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

[Petr Mladek <pmladek@suse.com>]

On Thu 2025-02-27 06:30:48, Aditya Garg 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: Aditya Garg <gargaditya08@live.com>
>

> --- 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[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 am a bit confused by the description like I was in the past, see
https://lore.kernel.org/r/Y3zhhLoqAOaZ7rMz@alley  ;-)

I wonder if the following sounds better:

<proposa>
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 bytes. The data might be interpreted
using the host-endian, reverse-host-endian, big-endian, or little endian.
</proposal>

> +Passed by reference.
> +
> +Examples for a little-endian machine, given &(u32)0x67503030::
> +
> +	%p4ch	gP00 (0x67503030)
> +	%p4cr	00Pg (0x30305067)
> +	%p4cb	00Pg (0x30305067)
> +	%p4cl	gP00 (0x67503030)
> +
> +Examples for a big-endian machine, given &(u32)0x67503030::
> +
> +	%p4ch	gP00 (0x67503030)
> +	%p4cr	00Pg (0x30305067)
> +	%p4cb	gP00 (0x67503030)
> +	%p4cl	00Pg (0x30305067)
> +
> Rust

The patch has been malformed. I guess that your mail client
removed spaces at the beginning of some lines.

> ----
> 
> diff --git a/lib/test_printf.c b/lib/test_printf.c
> index 59dbe4f9a..056929c06 100644
> --- a/lib/test_printf.c
> +++ b/lib/test_printf.c
> @@ -776,21 +776,46 @@ static void __init fwnode_pointer(void)
> 	software_node_unregister_node_group(group);
> }
> 
> +struct fourcc_struct {
> +	u32 code;
> +	const char *str;
> +};
> +
> +static void __init fourcc_pointer_test(const struct fourcc_struct *fc, size_t n,
> +				       const char *fmt)
> +{
> +	size_t i;
> +
> +	for (i = 0; i < n; i++)
> +		test(fc[i].str, fmt, &fc[i].code);
> +}
> +
> static void __init fourcc_pointer(void)
> {
> -	struct {
> -		u32 code;
> -		char *str;
> -	} const try[] = {
> +	static const struct fourcc_struct try_cc[] = {
> 		{ 0x3231564e, "NV12 little-endian (0x3231564e)", },
> 		{ 0xb231564e, "NV12 big-endian (0xb231564e)", },
> 		{ 0x10111213, ".... little-endian (0x10111213)", },
> 		{ 0x20303159, "Y10  little-endian (0x20303159)", },
> 	};
> -	unsigned int i;
> +	static const struct fourcc_struct try_ch = {
> +		0x41424344, "ABCD (0x41424344)",
> +	};
> +	static const struct fourcc_struct try_cr = {
> +		0x41424344, "DCBA (0x44434241)",
> +	};
> +	static const struct fourcc_struct try_cl = {
> +		le32_to_cpu(0x41424344), "ABCD (0x41424344)",
> +	};
> +	static const struct fourcc_struct try_cb = {
> +		be32_to_cpu(0x41424344), "ABCD (0x41424344)",
> +	};
> 
> -	for (i = 0; i < ARRAY_SIZE(try); i++)
> -		test(try[i].str, "%p4cc", &try[i].code);
> +	fourcc_pointer_test(try_cc, ARRAY_SIZE(try_cc), "%p4cc");
> +	fourcc_pointer_test(&try_ch, 1, "%p4ch");
> +	fourcc_pointer_test(&try_cr, 1, "%p4cr");
> +	fourcc_pointer_test(&try_cl, 1, "%p4cl");
> +	fourcc_pointer_test(&try_cb, 1, "%p4cb");

Nit: I would use ARRAY_SIZE() instead of the hardcoded 1 in all cases.
     But it might be a matter of taste.

> }
> 

Otherwise, it looks good to me.

Best Regards,
Petr