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

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Date:   Sat, 4 Apr 2020 02:36:47 +0300
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Petr Mladek <pmladek@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.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, mchehab@...nel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>
Subject: Re: [PATCH v2 1/1] lib/vsprintf: Add support for printing V4L2 and
 DRM fourccs

Hi Joe,

On Fri, Apr 03, 2020 at 11:38:29AM -0700, Joe Perches wrote:
> On Fri, 2020-04-03 at 13:47 +0300, Sakari Ailus wrote:
> > On Fri, Apr 03, 2020 at 01:24:49PM +0300, Laurent Pinchart 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.
> []
> > > > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> []
> > > > +static noinline_for_stack
> > > > +char *fourcc_string(char *buf, char *end, const u32 *fourcc,
> > > > +		    struct printf_spec spec, const char *fmt)
> []
> > > > +	if (fmt[1] != 'c' || fmt[2] != 'c')
> 
> This could check outside a format string if
> the %p4 is at the end of the format string.
> 
> 	printk("%p4", fourcc)
> 
> So this should verify
> 
> 	if (!(fmt[1] == 'c' && fmt[2] == 'c'))

How would these be different in functionality? fmt[2] won't be accessed if
fmt[1] is not 'c' (including '\0'), just like on the line above. I find the
original easier to read.

-- 
Regards,

Sakari Ailus