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

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Date:   Thu, 2 Apr 2020 10:32:20 +0300
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Hans Verkuil <hverkuil@...all.nl>, 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,
        laurent.pinchart@...asonboard.com, mchehab@...nel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 1/1] lib/vsprintf: Add support for printing V4L2 and DRM
 fourccs

Hi Andy,

Thanks for the review.

On Wed, Apr 01, 2020 at 06:13:32PM +0300, Andy Shevchenko wrote:
> On Wed, Apr 01, 2020 at 04:13:51PM +0200, Hans Verkuil wrote:
> > On 4/1/20 4:05 PM, 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.
> 
> %p4cc ?

Sounds good. Numbers have special handling but AFAIR only right after %
sign, so this should be possible.

> 
> > > +	char ch[2] = { 0 };
> > 
> > This can just be '{ };'
> 
> The latter is GCC extension, while above is C standard. Former is slightly
> better I think. Though see below.
> 
> > > +	unsigned int i;
> > > +
> > > +	if (check_pointer(&buf, end, fourcc, spec))
> > > +		return buf;
> > > +
> > > +	switch (fmt[1]) {
> > > +	case 'f':
> 
> > > +		for (i = 0; i < sizeof(*fourcc); i++) {
> > > +			ch[0] = *fourcc >> (i << 3);
> > 
> > You need to AND with 0x7f, otherwise a big endian fourcc (bit 31 is set)
> > will look wrong. Also, each character is standard 7 bit ascii, bit 7 isn't
> > used except to indicate a BE variant.
> 
> Why not to do it once by a flag and do reset it once?
> 
> 	u32 tmp = *fourcc;
> 	bool be4cc = tmp & BIT(31);
> 
> 	tmp &= BIT(31);

I had two extra temporary variables in a version I didn't send but I
figured they could be removed. :-)

> 
> On top of that, as promised above, why not simple do it in a simpler way, i.e.
> using standard idiom:
> 
> 	for (i = 0; i < sizeof(*fourcc); i++) {
> 		if (buf < end)
> 			*buf = tmp >> (i * 8);
> 		buf++;
> 	}
> ?

I guess that's at least more efficient, and comparing buf to end is
trivial. I'll do that in v2.

> 
> > > +			buf = string(buf, end, ch, spec);
> > > +		}
> > > +
> > > +		if (*fourcc & BIT(31))
> > > +			buf = string(buf, end, "-BE", spec);
> 
> Another possibility
> 
> 	u8 ch[8];
> 
> 	if (*fourcc & BIT(31)) {
> 		put_unaligned_be32(tmp, &ch[0]);
> 		strcpy(&ch[4], "-BE");
> 	} else {
> 		put_unaligned_le32(tmp, &ch[0]);
> 		strcpy(&ch[4], "-LE");
> 	}
> 	return string(buf, end, &ch[0], spec);

I think I prefer the loop. I figured you can only call string once,
otherwise field width handling will be broken. Let's see.

> 
> > > +		return buf;
> > > +	default:
> > > +		return error_string(buf, end, "(%pp?)", spec);
> > > +	}
> > > +}
> 

-- 
Kind regards,

Sakari Ailus