Re: [RESEND PATCH v3 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

[Joe Perches]

On Mon, 2020-04-27 at 09:02 -0700, Joe Perches wrote:
> On Mon, 2020-04-27 at 17:53 +0300, Sakari Ailus wrote:
> > 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.
> []
> > - Added WARN_ON_ONCE() sanity checks. Comments on these are welcome; I'd
> >   expect them mostly be covered by the tests.

perhaps this is simpler?
---
 lib/vsprintf.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 7c488a..3e1dbd7 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1721,6 +1721,46 @@ 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;
+	int i;
+	u32 val;
+
+	if (check_pointer(&buf, end, fourcc, spec))
+		return buf;
+
+	if (fmt[1] != 'c' || fmt[2] != 'c')
+		return error_string(buf, end, "(%p4?)", spec);
+
+	val = *fourcc & ~BIT(31);
+
+	for (i = 0; i < 4; i++) {
+		unsigned char c = val >> (i * 8);
+
+		if (isascii(c) && isprint(c)) {
+			*p++ = c;
+		} else {
+			*p++ = '(';
+			p = hex_byte_pack(p, c);
+			*p++ = ')';
+		}
+	}
+
+	strcpy(p, *fourcc & BIT(31) ? "big endian" : "little endian");
+	p += strlen(p);
+	*p++ = ' ';
+	*p++ = '(';
+	p = special_hex_number(p, p + 10, val, 4);
+	*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)
@@ -2131,6 +2171,7 @@ 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
@@ -2223,6 +2264,8 @@ 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':