[PATCH v3 2/7] lib/vsprintf: Add support for generic FOURCCs by extending %p4cc

[Petr Mladek]

On Mon 2022-11-14 16:15:50, Russell King (Oracle) wrote:
> On Mon, Nov 14, 2022 at 04:34:07PM +0100, Petr Mladek wrote:
> > On Tue 2022-11-08 16:33:22, Russell King wrote:
> > > From: Hector Martin <marcan at marcan.st>
> > > 
> > > %p4cc is designed for DRM/V4L2 FOURCCs with their specific quirks, but
> > > it's useful to be able to print generic 4-character codes formatted as
> > > an integer. Extend it to add format specifiers for printing generic
> > > 32-bit FOURCCs with various endian semantics:
> > > 
> > > %p4ch   Host-endian
> > > %p4cl	Little-endian
> > > %p4cb	Big-endian
> > > %p4cr	Reverse-endian
> > > 
> > > The endianness determines how bytes are interpreted as a u32, and the
> > > FOURCC is then always printed MSByte-first (this is the opposite of
> > > V4L/DRM FOURCCs). This covers most practical cases, e.g. %p4cr would
> > > allow printing LSByte-first FOURCCs stored in host endian order
> > > (other than the hex form being in character order, not the integer
> > > value).
> > > 
> > > Signed-off-by: Hector Martin <marcan at marcan.st>
> > > Signed-off-by: Russell King (Oracle) <rmk+kernel at armlinux.org.uk>
> > 
> > Reviewed-by: Petr Mladek <pmladek at suse.com>
> > 
> > See one nit below.
> > 
> > > --- a/lib/vsprintf.c
> > > +++ b/lib/vsprintf.c
> > > @@ -1762,27 +1762,50 @@ char *fourcc_string(char *buf, char *end, const u32 *fourcc,
> > >  	char output[sizeof("0123 little-endian (0x01234567)")];
> > >  	char *p = output;
> > >  	unsigned int i;
> > > +	bool pixel_fmt = false;
> > >  	u32 orig, val;
> > >  
> > > -	if (fmt[1] != 'c' || fmt[2] != 'c')
> > > +	if (fmt[1] != 'c')
> > >  		return error_string(buf, end, "(%p4?)", spec);
> > >  
> > >  	if (check_pointer(&buf, end, fourcc, spec))
> > >  		return buf;
> > >  
> > >  	orig = get_unaligned(fourcc);
> > > -	val = orig & ~BIT(31);
> > > +	switch (fmt[2]) {
> > > +	case 'h':
> > > +		val = orig;
> > > +		break;
> > > +	case 'r':
> > > +		val = orig = swab32(orig);
> > 
> > I do not like much these multi assignments. I think that the result
> > was not even defined in some older C standards. Though, I can't find
> > it now. And even make W=3 does not warn about it.
> 
> Err.
> 
> It's been supported for decades. I learnt about it back in 1992 when
> I was introduced to C by another experienced C programmer. It's been
> supported in ANSI C compilers. The Norcroft C compiler (which is
> strict ANSI) on Acorn platforms back in the late 1980s/1990s even
> supported it.

Ah, the problem probably was with a more complicated assignment.
For example, the result of the following code is not obvious:

	  a = b = a++;

Best Regards,
Petr