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

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Message-ID: <CAMuHMdWpqHLest0oqiB+hG47t=G7OScLmHz5zr2u0ZgED_+Obg@mail.gmail.com>
Date: Tue, 22 Apr 2025 10:43:59 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Aditya Garg <gargaditya08@...e.com>
Cc: Hector Martin <marcan@...can.st>, alyssa@...enzweig.io, Petr Mladek <pmladek@...e.com>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Sven Peter <sven@...npeter.dev>, 
	Thomas Zimmermann <tzimmermann@...e.de>, Aun-Ali Zaidi <admin@...eit.net>, 
	Maxime Ripard <mripard@...nel.org>, airlied@...hat.com, Simona Vetter <simona@...ll.ch>, 
	Steven Rostedt <rostedt@...dmis.org>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>, Jonathan Corbet <corbet@....net>, 
	Andrew Morton <akpm@...ux-foundation.org>, apw@...onical.com, joe@...ches.com, 
	dwaipayanray1@...il.com, lukas.bulwahn@...il.com, Kees Cook <kees@...nel.org>, 
	tamird@...il.com, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org, 
	Asahi Linux Mailing List <asahi@...ts.linux.dev>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 1/3] lib/vsprintf: Add support for generic FourCCs by
 extending %p4cc

Hi Aditya,

CC netdev

On Tue, 22 Apr 2025 at 10:30, Aditya Garg <gargaditya08@...e.com> wrote:
> On 22-04-2025 01:37 pm, Geert Uytterhoeven wrote:
> > On Tue, 8 Apr 2025 at 08:48, Aditya Garg <gargaditya08@...e.com> wrote:
> >> From: Hector Martin <marcan@...can.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@...musvillemoes.dk>
> >> Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> >> Reviewed-by: Petr Mladek <pmladek@...e.com>
> >> Tested-by: Petr Mladek <pmladek@...e.com>
> >> Signed-off-by: Hector Martin <marcan@...can.st>
> >> Signed-off-by: Aditya Garg <gargaditya08@...e.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.
>
> Originally, it was %p4cr (reverse-endian), but on the request of the maintainers, it was changed to %p4cn.

Ah, I found it[1]:

| so, it needs more information that this mimics htonl() / ntohl() for
networking.

IMHO this does not mimic htonl(), as htonl() is a no-op on big-endian.
while %p4ch and %p4cl yield different results on big-endian.

> So here network means reverse of host, not strictly big-endian.

Please don't call it "network byte order" if that does not have the same
meaning as in the network subsystem.

Personally, I like "%p4r" (reverse) more...
(and "%p4ch" might mean human-readable ;-)

[1] https://lore.kernel.org/all/Z8B6DwcRbV-8D8GB@smile.fi.intel.com

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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