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

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Message-ID:
 <PN3PR01MB9597B3AE75E009857AA12D4DB8BB2@PN3PR01MB9597.INDPRD01.PROD.OUTLOOK.COM>
Date: Tue, 22 Apr 2025 13:59:57 +0530
From: Aditya Garg <gargaditya08@...e.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>,
 Hector Martin <marcan@...can.st>
Cc: 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>
Subject: Re: [PATCH v4 1/3] lib/vsprintf: Add support for generic FourCCs by
 extending %p4cc



On 22-04-2025 01:37 pm, Geert Uytterhoeven wrote:
> Hi Aditya, Hector,
> 
> 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.

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

> 
>> +       %p4cl   00Pg (0x30305067)
>> +       %p4cb   gP00 (0x67503030)
>> +
>>  Rust
>>  ----
>>
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
>