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

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Date:   Fri, 3 Apr 2020 12:31:03 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Petr Mladek <pmladek@...e.com>, linux-media@...r.kernel.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        hverkuil@...all.nl, laurent.pinchart@...asonboard.com,
        mchehab@...nel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joe Perches <joe@...ches.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>
Subject: Re: [PATCH v2 1/1] lib/vsprintf: Add support for printing V4L2 and
 DRM fourccs

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.

...

> +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 };

I guess it makes it too complicated.

	char s[8];

> +	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);

Can you elaborate what the difference in output with this bit set over cleared?
I.o.w. why don't we need to put it as BE and for LE case addd "-LE"?

> +	if (*fourcc & BIT(31))
> +		strscpy(s + sizeof(*fourcc), FOURCC_STRING_BE,
> +			sizeof(FOURCC_STRING_BE));

We know the size, and we may put '\0' as well
	if (*fourcc & BIT(31))
		strscpy(&s[4], "-BE", sizeof("-BE"));
	else
		strscpy(&s[4], "", sizeof(""));


> +	return string(buf, end, s, spec);
> +}

-- 
With Best Regards,
Andy Shevchenko