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

[Andy Shevchenko]

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 ?

> > +	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);

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++;
	}
?

> > +			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);

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

-- 
With Best Regards,
Andy Shevchenko