lib/printbuf: New data structure for printing strings

This adds printbufs: a printbuf points to a char * buffer and knows the
size of the output buffer as well as the current output position.

Future patches will be adding more features to printbuf, but initially
printbufs are targeted at refactoring and improving our existing code in
lib/vsprintf.c - so this initial printbuf patch has the features
required for that.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>

1 files changed, 93 insertions, 0 deletions

diff --git a/include/linux/printbuf.h b/include/linux/printbuf.h
new file mode 100644
index 000000000000..c5af5d5d607d
--- /dev/null
+++ b/include/linux/printbuf.h
@@ -0,0 +1,93 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+/* Copyright (C) 2022 Kent Overstreet */
+
+#ifndef _LINUX_PRINTBUF_H
+#define _LINUX_PRINTBUF_H
+
+#include <linux/string.h>
+
+/*
+ * Printbufs: String buffer for outputting (printing) to, for vsnprintf
+ */
+
+struct printbuf {
+	char			*buf;
+	unsigned		size;
+	unsigned		pos;
+};
+
+static inline unsigned printbuf_remaining(struct printbuf *out)
+{
+	return out->pos < out->size ? out->size - out->pos : 0;
+}
+
+static inline unsigned printbuf_written(struct printbuf *out)
+{
+	return min(out->pos, out->size);
+}
+
+static inline void printbuf_nul_terminate(struct printbuf *out)
+{
+	if (out->pos < out->size)
+		out->buf[out->pos] = 0;
+	else if (out->size)
+		out->buf[out->size - 1] = 0;
+}
+
+static inline void pr_chars(struct printbuf *out, char c, unsigned n)
+{
+	memset(out->buf + out->pos,
+	       c,
+	       min(n, printbuf_remaining(out)));
+	out->pos += n;
+	printbuf_nul_terminate(out);
+}
+
+static inline void __pr_char(struct printbuf *out, char c)
+{
+	if (printbuf_remaining(out))
+		out->buf[out->pos] = c;
+	out->pos++;
+}
+
+static inline void pr_char(struct printbuf *out, char c)
+{
+	__pr_char(out, c);
+	printbuf_nul_terminate(out);
+}
+
+static inline void pr_bytes(struct printbuf *out, const void *b, unsigned n)
+{
+	memcpy(out->buf + out->pos,
+	       b,
+	       min(n, printbuf_remaining(out)));
+	out->pos += n;
+	printbuf_nul_terminate(out);
+}
+
+static inline void pr_str(struct printbuf *out, const char *str)
+{
+	pr_bytes(out, str, strlen(str));
+}
+
+static inline void pr_hex_byte(struct printbuf *out, u8 byte)
+{
+	__pr_char(out, hex_asc_hi(byte));
+	__pr_char(out, hex_asc_lo(byte));
+	printbuf_nul_terminate(out);
+}
+
+static inline void pr_hex_byte_upper(struct printbuf *out, u8 byte)
+{
+	__pr_char(out, hex_asc_upper_hi(byte));
+	__pr_char(out, hex_asc_upper_lo(byte));
+	printbuf_nul_terminate(out);
+}
+
+#define PRINTBUF_EXTERN(_buf, _size)			\
+((struct printbuf) {					\
+	.buf	= _buf,					\
+	.size	= _size,				\
+})
+
+#endif /* _LINUX_PRINTBUF_H */