linux-kernel - Re: [PATCH v2 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

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Message-ID: <1105bfe5-88f1-040e-db40-54d7761747d5@rasmusvillemoes.dk>
Date:   Fri, 3 Apr 2020 14:10:53 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Petr Mladek <pmladek@...e.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-media@...r.kernel.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        hverkuil@...all.nl, laurent.pinchart@...asonboard.com,
        mchehab@...nel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joe Perches <joe@...ches.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>
Subject: Re: [PATCH v2 1/1] lib/vsprintf: Add support for printing V4L2 and
 DRM fourccs

On 03/04/2020 11.11, Sakari Ailus wrote:
> Add a printk modifier %ppf (for pixel format) for printing V4L2 and DRM
> pixel formats denoted by 4ccs. The 4cc encoding is the same for both so
> the same implementation can be used.

This seems quite niche to me, I'm not sure that belongs in vsprintf.c.
What's wrong with having a

char *fourcc_string(char *buf, u32 x)

that formats x into buf and returns buf, so it can be used in a

char buf[8];
pr_debug("bla: %s\n", fourcc_string(buf, x))

Or, for that matter, since it's for debugging, why not just print x with
0x%08x?

At the very least, the "case '4'" in pointer() should be guarded by
appropriate CONFIG_*.

Good that Documentation/ gets updated, but test_printf needs updating as
well.


> Suggested-by: Mauro Carvalho Chehab <mchehab@...nel.org>
> Signed-off-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> ---
> since v1:
> 
> - Improve documentation (add -BE suffix, refer to "FourCC".
> 
> - Use '%p4cc' conversion specifier instead of '%ppf'.

Cute. Remember to update the commit log (which still says %ppf).

> - Fix 31st bit handling in printing FourCC codes.
> 
> - Use string() correctly, to allow e.g. proper field width handling.
> 
> - Remove loop, use put_unaligned_le32() instead.
> 
>  Documentation/core-api/printk-formats.rst | 12 +++++++++++
>  lib/vsprintf.c                            | 25 +++++++++++++++++++++++
>  2 files changed, 37 insertions(+)
> 
> diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
> index 8ebe46b1af39..550568520ab6 100644
> --- a/Documentation/core-api/printk-formats.rst
> +++ b/Documentation/core-api/printk-formats.rst
> @@ -545,6 +545,18 @@ For printing netdev_features_t.
>  
>  Passed by reference.
>  
> +V4L2 and DRM FourCC code (pixel format)
> +---------------------------------------
> +
> +::
> +
> +	%p4cc
> +
> +Print a FourCC code used by V4L2 or DRM. The "-BE" suffix is added on big endian
> +formats.
> +
> +Passed by reference.

Maybe it's obvious to anyone in that business, but perhaps make it more
clear the 4cc is stored in a u32 (and not, e.g., a __le32 or some other
integer), that obviously matters when the code treats the pointer as a u32*.
> +
> +	put_unaligned_le32(*fourcc & ~BIT(31), s);
> +
> +	if (*fourcc & BIT(31))
> +		strscpy(s + sizeof(*fourcc), FOURCC_STRING_BE,
> +			sizeof(FOURCC_STRING_BE));

put_unaligned_le32(0x0045422d, s + 4) probably generates smaller code,
and is more in line with building the first part of the string with
put_unaligned_le32().

Rasmus