Re: [PATCH v6 1/3] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

[Sakari Ailus]

Hi Andy,

On Mon, Feb 08, 2021 at 10:43:30PM +0200, Andy Shevchenko wrote:
> On Mon, Feb 8, 2021 at 10:11 PM Sakari Ailus
> <sakari.ailus@xxxxxxxxxxxxxxx> 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.
> 
> Thank you for an update with the examples how current users will be
> converted. Below review is based on the users I had seen so far and
> assumptions made in this code. I see that it's tagged by maintainers,
> but I can't help to comment again on this. In any case the decision is
> up to them.
> 
> ...
> 
> > +V4L2 and DRM FourCC code (pixel format)
> > +---------------------------------------
> > +
> > +::
> > +
> > +       %p4cc
> > +
> > +Print a FourCC code used by V4L2 or DRM, including format endianness and
> > +its numerical value as hexadecimal.
> > +
> > +Passed by reference.
> > +
> > +Examples::
> > +
> > +       %p4cc   BG12 little-endian (0x32314742)
> 
> This misses examples of the (strange) escaping cases and wiped
> whitespaces to make sure everybody understands that 'D 12' will be the
> same as 'D1 2' (side note: which I disagree on, perhaps something
> should be added into documentation why).

The spaces are expected to be at the end only. I can add such example if
you like. There are no fourcc codes with spaces in the middle in neither
V4L2 nor DRM, and I don't think the expectation is to have them either.

> 
> ...
> 
> > +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)")];
> 
> Do we have any evidence / document / standard that the above format is
> what people would find good? From existing practices (I consider other
> printings elsewhere and users in this series) I find '(xx)' form for
> hex numbers is weird. The standard practice is to use \xHH (without
> parentheses).

Earlier in the review it was proposed that special handling of codes below
32 should be added, which I did. Using the parentheses is apparently an
existing practice elsewhere.

Note that neither DRM nor V4L2 currently has such fourcc codes currently.

> 
> > +       char *p = output;
> > +       unsigned int i;
> > +       u32 val;
> > +
> > +       if (fmt[1] != 'c' || fmt[2] != 'c')
> > +               return error_string(buf, end, "(%p4?)", spec);
> > +
> > +       if (check_pointer(&buf, end, fourcc, spec))
> > +               return buf;
> > +
> > +       val = *fourcc & ~BIT(31);
> > +
> > +       for (i = 0; i < sizeof(*fourcc); i++) {
> > +               unsigned char c = val >> (i * 8);
> 
> ...
> 
> > +               /* Weed out spaces */
> > +               if (c == ' ')
> > +                       continue;
> 
> None of the existing users does that. Why?

The existing instances of printing fourcc codes in V4L2 are scattered
around with priority on implementation simplicity. As we have a single
simplementation here, I'm not really worried about that.

> 
> > +               /* Print non-control ASCII characters as-is */
> > +               if (isascii(c) && isprint(c)) {
> > +                       *p++ = c;
> > +                       continue;
> > +               }
> > +
> > +               *p++ = '(';
> > +               p = hex_byte_pack(p, c);
> > +               *p++ = ')';
> > +       }
> > +
> > +       strcpy(p, *fourcc & BIT(31) ? " big-endian" : " little-endian");
> > +       p += strlen(p);
> > +
> > +       *p++ = ' ';
> > +       *p++ = '(';
> 
> > +       p = special_hex_number(p, output + sizeof(output) - 2, *fourcc,
> > +                              sizeof(u32));
> 
> This is perfectly one line (in this file we have even longer lines).

Sure, you can do that, and I can then review your patch and point to the
coding style documentation. :-)

> 
> > +       *p++ = ')';
> > +       *p = '\0';
> > +
> > +       return string(buf, end, output, spec);
> > +}
> 

-- 
Kind regards,

Sakari Ailus