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

[Sakari Ailus]

Hi Hans,

On Tue, Oct 17, 2017 at 11:28:46AM +0200, Hans Verkuil wrote:
> On 10/17/17 11:17, Dave Stevenson wrote:
> > Hi Sakari.
> > 
> > On 13 October 2017 at 22:09, Sakari Ailus <sakari.ailus at iki.fi> wrote:
> >> 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" : ""
> 
> 'fourcc' should be in parenthesis '(fourcc)'.

Yes, I omitted those for readability here.

> 
> >>
> >> 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.
> > 
> > I was following Hans' suggestion made in
> > https://www.spinics.net/lists/linux-media/msg117046.html
> > 
> > Hans: Do you agree with Sakari here to make it to a macro?
> 
> Not a bad idea. But I think we should add it to the public videodev2.h
> header in that case, allowing it to be used by applications as well.

Sounds good.

> 
> How about calling the defines V4L2_FOURCC_FMT and V4L2_FOURCC_FMT_ARGS?

printf(3) describes conversion specifiers (%...) and I thought of using
"CONV" there for that purpose to make it explicit. Fourcc, implicitly,
already is about formats. What would you think of V4L2_FOURCC_CONV and
V4L2_FOURCC_CONV_ARGS (or just V4L2_FOURCC_ARGS)?

-- 
Kind regards,

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