Re: [PATCH v4 1/3] lib/vsprintf: Add support for generic FourCCs by extending %p4cc

Geert Uytterhoeven
2025-04-22T08:14:37Z

Hi Aditya, Hector,

On Tue, 8 Apr 2025 at 08:48, 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 byte order
> %p4cn Network byte order
> %p4cl Little-endian
> %p4cb Big-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. %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@rasmusvillemoes.dk>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Reviewed-by: Petr Mladek <pmladek@suse.com>
> Tested-by: Petr Mladek <pmladek@suse.com>
> Signed-off-by: Hector Martin <marcan@marcan.st>
> Signed-off-by: Aditya Garg <gargaditya08@live.com>

Thanks for your patch, which is now commit 1938479b2720ebc0
("lib/vsprintf: Add support for generic FourCCs by extending %p4cc")
in drm-misc-next/

> --- 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)

This doesn't look right to me, as network byte order is big endian?
Note that I didn't check the code.

> + %p4cl 00Pg (0x30305067)
> + %p4cb gP00 (0x67503030)
> +
> Rust
> ----
>

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds