linux-kernel - Re: [PATCH v6 1/3] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

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Message-ID: <YCZvY7tc2xCmtrZ6@alley>
Date:   Fri, 12 Feb 2021 13:06:59 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.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 v6 1/3] lib/vsprintf: Add support for printing V4L2 and
 DRM fourccs

On Fri 2021-02-12 13:28:56, Sakari Ailus wrote:
> On Thu, Feb 11, 2021 at 06:14:28PM +0100, Petr Mladek wrote:
> > On Tue 2021-02-09 19:47:55, Sakari Ailus wrote:
> > > Hi Andy,
> > > 
> > > On Tue, Feb 09, 2021 at 11:58:40AM +0200, Andy Shevchenko wrote:
> > > > On Tue, Feb 09, 2021 at 11:20:32AM +0200, Sakari Ailus wrote:
> > > > > On Mon, Feb 08, 2021 at 10:43:30PM +0200, Andy Shevchenko wrote:
> > > > > > On Mon, Feb 8, 2021 at 10:11 PM Sakari Ailus
> > > > > > <sakari.ailus@...ux.intel.com> wrote:
> > > > 
> > > > > > > +       %p4cc   BG12 little-endian (0x32314742)
> > > > > > 
> > > > > > This misses examples of the (strange) escaping cases and wiped
> > > > > > whitespaces to make sure everybody understands that 'D 12' will be the
> > > > > > same as 'D1 2' (side note: which I disagree on, perhaps something
> > > > > > should be added into documentation why).
> 
> I discussed this with Hans and we concluded spaces shouldn't be dropped.
> 
> Spaces can be present in the codes themselves in any case.

Great!

> > 
> > > > 
> > > > ...
> > > > 
> > > > > > > +static noinline_for_stack
> > > > > > > +char *fourcc_string(char *buf, char *end, const u32 *fourcc,
> > > > > > > +                   struct printf_spec spec, const char *fmt)
> > > > > > > +{
> > > > > > 
> > > > > > > +       char output[sizeof("(xx)(xx)(xx)(xx) little-endian (0x01234567)")];
> > > > > > 
> > > > > > Do we have any evidence / document / standard that the above format is
> > > > > > what people would find good? From existing practices (I consider other
> > > > > > printings elsewhere and users in this series) I find '(xx)' form for
> > > > > > hex numbers is weird. The standard practice is to use \xHH (without
> > > > > > parentheses).
> > > > > 
> > > > > Earlier in the review it was proposed that special handling of codes below
> > > > > 32 should be added, which I did. Using the parentheses is apparently an
> > > > > existing practice elsewhere.
> > > > 
> > > > Where? \xHH is quite well established format for escaping. Never heard about
> > > > '(xx)' variant before this very series.
> > 
> > What about using the same approach as drm_get_format_name()?
> > I mean printing '?' when the character is not printable.
> > The exact value is printed later anyway.
> > 
> > The advantage is that it will always printk 4 characters.
> 
> "?" can be expanded by the shell. We (me and Hans) both though a dot (".")
> would be good. These aren't going to be present in valid fourcc codes in
> any case.

The dot (".") looks fine to me.

Best Regards,
Petr