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Date: Wed, 23 Apr 2025 13:47:24 +0000
From: Aditya Garg <gargaditya08@...e.com>
To: Petr Mladek <pmladek@...e.com>
CC: Geert Uytterhoeven <geert@...ux-m68k.org>, Hector Martin
	<marcan@...can.st>, "alyssa@...enzweig.io" <alyssa@...enzweig.io>, Andy
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 Mailing List <linux-kernel@...r.kernel.org>,
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	<netdev@...r.kernel.org>
Subject: Re: [PATCH v4 1/3] lib/vsprintf: Add support for generic FourCCs by
 extending %p4cc



> On 23 Apr 2025, at 7:09 PM, Petr Mladek <pmladek@...e.com> wrote:
> 
> On Tue 2025-04-22 10:43:59, Geert Uytterhoeven wrote:
>> 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
> 
> 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 tried to implement it and the complexity of the code is similar:
> 
> From f6aa2213cec9b9d25c0506e3112f32e90a18aa7f Mon Sep 17 00:00:00 2001
> From: Petr Mladek <pmladek@...e.com>
> Date: Wed, 23 Apr 2025 15:02:10 +0200
> Subject: [PATCH] vsprintf: Use %p4chR instead of %p4cn for reading data in
> reversed host ordering
> 
> The generic FourCC format always prints the data using the big endian
> order. It is generic because it allows to read the data using a custom
> ordering.
> 
> The current code uses "n" for reading data in the reverse host ordering.
> It makes the 4 variants [hnbl] consistent with the generic printing
> of IPv4 addresses.
> 
> Unfortunately, it creates confusion on big endian systems. For example,
> it shows the data &(u32)0x67503030 as
> 
>    %p4cn    00Pg (0x30305067)
> 
> But people expect that the ordering stays the same. The network ordering
> is a big-endian ordering.
> 
> 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.
> 
> Avoid the confusion by replacing the "n" modifier with "hR", aka
> reverse host ordering.
> 
> Signed-off-by: Petr Mladek <pmladek@...e.com>
> ---
> Documentation/core-api/printk-formats.rst | 10 +++++-----
> lib/tests/printf_kunit.c                  |  2 +-
> lib/vsprintf.c                            | 11 ++++++++---
> 3 files changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
> index 125fd0397510..f531873bb3c9 100644
> --- a/Documentation/core-api/printk-formats.rst
> +++ b/Documentation/core-api/printk-formats.rst
> @@ -652,7 +652,7 @@ Generic FourCC code
> -------------------
> 
> ::
> -    %p4c[hnlb]    gP00 (0x67503030)
> +    %p4c[h[R]lb]    gP00 (0x67503030)
> 
> Print a generic FourCC code, as both ASCII characters and its numerical
> value as hexadecimal.
> @@ -660,23 +660,23 @@ 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
> +The additional ``h``, ``hR``, ``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.
> +using the host, reversed host 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)
> +    %p4chR    00Pg (0x30305067)
>    %p4cl    gP00 (0x67503030)
>    %p4cb    00Pg (0x30305067)
> 
> Examples for a big-endian machine, given &(u32)0x67503030::
> 
>    %p4ch    gP00 (0x67503030)
> -    %p4cn    00Pg (0x30305067)
> +    %p4chR    00Pg (0x30305067)
>    %p4cl    00Pg (0x30305067)
>    %p4cb    gP00 (0x67503030)
> 
> diff --git a/lib/tests/printf_kunit.c b/lib/tests/printf_kunit.c
> index b1fa0dcea52f..b8a4b5006f9c 100644
> --- a/lib/tests/printf_kunit.c
> +++ b/lib/tests/printf_kunit.c
> @@ -738,7 +738,7 @@ static void fourcc_pointer(struct kunit *kunittest)
> 
>    fourcc_pointer_test(kunittest, try_cc, ARRAY_SIZE(try_cc), "%p4cc");
>    fourcc_pointer_test(kunittest, try_ch, ARRAY_SIZE(try_ch), "%p4ch");
> -    fourcc_pointer_test(kunittest, try_cn, ARRAY_SIZE(try_cn), "%p4cn");
> +    fourcc_pointer_test(kunittest, try_cn, ARRAY_SIZE(try_cn), "%p4chR");

Maybe rename try_cn to try_chR?

In any case, it looks good to me.

>    fourcc_pointer_test(kunittest, try_cl, ARRAY_SIZE(try_cl), "%p4cl");
>    fourcc_pointer_test(kunittest, try_cb, ARRAY_SIZE(try_cb), "%p4cb");
> }
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 2c5de4216415..34587b2dbdb1 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1804,9 +1804,8 @@ char *fourcc_string(char *buf, char *end, const u32 *fourcc,
>    orig = get_unaligned(fourcc);
>    switch (fmt[2]) {
>    case 'h':
> -        break;
> -    case 'n':
> -        orig = swab32(orig);
> +        if (fmt[3] == 'R')
> +            orig = swab32(orig);
>        break;
>    case 'l':
>        orig = (__force u32)cpu_to_le32(orig);
> @@ -2396,6 +2395,12 @@ early_param("no_hash_pointers", no_hash_pointers_enable);
>  *       read the documentation (path below) first.
>  * - 'NF' For a netdev_features_t
>  * - '4cc' V4L2 or DRM FourCC code, with endianness and raw numerical value.
> + * - '4c[h[R]lb]' For generic FourCC code with raw numerical value. Both are
> + *     displayed in the big-endian format. This is the opposite of V4L2 or
> + *     DRM FourCCs.
> + *     The additional specifiers define what endianness is used to load
> + *     the stored bytes. The data might be interpreted using the host,
> + *     reversed host byte order, little-endian, or big-endian.
>  * - 'h[CDN]' For a variable-length buffer, it prints it as a hex string with
>  *            a certain separator (' ' by default):
>  *              C colon
> --
> 2.49.0
> 

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