[PATCH v3 2/7] lib/vsprintf: Add support for generic FOURCCs by extending %p4cc

[Russell King (Oracle)]

From: Hector Martin <marcan at marcan.st>

%p4cc is designed for DRM/V4L2 FOURCCs with their specific quirks, but
it's useful to be able to print generic 4-character codes formatted as
an integer. Extend it to add format specifiers for printing generic
32-bit FOURCCs with various endian semantics:

%p4ch   Host-endian
%p4cl	Little-endian
%p4cb	Big-endian
%p4cr	Reverse-endian

The endianness determines how bytes are interpreted as a u32, and the
FOURCC is then always printed MSByte-first (this is the opposite of
V4L/DRM FOURCCs). This covers most practical cases, e.g. %p4cr would
allow printing LSByte-first FOURCCs stored in host endian order
(other than the hex form being in character order, not the integer
value).

Signed-off-by: Hector Martin <marcan at marcan.st>
Signed-off-by: Russell King (Oracle) <rmk+kernel at armlinux.org.uk>
---
 Documentation/core-api/printk-formats.rst | 32 +++++++++++++++++++
 lib/test_printf.c                         | 39 +++++++++++++++++++----
 lib/vsprintf.c                            | 35 ++++++++++++++++----
 3 files changed, 93 insertions(+), 13 deletions(-)

diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
index dbe1aacc79d0..92a488884cf8 100644
--- a/Documentation/core-api/printk-formats.rst
+++ b/Documentation/core-api/printk-formats.rst
@@ -625,6 +625,38 @@ Passed by reference.
 	%p4cc	Y10  little-endian (0x20303159)
 	%p4cc	NV12 big-endian (0xb231564e)
 
+Generic FourCC code
+-------------------
+
+::
+	%p4c[hrbl]	gP00 (0x67503030)
+
+Print a generic FourCC code, as both ASCII characters and its numerical
+value as hexadecimal.
+
+The additional ``h``, ``r``, ``b``, and ``l`` specifiers are used to specify
+host, reversed, big or little endian order data respectively. Host endian
+order means the data is interpreted as a 32-bit integer and the most
+significant byte is printed first; that is, the character code as printed
+matches the byte order stored in memory on big-endian systems, and is reversed
+on little-endian systems.
+
+Passed by reference.
+
+Examples for a little-endian machine, given &(u32)0x67503030::
+
+	%p4ch	gP00 (0x67503030)
+	%p4cr	00Pg (0x30305067)
+	%p4cb	00Pg (0x30305067)
+	%p4cl	gP00 (0x67503030)
+
+Examples for a big-endian machine, given &(u32)0x67503030::
+
+	%p4ch	gP00 (0x67503030)
+	%p4cr	00Pg (0x30305067)
+	%p4cb	gP00 (0x67503030)
+	%p4cl	00Pg (0x30305067)
+
 Rust
 ----
 
diff --git a/lib/test_printf.c b/lib/test_printf.c
index 4bd15a593fbd..77a9128a6b5a 100644
--- a/lib/test_printf.c
+++ b/lib/test_printf.c
@@ -731,21 +731,46 @@ static void __init fwnode_pointer(void)
 	software_node_unregister_nodes(softnodes);
 }
 
+struct fourcc_struct {
+	u32 code;
+	const char *str;
+};
+
+static void __init fourcc_pointer_test(const struct fourcc_struct *fc, size_t n,
+				       const char *fmt)
+{
+	size_t i;
+
+	for (i = 0; i < n; i++)
+		test(fc[i].str, fmt, &fc[i].code);
+}
+
 static void __init fourcc_pointer(void)
 {
-	struct {
-		u32 code;
-		char *str;
-	} const try[] = {
+	struct fourcc_struct const try_cc[] = {
 		{ 0x3231564e, "NV12 little-endian (0x3231564e)", },
 		{ 0xb231564e, "NV12 big-endian (0xb231564e)", },
 		{ 0x10111213, ".... little-endian (0x10111213)", },
 		{ 0x20303159, "Y10  little-endian (0x20303159)", },
 	};
-	unsigned int i;
+	struct fourcc_struct const try_ch = {
+		0x41424344, "ABCD (0x41424344)",
+	};
+	struct fourcc_struct const try_cr = {
+		0x41424344, "DCBA (0x44434241)",
+	};
+	struct fourcc_struct const try_cl = {
+		le32_to_cpu(0x41424344), "ABCD (0x41424344)",
+	};
+	struct fourcc_struct const try_cb = {
+		be32_to_cpu(0x41424344), "ABCD (0x41424344)",
+	};
 
-	for (i = 0; i < ARRAY_SIZE(try); i++)
-		test(try[i].str, "%p4cc", &try[i].code);
+	fourcc_pointer_test(try_cc, ARRAY_SIZE(try_cc), "%p4cc");
+	fourcc_pointer_test(&try_ch, 1, "%p4ch");
+	fourcc_pointer_test(&try_cr, 1, "%p4cr");
+	fourcc_pointer_test(&try_cl, 1, "%p4cl");
+	fourcc_pointer_test(&try_cb, 1, "%p4cb");
 }
 
 static void __init
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 24f37bab8bc1..17064b839f19 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1762,27 +1762,50 @@ char *fourcc_string(char *buf, char *end, const u32 *fourcc,
 	char output[sizeof("0123 little-endian (0x01234567)")];
 	char *p = output;
 	unsigned int i;
+	bool pixel_fmt = false;
 	u32 orig, val;
 
-	if (fmt[1] != 'c' || fmt[2] != 'c')
+	if (fmt[1] != 'c')
 		return error_string(buf, end, "(%p4?)", spec);
 
 	if (check_pointer(&buf, end, fourcc, spec))
 		return buf;
 
 	orig = get_unaligned(fourcc);
-	val = orig & ~BIT(31);
+	switch (fmt[2]) {
+	case 'h':
+		val = orig;
+		break;
+	case 'r':
+		val = orig = swab32(orig);
+		break;
+	case 'l':
+		val = orig = le32_to_cpu(orig);
+		break;
+	case 'b':
+		val = orig = be32_to_cpu(orig);
+		break;
+	case 'c':
+		/* Pixel formats are printed LSB-first */
+		val = swab32(orig & ~BIT(31));
+		pixel_fmt = true;
+		break;
+	default:
+		return error_string(buf, end, "(%p4?)", spec);
+	}
 
 	for (i = 0; i < sizeof(u32); i++) {
-		unsigned char c = val >> (i * 8);
+		unsigned char c = val >> ((3 - i) * 8);
 
 		/* Print non-control ASCII characters as-is, dot otherwise */
 		*p++ = isascii(c) && isprint(c) ? c : '.';
 	}
 
-	*p++ = ' ';
-	strcpy(p, orig & BIT(31) ? "big-endian" : "little-endian");
-	p += strlen(p);
+	if (pixel_fmt) {
+		*p++ = ' ';
+		strcpy(p, orig & BIT(31) ? "big-endian" : "little-endian");
+		p += strlen(p);
+	}
 
 	*p++ = ' ';
 	*p++ = '(';
-- 
2.30.2