vsprintf: Use %p4chR instead of %p4cn for reading data in reversed host ordering

The generic FourCC format always prints the data using the big endian
order. It is generic because it allows to read the data using a custom
ordering.

The current code uses "n" for reading data in the reverse host ordering.
It makes the 4 variants [hnbl] consistent with the generic printing
of IPv4 addresses.

Unfortunately, it creates confusion on big endian systems. For example,
it shows the data &(u32)0x67503030 as

	%p4cn	00Pg (0x30305067)

But people expect that the ordering stays the same. The network ordering
is a big-endian ordering.

The problem is that the semantic is not the same. The modifiers affect
the output ordering of IPv4 addresses while they affect the reading order
in case of FourCC code.

Avoid the confusion by replacing the "n" modifier with "hR", aka
reverse host ordering. It is inspired by the existing %p[mM]R printf
format.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Closes: https://lore.kernel.org/r/CAMuHMdV9tX=TG7E_CrSF=2PY206tXf+_yYRuacG48EWEtJLo-Q@mail.gmail.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Aditya Garg <gargaditya08@live.com>
Link: https://lore.kernel.org/r/20250428123132.578771-1-pmladek@suse.com
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>

diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
index 125fd03..f531873 100644 (file)
--- a/Documentation/core-api/printk-formats.rst
+++ b/Documentation/core-api/printk-formats.rst
@@ -652,7 +652,7 @@ Generic FourCC code
 -------------------
 
 ::
-       %p4c[hnlb]      gP00 (0x67503030)
+       %p4c[h[R]lb]    gP00 (0x67503030)
 
 Print a generic FourCC code, as both ASCII characters and its numerical
 value as hexadecimal.
@@ -660,23 +660,23 @@ 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
+The additional ``h``, ``hR``, ``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.
+using the host, reversed host 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)
+       %p4chR  00Pg (0x30305067)
        %p4cl   gP00 (0x67503030)
        %p4cb   00Pg (0x30305067)
 
 Examples for a big-endian machine, given &(u32)0x67503030::
 
        %p4ch   gP00 (0x67503030)
-       %p4cn   00Pg (0x30305067)
+       %p4chR  00Pg (0x30305067)
        %p4cl   00Pg (0x30305067)
        %p4cb   gP00 (0x67503030)
 

diff --git a/lib/tests/printf_kunit.c b/lib/tests/printf_kunit.c
index b1fa0dc..bc54cca 100644 (file)
--- a/lib/tests/printf_kunit.c
+++ b/lib/tests/printf_kunit.c
@@ -726,7 +726,7 @@ static void fourcc_pointer(struct kunit *kunittest)
        static const struct fourcc_struct try_ch[] = {
                { 0x41424344, "ABCD (0x41424344)", },
        };
-       static const struct fourcc_struct try_cn[] = {
+       static const struct fourcc_struct try_chR[] = {
                { 0x41424344, "DCBA (0x44434241)", },
        };
        static const struct fourcc_struct try_cl[] = {
@@ -738,7 +738,7 @@ static void fourcc_pointer(struct kunit *kunittest)
 
        fourcc_pointer_test(kunittest, try_cc, ARRAY_SIZE(try_cc), "%p4cc");
        fourcc_pointer_test(kunittest, try_ch, ARRAY_SIZE(try_ch), "%p4ch");
-       fourcc_pointer_test(kunittest, try_cn, ARRAY_SIZE(try_cn), "%p4cn");
+       fourcc_pointer_test(kunittest, try_chR, ARRAY_SIZE(try_chR), "%p4chR");
        fourcc_pointer_test(kunittest, try_cl, ARRAY_SIZE(try_cl), "%p4cl");
        fourcc_pointer_test(kunittest, try_cb, ARRAY_SIZE(try_cb), "%p4cb");
 }

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 6bc64ae..d8a2ec0 100644 (file)
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1805,9 +1805,8 @@ char *fourcc_string(char *buf, char *end, const u32 *fourcc,
        orig = get_unaligned(fourcc);
        switch (fmt[2]) {
        case 'h':
-               break;
-       case 'n':
-               orig = swab32(orig);
+               if (fmt[3] == 'R')
+                       orig = swab32(orig);
                break;
        case 'l':
                orig = (__force u32)cpu_to_le32(orig);
@@ -2397,6 +2396,12 @@ early_param("no_hash_pointers", no_hash_pointers_enable);
  *       read the documentation (path below) first.
  * - 'NF' For a netdev_features_t
  * - '4cc' V4L2 or DRM FourCC code, with endianness and raw numerical value.
+ * - '4c[h[R]lb]' For generic FourCC code with raw numerical value. Both are
+ *      displayed in the big-endian format. This is the opposite of V4L2 or
+ *      DRM FourCCs.
+ *      The additional specifiers define what endianness is used to load
+ *      the stored bytes. The data might be interpreted using the host,
+ *      reversed host byte order, little-endian, or big-endian.
  * - 'h[CDN]' For a variable-length buffer, it prints it as a hex string with
  *            a certain separator (' ' by default):
  *              C colon