linux-kernel - Re: [PATCH v5] lib/vsprintf: Add support for generic FourCCs by extending %p4cc

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Message-ID: <AD17906E-5848-4274-B732-C97A7FECE900@live.com>
Date: Fri, 28 Feb 2025 16:41:11 +0000
From: Aditya Garg <gargaditya08@...e.com>
To: Petr Mladek <pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>,
	"andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>, "senozhatsky@...omium.org"
	<senozhatsky@...omium.org>, Jonathan Corbet <corbet@....net>, Andrew Morton
	<akpm@...ux-foundation.org>, "apw@...onical.com" <apw@...onical.com>,
	"joe@...ches.com" <joe@...ches.com>, "dwaipayanray1@...il.com"
	<dwaipayanray1@...il.com>, "lukas.bulwahn@...il.com"
	<lukas.bulwahn@...il.com>
CC: "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>, Linux Kernel
 Mailing List <linux-kernel@...r.kernel.org>, Hector Martin
	<marcan@...can.st>, Sven Peter <sven@...npeter.dev>, Janne Grunau
	<j@...nau.net>, Alyssa Rosenzweig <alyssa@...enzweig.io>, Asahi Linux Mailing
 List <asahi@...ts.linux.dev>
Subject: Re: [PATCH v5] lib/vsprintf: Add support for generic FourCCs by
 extending %p4cc



> On 28 Feb 2025, at 9:59 PM, 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-endian
> %p4cn Reverse-endian
> %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).
> 
> Signed-off-by: Hector Martin <marcan@...can.st>
> Signed-off-by: Aditya Garg <gargaditya08@...e.com>
> ---
> v2 ->
> - Add this patch to appletbdrm patchset
> 
> v3 ->
> - Make array static
> 
> v4 ->
> - Fix code error
> - Fix sparse warnings
> - Make this patch separate from drm
> 
> v5 ->
> - Improve documentation
> - Use better logic to assign value of val
> - Use 'n' instead of 'r' for reverse endian
> - Use ARRAY_SIZE() instead of hardcoding 1
> 
> Documentation/core-api/printk-formats.rst | 32 +++++++++++++++++++
> lib/test_printf.c                         | 39 +++++++++++++++++++----
> lib/vsprintf.c                            | 35 ++++++++++++++++----
> scripts/checkpatch.pl                     |  2 +-
> 4 files changed, 94 insertions(+), 14 deletions(-)
> 
> diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
> index ecccc0473..14853295a 100644
> --- 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-endian, reverse-host-endian, 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)
> + %p4cl 00Pg (0x30305067)
> + %p4cb gP00 (0x67503030)
> +
> Rust
> ----
> 
> diff --git a/lib/test_printf.c b/lib/test_printf.c
> index 59dbe4f9a..80d9fd374 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_cn = {
> + { 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)", },

Ugh, how did the [] get ruined here now. Pretty sure I added them. Anyways, I’ll wait for a review in case more changes are needed as well.