On 2025-06-12 7:56 pm, Nicolas Frattaroli wrote:
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.
Add two macros: HWORD_UPDATE, and HWORD_UPDATE_CONST. The latter is a
version that can be used in initializers, like FIELD_PREP_CONST. The
macro names are chosen to not clash with any potential other macros that
drivers may already have implemented themselves, while retaining a
familiar name.
Nit: while from one angle it indeed looks similar, from another it's
even more opaque and less meaningful than what we have already.
Personally I cannot help but see "hword" as "halfword", so logically if
we want 32+32-bit or 8+8-bit variants in future those would be
WORD_UPDATE() and BYTE_UPDATE(), right? ;)
It's also confounded by "update" not actually having any obvious meaning
at this level without all the implicit usage context. FWIW my suggestion
would be FIELD_PREP_WM_U16, such that the reader instantly sees
"FIELD_PREP with some additional semantics", even if they then need to
glance at the kerneldoc for clarification that WM stands for writemask
(or maybe WE for write-enable if people prefer). Plus it then leaves
room to easily support different sizes (and potentially even bonkers
upside-down Ux_WM variants?!) without any bother if we need to.
Thanks,
Robin.
Signed-off-by: Nicolas Frattaroli <nicolas.frattar...@collabora.com>
---
include/linux/bitfield.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 47 insertions(+)
diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index
6d9a53db54b66c0833973c880444bd289d9667b1..b90d88db7405f95b78cdd6f3426263086bab5aa6
100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -8,6 +8,7 @@
#define _LINUX_BITFIELD_H
#include <linux/build_bug.h>
+#include <linux/limits.h>
#include <linux/typecheck.h>
#include <asm/byteorder.h>
@@ -142,6 +143,52 @@
(((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask)) \
)
+/**
+ * HWORD_UPDATE() - 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
+ *
+ * HWORD_UPDATE() 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 HWORD_UPDATE(_mask, _val) \
+ ({ \
+ __BF_FIELD_CHECK(_mask, ((u16) 0U), _val, \
+ "HWORD_UPDATE: "); \
+ (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask)) | \
+ ((_mask) << 16); \
+ })
+
+/**
+ * HWORD_UPDATE_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
+ *
+ * HWORD_UPDATE_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 HWORD_UPDATE(), this is a constant expression and can therefore
+ * be used in initializers. Error checking is less comfortable for this
+ * version.
+ */
+#define HWORD_UPDATE_CONST(_mask, _val)
\
+ ( \
+ FIELD_PREP_CONST(_mask, _val) | \
+ (BUILD_BUG_ON_ZERO(const_true((u64) (_mask) > U16_MAX)) + \
+ ((_mask) << 16)) \
+ )
+
/**
* FIELD_GET() - extract a bitfield element
* @_mask: shifted mask defining the field's length and position
