[PATCH v3 1/4] [media] v4l2-common: Add helper function for fourcc to string

[Sakari Ailus]

Hi Dave,

On Wed, Sep 20, 2017 at 05:07:54PM +0100, Dave Stevenson wrote:
> New helper function char *v4l2_fourcc2s(u32 fourcc, char *buf)
> that converts a fourcc into a nice printable version.
> 
> Signed-off-by: Dave Stevenson <dave.stevenson at raspberrypi.org>
> ---
> 
> No changes from v2 to v3
> 
>  drivers/media/v4l2-core/v4l2-common.c | 18 ++++++++++++++++++
>  include/media/v4l2-common.h           |  3 +++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-common.c b/drivers/media/v4l2-core/v4l2-common.c
> index a5ea1f5..0219895 100644
> --- a/drivers/media/v4l2-core/v4l2-common.c
> +++ b/drivers/media/v4l2-core/v4l2-common.c
> @@ -405,3 +405,21 @@ void v4l2_get_timestamp(struct timeval *tv)
>  	tv->tv_usec = ts.tv_nsec / NSEC_PER_USEC;
>  }
>  EXPORT_SYMBOL_GPL(v4l2_get_timestamp);
> +
> +char *v4l2_fourcc2s(u32 fourcc, char *buf)
> +{
> +	buf[0] = fourcc & 0x7f;
> +	buf[1] = (fourcc >> 8) & 0x7f;
> +	buf[2] = (fourcc >> 16) & 0x7f;
> +	buf[3] = (fourcc >> 24) & 0x7f;
> +	if (fourcc & (1 << 31)) {
> +		buf[4] = '-';
> +		buf[5] = 'B';
> +		buf[6] = 'E';
> +		buf[7] = '\0';
> +	} else {
> +		buf[4] = '\0';
> +	}
> +	return buf;
> +}
> +EXPORT_SYMBOL_GPL(v4l2_fourcc2s);
> diff --git a/include/media/v4l2-common.h b/include/media/v4l2-common.h
> index aac8b7b..5b0fff9 100644
> --- a/include/media/v4l2-common.h
> +++ b/include/media/v4l2-common.h
> @@ -264,4 +264,7 @@ const struct v4l2_frmsize_discrete *v4l2_find_nearest_format(
>  
>  void v4l2_get_timestamp(struct timeval *tv);
>  
> +#define V4L2_FOURCC_MAX_SIZE 8
> +char *v4l2_fourcc2s(u32 fourcc, char *buf);
> +
>  #endif /* V4L2_COMMON_H_ */

I like the idea but the use of a character pointer and assuming a length
looks a bit scary.

As this seems to be used uniquely for printing stuff, a couple of macros
could be used instead. Something like

#define V4L2_FOURCC_CONV "%c%c%c%c%s"
#define V4L2_FOURCC_TO_CONV(fourcc) \
	fourcc & 0x7f, (fourcc >> 8) & 0x7f, (fourcc >> 16) & 0x7f, \
	(fourcc >> 24) & 0x7f, fourcc & BIT(31) ? "-BE" : ""

You could use it with printk-style functions, e.g.

	printk("blah fourcc " V4L2_FOURCC_CONV " is a nice format",
	       V4L2_FOURCC_TO_CONV(fourcc));

I guess it'd be better to add more parentheses around "fourcc" but it'd be
less clear here that way.

-- 
Regards,

Sakari Ailus
e-mail: sakari.ailus at iki.fi