[PATCH v4 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

[Sakari Ailus]

Add a printk modifier %p4cc (for pixel format) for printing V4L2 and DRM
pixel formats denoted by fourccs. The fourcc encoding is the same for both
so the same implementation can be used.

Suggested-by: Mauro Carvalho Chehab <mchehab at kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus at linux.intel.com>
---
Hi folks,

I believe I've addressed comments from Rasmus, Joe and Andy --- variables
that don't need to be negative have remained unsigned though. Thanks for
the suggestions, I believe this is much cleaner than v3.

since v3:

- Remove use of WARN_ON, assume the code is correct instead. A side effect
  of this is the code is much easier to understand.

- Check the modifier first before validating the buf pointer.

- Use isascii() and isprint() functions to weed out non-printable
  characters.

- Use hex_byte_pack() for printing 8-bit hexadecimal numbers.

- Remove macros, and instead use plain strings.

- Use strcpy() to copy the endianness string, and then strlen() to add its
  length.

- Strip the MSB (endianness bit), then use the value as such.

- Assign characters to the buffer and increment active pointer at the same
  time.

- Drop __ from variable names.

- Add example to documentation.

 Documentation/core-api/printk-formats.rst | 16 +++++++
 lib/test_printf.c                         | 17 ++++++++
 lib/vsprintf.c                            | 52 +++++++++++++++++++++++
 3 files changed, 85 insertions(+)

diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
index 6d26c5c6ac48..080262d2e030 100644
--- a/Documentation/core-api/printk-formats.rst
+++ b/Documentation/core-api/printk-formats.rst
@@ -565,6 +565,22 @@ For printing netdev_features_t.
 
 Passed by reference.
 
+V4L2 and DRM FourCC code (pixel format)
+---------------------------------------
+
+::
+
+	%p4cc
+
+Print a FourCC code used by V4L2 or DRM, including format endianness and
+its numerical value as hexadecimal.
+
+Passed by reference.
+
+Examples::
+
+	%p4cc	BG12 little-endian (0x32314742)
+
 Thanks
 ======
 
diff --git a/lib/test_printf.c b/lib/test_printf.c
index 7ac87f18a10f..7fb042542660 100644
--- a/lib/test_printf.c
+++ b/lib/test_printf.c
@@ -649,6 +649,22 @@ static void __init fwnode_pointer(void)
 	software_node_unregister(&softnodes[0]);
 }
 
+static void __init fourcc_pointer(void)
+{
+	struct {
+		u32 code;
+		char *str;
+	} const try[] = {
+		{ 0x20104646, "FF(10) little-endian (0x20104646)", },
+		{ 0xa0104646, "FF(10) big-endian (0xa0104646)", },
+		{ 0x10111213, "(13)(12)(11)(10) little-endian (0x10111213)", },
+	};
+	unsigned int i;
+
+	for (i = 0; i < ARRAY_SIZE(try); i++)
+		test(try[i].str, "%p4cc", &try[i].code);
+}
+
 static void __init
 errptr(void)
 {
@@ -694,6 +710,7 @@ test_pointer(void)
 	flags();
 	errptr();
 	fwnode_pointer();
+	fourcc_pointer();
 }
 
 static void __init selftest(void)
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 14c9a6af1b23..2be86b302c88 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1733,6 +1733,55 @@ char *netdev_bits(char *buf, char *end, const void *addr,
 	return special_hex_number(buf, end, num, size);
 }
 
+static noinline_for_stack
+char *fourcc_string(char *buf, char *end, const u32 *fourcc,
+		    struct printf_spec spec, const char *fmt)
+{
+	char output[sizeof("(xx)(xx)(xx)(xx) little-endian (0x01234567)")];
+	char *p = output;
+	unsigned int i;
+	u32 val;
+
+	if (fmt[1] != 'c' || fmt[2] != 'c')
+		return error_string(output, end, "(%p4?)", spec);
+
+	if (check_pointer(&buf, end, fourcc, spec))
+		return buf;
+
+	val = *fourcc & ~BIT(31);
+
+	for (i = 0; i < sizeof(*fourcc); i++) {
+		unsigned char c = val >> (i * 8);
+
+		/* Weed out spaces */
+		if (c == ' ')
+			continue;
+
+		/* Print non-control ASCII characters as-is */
+		if (isascii(c) && isprint(c)) {
+			*p++ = c;
+			continue;
+		}
+
+		*p++ = '(';
+		p = hex_byte_pack(p, c);
+		*p++ = ')';
+	}
+
+	strcpy(p, *fourcc & BIT(31) ? " big-endian" : " little-endian");
+	p += strlen(p);
+
+	*p++ = ' ';
+	*p++ = '(';
+	/* subtract parenthesis and the space from the size */
+	p = special_hex_number(p, output + sizeof(output) - 2, *fourcc,
+			       sizeof(u32));
+	*p++ = ')';
+	*p = '\0';
+
+	return string(buf, end, output, spec);
+}
+
 static noinline_for_stack
 char *address_val(char *buf, char *end, const void *addr,
 		  struct printf_spec spec, const char *fmt)
@@ -2162,6 +2211,7 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
  *       correctness of the format string and va_list arguments.
  * - 'K' For a kernel pointer that should be hidden from unprivileged users
  * - 'NF' For a netdev_features_t
+ * - '4cc' V4L2 or DRM FourCC code, with endianness and raw numerical value.
  * - 'h[CDN]' For a variable-length buffer, it prints it as a hex string with
  *            a certain separator (' ' by default):
  *              C colon
@@ -2259,6 +2309,8 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 		return restricted_pointer(buf, end, ptr, spec);
 	case 'N':
 		return netdev_bits(buf, end, ptr, spec, fmt);
+	case '4':
+		return fourcc_string(buf, end, ptr, spec, fmt);
 	case 'a':
 		return address_val(buf, end, ptr, spec, fmt);
 	case 'd':
-- 
2.27.0