.h
F: include/linux/cpumask_types.h
F: include/linux/find.h
+F: include/linux/hw_bitfield.h
F: include/linux/nodemask.h
F: include/linux/nodemask_types.h
F: include/uapi/linux/bits.h
diff --git a/include/linux/hw_bitfield.h b/include/linux/hw_bitfield.h
new file mode 100644
index 0000000000000000000000000000000000000000..df202e167ce487122e4440962eacb2e44817ad9f
--- /dev/null
+++ b/include/linux/hw_bitfield.h
@@ -0,0 +1,62 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright (C) 2025, Collabora Ltd.
+ */
+
+#ifndef _LINUX_HW_BITFIELD_H
+#define _LINUX_HW_BITFIELD_H
+
+#include <linux/bitfield.h>
+#include <linux/build_bug.h>
+#include <linux/limits.h>
+
+/**
+ * FIELD_PREP_WM16() - prepare a bitfield element with a mask in the upper half
+ * @_mask: shifted mask defining the field's length and position
+ * @_val: value to put in the field
+ *
+ * FIELD_PREP_WM16() masks and shifts up the value, as well as bitwise ORs the
+ * result with the mask shifted up by 16.
+ *
+ * This is useful for a common design of hardware registers where the upper
+ * 16-bit half of a 32-bit register is used as a write-enable mask. In such a
+ * register, a bit in the lower half is only updated if the corresponding bit
+ * in the upper half is high.
+ */
+#define FIELD_PREP_WM16(_mask, _val) \
+ ({ \
+ typeof(_val) __val = _val; \
+ typeof(_mask) __mask = _mask; \
+ __BF_FIELD_CHECK(__mask, ((u16)0U), __val, \
+ "HWORD_UPDATE: "); \
+ (((typeof(__mask))(__val) << __bf_shf(__mask)) & (__mask)) | \
+ ((__mask) << 16); \
+ })
+
+/**
+ * FIELD_PREP_WM16_CONST() - prepare a constant bitfield element with a mask in
+ * the upper half
+ * @_mask: shifted mask defining the field's length and position
+ * @_val: value to put in the field
+ *
+ * FIELD_PREP_WM16_CONST() masks and shifts up the value, as well as bitwise ORs
+ * the result with the mask shifted up by 16.
+ *
+ * This is useful for a common design of hardware registers where the upper
+ * 16-bit half of a 32-bit register is used as a write-enable mask. In such a
+ * register, a bit in the lower half is only updated if the corresponding bit
+ * in the upper half is high.
+ *
+ * Unlike FIELD_PREP_WM16(), this is a constant expression and can therefore
+ * be used in initializers. Error checking is less comfortable for this
+ * version.
+ */
+#define FIELD_PREP_WM16_CONST(_mask, _val) \
+ ( \
+ FIELD_PREP_CONST(_mask, _val) | \
+ (BUILD_BUG_ON_ZERO(const_true((u64)(_mask) > U16_MAX)) + \
+ ((_mask) << 16)) \
+ )
+
+
+#endif /* _LINUX_HW_BITFIELD_H */

--
2.51.0