[PATCH v3 1/4] media: rkvdec: Introduce a global bitwriter helper

[Detlev Casanova <detlev.casanova@collabora.com>]

The use of structures with bitfields is good when the values are
somewhat aligned.
More mis-alignement means that compilers need to do more gymanstics
to edit the fields values.

Some cases have been reported with CLang on specific architectures
like armhf and hexagon, where the compiler would allocate a bigger
local stack than needed or even completely freeze during compilation.

Some fixes have been provided to ease the issues, but the real fix
here is to use a bitwriter instead of heavily unaligned bitfields.

This is a preparation commit to provide a global bitwriter interface
for the whole driver.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
---
 .../platform/rockchip/rkvdec/rkvdec-bitwriter.h    | 39 ++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/drivers/media/platform/rockchip/rkvdec/rkvdec-bitwriter.h b/drivers/media/platform/rockchip/rkvdec/rkvdec-bitwriter.h
new file mode 100644
index 000000000000..2a5c271ade91
--- /dev/null
+++ b/drivers/media/platform/rockchip/rkvdec/rkvdec-bitwriter.h
@@ -0,0 +1,39 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Rockchip Video Decoder bit writer
+ *
+ * Copyright (C) 2026 Collabora, Ltd.
+ *      Detlev Casanova <detlev.casanova@collabora.com>
+ * Copyright (C) 2019 Collabora, Ltd.
+ *	Boris Brezillon <boris.brezillon@collabora.com>
+ */
+
+#ifndef RKVDEC_BIT_WRITER_H_
+#define RKVDEC_BIT_WRITER_H_
+
+#include <linux/types.h>
+#include <linux/bits.h>
+
+struct rkvdec_bw_field {
+	u16 offset;
+	u8 len;
+};
+
+#define BW_FIELD(_offset, _len) ((struct rkvdec_bw_field){ _offset, _len })
+
+static inline void rkvdec_set_bw_field(u32 *buf, struct rkvdec_bw_field field, u32 value)
+{
+	u8 bit = field.offset % 32;
+	u16 word = field.offset / 32;
+	u64 mask = GENMASK_ULL(bit + field.len - 1, bit);
+	u64 val = ((u64)value << bit) & mask;
+
+	buf[word] &= ~mask;
+	buf[word] |= val;
+	if (bit + field.len > 32) {
+		buf[word + 1] &= ~(mask >> 32);
+		buf[word + 1] |= val >> 32;
+	}
+}
+
+#endif /* RKVDEC_BIT_WRITER_H_ */

-- 
2.53.0