lib/vsprintf: Add support for generic FourCCs by extending %p4cc

%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 byte order
%p4cn	Network byte order
%p4cl	Little-endian
%p4cb	Big-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. %p4cn would
allow printing LSByte-first FourCCs stored in host endian order
(other than the hex form being in character order, not the integer
value).

Acked-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Aditya Garg <gargaditya08@live.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/PN3PR01MB9597B01823415CB7FCD3BC27B8B52@PN3PR01MB9597.INDPRD01.PROD.OUTLOOK.COM
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>

Documentation/core-api/printk-formats.rst

@@ -648,6 +648,38 @@ Examples::
 	%p4cc	Y10  little-endian (0x20303159)
 	%p4cc	NV12 big-endian (0xb231564e)
 
+Generic FourCC code
+-------------------
+
+::
+	%p4c[hnlb]	gP00 (0x67503030)
+
+Print a generic FourCC code, as both ASCII characters and its numerical
+value as hexadecimal.
+
+The generic FourCC code is always printed in the big-endian format,
+the most significant byte first. This is the opposite of V4L/DRM FourCCs.
+
+The additional ``h``, ``n``, ``l``, and ``b`` specifiers define what
+endianness is used to load the stored bytes. The data might be interpreted
+using the host byte order, network byte order, little-endian, or big-endian.
+
+Passed by reference.
+
+Examples for a little-endian machine, given &(u32)0x67503030::
+
+	%p4ch	gP00 (0x67503030)
+	%p4cn	00Pg (0x30305067)
+	%p4cl	gP00 (0x67503030)
+	%p4cb	00Pg (0x30305067)
+
+Examples for a big-endian machine, given &(u32)0x67503030::
+
+	%p4ch	gP00 (0x67503030)
+	%p4cn	00Pg (0x30305067)
+	%p4cl	00Pg (0x30305067)
+	%p4cb	gP00 (0x67503030)
+
 Rust
 ----
 

lib/vsprintf.c

@@ -1793,27 +1793,50 @@ 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':
+		break;
+	case 'n':
+		orig = swab32(orig);
+		break;
+	case 'l':
+		orig = (__force u32)cpu_to_le32(orig);
+		break;
+	case 'b':
+		orig = (__force u32)cpu_to_be32(orig);
+		break;
+	case 'c':
+		/* Pixel formats are printed LSB-first */
+		pixel_fmt = true;
+		break;
+	default:
+		return error_string(buf, end, "(%p4?)", spec);
+	}
+
+	val = pixel_fmt ? swab32(orig & ~BIT(31)) : orig;
 
 	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++ = '(';

scripts/checkpatch.pl

@@ -6891,7 +6891,7 @@ sub process {
 					    ($extension eq "f" &&
 					     defined $qualifier && $qualifier !~ /^w/) ||
 					    ($extension eq "4" &&
-					     defined $qualifier && $qualifier !~ /^cc/)) {
+					     defined $qualifier && $qualifier !~ /^c[hnlbc]/)) {
 						$bad_specifier = $specifier;
 						last;
 					}