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

Geert Uytterhoeven
2025-04-23T14:58:06Z

Hi Petr,

On Wed, 23 Apr 2025 at 15:39, Petr Mladek <pmladek@suse.com> wrote:
> On Tue 2025-04-22 10:43:59, Geert Uytterhoeven wrote:
> > On Tue, 22 Apr 2025 at 10:30, Aditya Garg <gargaditya08@live.com> wrote:
> > > On 22-04-2025 01:37 pm, Geert Uytterhoeven wrote:
> > > > 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.
> > >
> > > 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
>
> I have to admit that I was always a bit confused by the meaning of the
> new modifiers. And I did give up at some point and decided to do not
> block the patch when it made sense to others.
>
> But I have to agree with Geert here. The current behavior of %p4ch
> is confusing on big endian system. I would expect that it does not
> revert the ordering.
>
> Well, I still think that people might find all 4 variants useful.
> Andy does not like "r". What about "hR"? It is inspired by
> the existing %pmR.

I am not a fan of complicating the format specifier even more by adding
more characters... But seeing %pmR, I have to admit it does make sense.

> The problem is that the semantic is not the same. The modifiers affect
> the output ordering of IPv4 addresses while they affect the reading order
> in case of FourCC code.

Note that for IPv4 addresses we have %pI4, which BTW also takes [hnbl]
modifiers.

> Avoid the confusion by replacing the "n" modifier with "hR", aka
> reverse host ordering.
>
> Signed-off-by: Petr Mladek <pmladek@suse.com>

Thanks, LGTM!

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