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

[Andy Shevchenko]

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).

...

> +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).

> +       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?

> +               /* 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).

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

-- 
With Best Regards,
Andy Shevchenko