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

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Date:   Mon, 8 Feb 2021 22:58:55 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        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 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:

...

> > +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).

> > +       val = *fourcc & ~BIT(31);
> > +
> > +       for (i = 0; i < sizeof(*fourcc); i++) {
> > +               unsigned char c = val >> (i * 8);
> 
> ...
> 
> > +               /* Weed out spaces */
> > +               if (c == ' ')
> > +                       continue;
> 
> None of the existing users does that. Why?
> 
> > +               /* Print non-control ASCII characters as-is */
> > +               if (isascii(c) && isprint(c)) {
> > +                       *p++ = c;
> > +                       continue;
> > +               }
> > +
> > +               *p++ = '(';
> > +               p = hex_byte_pack(p, c);
> > +               *p++ = ')';
> > +       }


To be on constructive side, I would propose to replace above with something
like:

__le32 val;

val = cpu_to_le32(*fourcc & ~BIT(31));

p += string_escape_mem(&val, sizeof(*fourcc), output, sizeof(output), ESCAPE_NP | ESCAPE_HEX, NULL);


-- 
With Best Regards,
Andy Shevchenko