bitmap: introduce hardware-specific bitfield operations

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>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>

Author: Nicolas Frattaroli

MAINTAINERS

@@ -4275,6 +4275,7 @@ 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

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 */