Re: [PATCH v2 2/3] lib/vsprintf: Add support for generic FOURCCs by extending %p4cc

[Aditya Garg <gargaditya08@live.com>]

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@live.com> wrote:
> 
> From: Hector Martin <marcan@marcan.st>
> 
> %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@marcan.st>
> Signed-off-by: Aditya Garg <gargaditya08@live.com>
> ---
> 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
>