Re: [PATCH v3 2/7] lib/vsprintf: Add support for generic FOURCCs by extending %p4cc

[Petr Mladek <pmladek@suse.com>]

On Tue 2022-11-08 16:33:22, Russell King 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-endian
> %p4cl	Little-endian
> %p4cb	Big-endian
> %p4cr	Reverse-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. %p4cr would
> allow printing LSByte-first FOURCCs stored in host endian order
> (other than the hex form being in character order, not the integer
> value).
> 
> Signed-off-by: Hector Martin <marcan@marcan.st>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
> ---
>  Documentation/core-api/printk-formats.rst | 32 +++++++++++++++++++
>  lib/test_printf.c                         | 39 +++++++++++++++++++----
>  lib/vsprintf.c                            | 35 ++++++++++++++++----
>  3 files changed, 93 insertions(+), 13 deletions(-)
> 
> diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
> index dbe1aacc79d0..92a488884cf8 100644
> --- a/Documentation/core-api/printk-formats.rst
> +++ b/Documentation/core-api/printk-formats.rst
> @@ -625,6 +625,38 @@ Passed by reference.
>  	%p4cc	Y10  little-endian (0x20303159)
>  	%p4cc	NV12 big-endian (0xb231564e)
>  
> +Generic FourCC code
> +-------------------
> +
> +::
> +	%p4c[hrbl]	gP00 (0x67503030)
> +
> +Print a generic FourCC code, as both ASCII characters and its numerical
> +value as hexadecimal.
> +
> +The additional ``h``, ``r``, ``b``, and ``l`` specifiers are used to specify
> +host, reversed, big or little endian order data respectively. Host endian
> +order means the data is interpreted as a 32-bit integer and the most
> +significant byte is printed first; that is, the character code as printed
> +matches the byte order stored in memory on big-endian systems, and is reversed
> +on little-endian systems.

I though a bit more about the semantic and got a bit confused.
It might be because I am not familiar with FourCC. Anyway,
the description in the commit message provided some more clues.

The following documentation looks be more clear to me:

<proposal>
Generic FourCC code
-------------------

::
	%p4c[hrbl]	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 the big-endian format,
the most significant byte first. This is the opposite of V4L/DRM
FOURCCs.

The additional ``h``, ``r``, ``b``, and ``l`` specifiers define what
endianes is used to load the stored value as 32-bit integer. The value
might be stored as host-endian, reverse-host-endian, big-endian,
or little endian.

Examples for a little-endian machine, host native load &(u32)0x67503030::

	%p4ch	gP00 (0x67503030)
	%p4cr	00Pg (0x30305067)
	%p4cb	00Pg (0x30305067)
	%p4cl	gP00 (0x67503030)

Examples for a big-endian machine, host native load &(u32)0x67503030::

	%p4ch	gP00 (0x67503030)
	%p4cr	00Pg (0x30305067)
	%p4cb	gP00 (0x67503030)
	%p4cl	00Pg (0x30305067)
</proposal>

Best Regards,
Petr