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

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Message-ID: <20201210140805.GI4077@smile.fi.intel.com>
Date:   Thu, 10 Dec 2020 16:08:05 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Hans Verkuil <hverkuil@...all.nl>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mauro Carvalho Chehab <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>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v5 1/1] lib/vsprintf: Add support for printing V4L2 and
 DRM fourccs

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@...e.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@...nel.org>
> > > > Signed-off-by: Sakari Ailus <sakari.ailus@...ux.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