Re: [PATCH 1/2] lib/vsprintf: Add support for generic FourCCs by extending %p4cc

From: Aditya Garg
Date: Wed Mar 12 2025 - 07:49:49 EST



> On 12 Mar 2025, at 5:16 PM, Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:
>
> Hi
>
>> Am 12.03.25 um 10:05 schrieb Aditya Garg:
>> 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 byte order
>> %p4cn Network byte order
>> %p4cl Little-endian
>> %p4cb Big-endian
>
> That looks like someone trying to be too clever for their own good. Just my 2 cts.

I don't understand what you are trying to say. Anyways, I thought it's obvious, but Petr's Ack is still left and thus cannot be merged into DRM for now unless he says so in this thread.
>
> Best regards
> Thomas
>
>>
>> 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. %p4cn would
>> allow printing LSByte-first FourCCs stored in host endian order
>> (other than the hex form being in character order, not the integer
>> value).
>>
>> Acked-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
>> Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
>> Signed-off-by: Hector Martin <marcan@xxxxxxxxx>
>> Signed-off-by: Aditya Garg <gargaditya08@xxxxxxxx>
>> ---
>> Documentation/core-api/printk-formats.rst | 32 +++++++++++++++++++
>> lib/test_printf.c | 39 +++++++++++++++++++----
>> lib/vsprintf.c | 35 ++++++++++++++++----
>> scripts/checkpatch.pl | 2 +-
>> 4 files changed, 94 insertions(+), 14 deletions(-)
>>
>> diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
>> index ecccc0473..bd420e8aa 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[hnlb] gP00 (0x67503030)
>> +
>> +Print a generic FourCC code, as both ASCII characters and its numerical
>> +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
>> +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.
>> +
>> +Passed by reference.
>> +
>> +Examples for a little-endian machine, given &(u32)0x67503030::
>> +
>> + %p4ch gP00 (0x67503030)
>> + %p4cn 00Pg (0x30305067)
>> + %p4cl gP00 (0x67503030)
>> + %p4cb 00Pg (0x30305067)
>> +
>> +Examples for a big-endian machine, given &(u32)0x67503030::
>> +
>> + %p4ch gP00 (0x67503030)
>> + %p4cn 00Pg (0x30305067)
>> + %p4cl 00Pg (0x30305067)
>> + %p4cb gP00 (0x67503030)
>> +
>> Rust
>> ----
>> diff --git a/lib/test_printf.c b/lib/test_printf.c
>> index 59dbe4f9a..b9e8afc01 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[] = {
>> + static const struct fourcc_struct try_cc[] = {
>> { 0x3231564e, "NV12 little-endian (0x3231564e)", },
>> { 0xb231564e, "NV12 big-endian (0xb231564e)", },
>> { 0x10111213, ".... little-endian (0x10111213)", },
>> { 0x20303159, "Y10 little-endian (0x20303159)", },
>> };
>> - unsigned int i;
>> + static const struct fourcc_struct try_ch[] = {
>> + { 0x41424344, "ABCD (0x41424344)", },
>> + };
>> + static const struct fourcc_struct try_cn[] = {
>> + { 0x41424344, "DCBA (0x44434241)", },
>> + };
>> + static const struct fourcc_struct try_cl[] = {
>> + { (__force u32)cpu_to_le32(0x41424344), "ABCD (0x41424344)", },
>> + };
>> + static const struct fourcc_struct try_cb[] = {
>> + { (__force u32)cpu_to_be32(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, ARRAY_SIZE(try_ch), "%p4ch");
>> + fourcc_pointer_test(try_cn, ARRAY_SIZE(try_cn), "%p4cn");
>> + fourcc_pointer_test(try_cl, ARRAY_SIZE(try_cl), "%p4cl");
>> + fourcc_pointer_test(try_cb, ARRAY_SIZE(try_cb), "%p4cb");
>> }
>> static void __init
>> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
>> index 56fe96319..56511a994 100644
>> --- a/lib/vsprintf.c
>> +++ b/lib/vsprintf.c
>> @@ -1781,27 +1781,50 @@ 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':
>> + break;
>> + case 'n':
>> + orig = swab32(orig);
>> + break;
>> + case 'l':
>> + orig = (__force u32)cpu_to_le32(orig);
>> + break;
>> + case 'b':
>> + orig = (__force u32)cpu_to_be32(orig);
>> + break;
>> + case 'c':
>> + /* Pixel formats are printed LSB-first */
>> + pixel_fmt = true;
>> + break;
>> + default:
>> + return error_string(buf, end, "(%p4?)", spec);
>> + }
>> +
>> + val = pixel_fmt ? swab32(orig & ~BIT(31)) : orig;
>> 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..5595a0898 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[hnlbc]/)) {
>> $bad_specifier = $specifier;
>> last;
>> }
>
> --
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Frankenstrasse 146, 90461 Nuernberg, Germany
> GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
> HRB 36809 (AG Nuernberg)
>