linux-kernel - Re: [PATCH v4 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

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Message-ID: <8d00647824e2ce8c05ee4447e158885e0134ed37.camel@perches.com>
Date:   Fri, 13 Nov 2020 08:46:57 -0800
From:   Joe Perches <joe@...ches.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        Petr Mladek <pmladek@...e.com>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        dri-devel@...ts.freedesktop.org, hverkuil@...all.nl,
        laurent.pinchart@...asonboard.com, mchehab@...nel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v4 1/1] lib/vsprintf: Add support for printing V4L2 and
 DRM fourccs

On Fri, 2020-11-13 at 12:54 +0200, Sakari Ailus wrote:
> Hi Joe,
> 
> On Tue, Nov 03, 2020 at 08:49:36AM -0800, Joe Perches wrote:
> > On Tue, 2020-11-03 at 16:56 +0200, Sakari Ailus wrote:
> > > On Tue, Nov 03, 2020 at 04:47:47PM +0200, Andy Shevchenko wrote:
> > > > On Tue, Nov 03, 2020 at 03:34:00PM +0200, 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.
> > > > 
> > > > ...
> > > > 
> > > > > +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)")];
> > > > 
> > > > I would add a comment that there is another possibility, i.e. big-endian, but
> > > > it occupies less space.
> > 
> > I think it's unnecessary as it's obvious and similarly done in other
> > <foo>_string type functions.
> > 
> > > > > +	p = special_hex_number(p, output + sizeof(output) - 2, *fourcc,
> > > > > +			       sizeof(u32));
> > > > 
> > > > I would go with one line here.
> > > 
> > > It's wrapped since the result would be over 80 otherwise.
> > 
> > Perhaps simpler as
> > 
> > 	p = special_hex_number(p, p + 10, *fourcc, sizeof(u32));
> 
> Yes. But having bugs elsewhere would have a magnified effect.

How's that?  Where would "elsewhere" be?

> I wouldn't be afraid of a newline here.

I'd prefer obvious code instead of indirected p vs output
and having to lookup whatever output is again.

special_hex_number is already known to fit in the buffer.