Re: [PATCH v4 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

[Andy Shevchenko]

On Tue, Nov 03, 2020 at 03:34:00PM +0200, Sakari Ailus 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.

...

> +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)")];

I would add a comment that there is another possibility, i.e. big-endian, but
it occupies less space.

> +	char *p = output;
> +	unsigned int i;
> +	u32 val;
> +
> +	if (fmt[1] != 'c' || fmt[2] != 'c')
> +		return error_string(output, 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;
> +
> +		/* Print non-control ASCII characters as-is */
> +		if (isascii(c) && isprint(c)) {
> +			*p++ = c;
> +			continue;
> +		}
> +
> +		*p++ = '(';
> +		p = hex_byte_pack(p, c);
> +		*p++ = ')';
> +	}

I guess above loop can be still optimized, but I have to think about it.

> +	strcpy(p, *fourcc & BIT(31) ? " big-endian" : " little-endian");
> +	p += strlen(p);
> +
> +	*p++ = ' ';
> +	*p++ = '(';
> +	/* subtract parenthesis and the space from the size */

This comment misleading. What you are subtracting is a room for closing
parenthesis and NUL.

> +	p = special_hex_number(p, output + sizeof(output) - 2, *fourcc,
> +			       sizeof(u32));

I would go with one line here.

The (theoretical) problem is here that the case when buffer size is not enough
to print a value will be like '(0xabc)' but should be rather '(0xabcd' like
snprintf() does in general.

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

-- 
With Best Regards,
Andy Shevchenko