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

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Message-ID: <20200403101043.GB3172@kekkonen.localdomain>
Date:   Fri, 3 Apr 2020 13:10:44 +0300
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Petr Mladek <pmladek@...e.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

Hi Andy,

On Fri, Apr 03, 2020 at 12:54:41PM +0300, Andy Shevchenko wrote:
> On Fri, Apr 03, 2020 at 12:39:16PM +0300, Sakari Ailus wrote:
> > On Fri, Apr 03, 2020 at 12:31:03PM +0300, Andy Shevchenko wrote:
> > > On Fri, Apr 03, 2020 at 12:11:56PM +0300, 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.
> > > 
> > > ...
> > > 
> > > > +static noinline_for_stack
> > > > +char *fourcc_string(char *buf, char *end, const u32 *fourcc,
> > > > +		    struct printf_spec spec, const char *fmt)
> > > > +{
> > > 
> > > > +#define FOURCC_STRING_BE	"-BE"
> > > > +	char s[sizeof(*fourcc) + sizeof(FOURCC_STRING_BE)] = { 0 };
> > > 
> > > I guess it makes it too complicated.
> > 
> > The above also clearly binds the size to the data that is expected to
> > contain there. I'd prefer keeping it as-is. And yes, 8 would be correct,
> > too.
> 
> OK.
> 
> > > 	char s[8];
> > > 
> > > > +	if (check_pointer(&buf, end, fourcc, spec))
> > > > +		return buf;
> > > > +
> > > > +	if (fmt[1] != 'c' || fmt[2] != 'c')
> > > > +		return error_string(buf, end, "(%p4?)", spec);
> > > > +
> > > 
> > > > +	put_unaligned_le32(*fourcc & ~BIT(31), s);
> > > 
> > > Can you elaborate what the difference in output with this bit set over cleared?
> > > I.o.w. why don't we need to put it as BE and for LE case addd "-LE"?
> > 
> > The established practice is that big endian formats have "-BE" suffix
> > whereas the little endian ones have nothing. (At least when it comes to
> > V4L2.)
> 
> What I meant by the first part of the question is ordering of the characters.
> That ordering of characters is not related to that flag, correct? So, bit
> actually defines the endianess of the data in the certain fourcc.
> 
> Probably you need to put a comment to explain this.

How about:

The 31st bit defines the endianness of the data, so save its printing to
the big endian suffix.

> 
> > > > +	if (*fourcc & BIT(31))
> > > > +		strscpy(s + sizeof(*fourcc), FOURCC_STRING_BE,
> > > > +			sizeof(FOURCC_STRING_BE));
> > > 
> > > We know the size, and we may put '\0' as well
> > > 	if (*fourcc & BIT(31))
> > > 		strscpy(&s[4], "-BE", sizeof("-BE"));
> > > 	else
> > > 		strscpy(&s[4], "", sizeof(""));
> > 
> > The rest of the struct memory has already been set to zero in variable
> > declaration.
> 
> Which is bogus in my opinion. strscpy() or direct '\0' termination will put it
> more explicit.

There's no need to assign nul a simple character using strscpy(). In that
case I'd just do

	s[sizeof(*fourcc)] = '\0';

and remove the initial assignment to zero.

-- 
Regards,

Sakari Ailus