I request the printk maintainers for their views on whether if they are ok with the sparse errors in this original patch.
> On 20 Feb 2025, at 10:09 PM, Aditya Garg <gargaditya08@xxxxxxxx> wrote:
>
> From: Hector Martin <marcan@xxxxxxxxx>
>
> %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@xxxxxxxxx>
> Signed-off-by: Aditya Garg <gargaditya08@xxxxxxxx>
> ---
> v2 -> Add this patch
> Documentation/core-api/printk-formats.rst | 32 +++++++++++++++++++
> lib/test_printf.c | 39 +++++++++++++++++++----
> lib/vsprintf.c | 38 ++++++++++++++++++----
> scripts/checkpatch.pl | 2 +-
> 4 files changed, 97 insertions(+), 14 deletions(-)
>
> diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
> index ecccc0473..9982861fa 100644
> --- a/Documentation/core-api/printk-formats.rst
> +++ b/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[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 59dbe4f9a..ee860327e 100644
> --- a/lib/test_printf.c
> +++ b/lib/test_printf.c
> @@ -776,21 +776,46 @@ static void __init fwnode_pointer(void)
> software_node_unregister_node_group(group);
> }
>
> +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 56fe96319..13733a4da 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1781,27 +1781,53 @@ 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':
> + orig = swab32(orig);
> + val = orig;
> + break;
> + case 'l':
> + orig = le32_to_cpu(orig);
> + val = orig;
> + break;
> + case 'b':
> + orig = be32_to_cpu(orig);
> + val = 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++ = '(';
> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
> index 7b28ad331..21516f753 100755
> --- a/scripts/checkpatch.pl
> +++ b/scripts/checkpatch.pl
> @@ -6904,7 +6904,7 @@ sub process {
> ($extension eq "f" &&
> defined $qualifier && $qualifier !~ /^w/) ||
> ($extension eq "4" &&
> - defined $qualifier && $qualifier !~ /^cc/)) {
> + defined $qualifier && $qualifier !~ /^c[chlbr]/)) {
> $bad_specifier = $specifier;
> last;
> }
> --
> 2.43.0
>
