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

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Laurent,

Thanks for the comments.

On Fri, Apr 03, 2020 at 01:24:49PM +0300, Laurent Pinchart wrote:
> Hi Sakari,
> 
> Thank you for the patch.
> 
> 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.
> > 
> > Suggested-by: Mauro Carvalho Chehab <mchehab@kernel.org>
> > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > ---
> > since v1:
> > 
> > - Improve documentation (add -BE suffix, refer to "FourCC".
> > 
> > - Use '%p4cc' conversion specifier instead of '%ppf'.
> > 
> > - Fix 31st bit handling in printing FourCC codes.
> > 
> > - Use string() correctly, to allow e.g. proper field width handling.
> > 
> > - Remove loop, use put_unaligned_le32() instead.
> > 
> >  Documentation/core-api/printk-formats.rst | 12 +++++++++++
> >  lib/vsprintf.c                            | 25 +++++++++++++++++++++++
> >  2 files changed, 37 insertions(+)
> > 
> > diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
> > index 8ebe46b1af39..550568520ab6 100644
> > --- a/Documentation/core-api/printk-formats.rst
> > +++ b/Documentation/core-api/printk-formats.rst
> > @@ -545,6 +545,18 @@ For printing netdev_features_t.
> >  
> >  Passed by reference.
> >  
> > +V4L2 and DRM FourCC code (pixel format)
> > +---------------------------------------
> > +
> > +::
> > +
> > +	%p4cc
> > +
> > +Print a FourCC code used by V4L2 or DRM. The "-BE" suffix is added on big endian
> > +formats.
> > +
> > +Passed by reference.
> > +
> >  Thanks
> >  ======
> >  
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index 7c488a1ce318..93eea6a320da 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -1721,6 +1721,28 @@ char *netdev_bits(char *buf, char *end, const void *addr,
> >  	return special_hex_number(buf, end, num, size);
> >  }
> >  
> > +static noinline_for_stack
> > +char *fourcc_string(char *buf, char *end, const u32 *fourcc,
> > +		    struct printf_spec spec, const char *fmt)
> > +{
> > +#define FOURCC_STRING_BE	"-BE"
> > +	char s[sizeof(*fourcc) + sizeof(FOURCC_STRING_BE)] = { 0 };
> > +
> > +	if (check_pointer(&buf, end, fourcc, spec))
> > +		return buf;
> > +
> > +	if (fmt[1] != 'c' || fmt[2] != 'c')
> > +		return error_string(buf, end, "(%p4?)", spec);
> > +
> > +	put_unaligned_le32(*fourcc & ~BIT(31), s);
> > +
> > +	if (*fourcc & BIT(31))
> > +		strscpy(s + sizeof(*fourcc), FOURCC_STRING_BE,
> > +			sizeof(FOURCC_STRING_BE));
> > +
> > +	return string(buf, end, s, spec);
> 
> Taking V4L2_PIX_FMT_Y16_BE as an example, this will print 'Y16 -BE'
> (without quotes). There are other 4CCs that contain spaces and would
> suffer from a similar issue. Even in little-endian format, it would
> result in additional spaces in the output string. Is this what we want ?
> Should the caller always enclose the 4CC in quotes or brackets for
> clarity ? Or should still be done here ?

Good question. Space is indeed a valid character in a 4cc code.

If I omit one or more spaces, it will no longer be a 4cc, but a 3cc or even
a 2cc. Jokes aside, there are probably fair arguments both ways.

I presume there's no 4cc code where the location of a space would make a
difference but all of the spaces are trailing spaces.

It's also worth noting that the formats printed are mostly for debugging
purpose and thus even getting a hypothetical case wrong is not a grave
issue. This would also support just printing them as-is though.

I'm leaning slightly towards omitting any spaces if the code has them. This
is something that couldn't be done by using a macro...

-- 
Regards,

Sakari Ailus