Re: [PATCH v5 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Petr Mladek <pmladek@suse.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux Media Mailing List <linux-media@vger.kernel.org>,
	Dave Stevenson <dave.stevenson@raspberrypi.com>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Hans Verkuil <hverkuil@xs4all.nl>,
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Joe Perches <joe@perches.com>,
	Jani Nikula <jani.nikula@linux.intel.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>
Subject: Re: [PATCH v5 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs
Date: Thu, 10 Dec 2020 16:08:05 +0200	[thread overview]
Message-ID: <20201210140805.GI4077@smile.fi.intel.com> (raw)
In-Reply-To: <20201210135526.GH25763@paasikivi.fi.intel.com>

On Thu, Dec 10, 2020 at 03:55:27PM +0200, Sakari Ailus wrote:
> On Thu, Dec 10, 2020 at 03:05:02PM +0200, Andy Shevchenko wrote:
> > On Thu, Dec 10, 2020 at 2:16 PM Petr Mladek <pmladek@suse.com> wrote:
> > > On Fri 2020-11-13 12:54:41, Sakari Ailus wrote:
> > > > Add a printk modifier %p4cc (for pixel format) for printing V4L2 and DRM
> > > > pixel formats denoted by fourccs. The fourcc encoding is the same for both
> > > > so the same implementation can be used.
> > > >
> > > > Suggested-by: Mauro Carvalho Chehab <mchehab@kernel.org>
> > > > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > >
> > > Andy, Rasmus,
> > >
> > > the last version looks fine to me. I am going to push it.
> > > Please, speak up if you are against it.
> > 
> > My concerns are:
> > - not so standard format of representation (why not to use
> > string_escape_mem() helper?) or is it?
> 
> The format string may contain spaces that are not meant to be printed.
> Other unprintable chacaters should not be present (at least not in V4L2
> pixelformats). The hexadecimal representation is there to convey the
> numerical value and that originally came from DRM, not V4L2.

Yes, but I mean that we usually anticipate the escaped characters in a form of
'\xNN' (hex) or '\NNN' (octal). The format '(NN)' is quite unusual to me.

> > - no compatibility with generic 4cc
> >   (I would rather have an additional specifier here for v4l2 cases.
> 
> What do you mean by "generic 4cc"? There are two users of 4cc codes in the
> kernel that I know of: V4L2 and DRM. Something that does not refer to
> in-memory pixel formats?

Of course. Everything else. 4cc is a generic term to describe something which
is of 4 characters long [1]. It's not limited by media file formats. And
moreover some (chip) vendors are using it as well (Synopsys).
Microsoft uses 4cc in CSRT ACPI table for vendor field and so on...

[1]: https://en.wikipedia.org/wiki/FourCC

-- 
With Best Regards,
Andy Shevchenko