Re: [RFC PATCH 1/3] media: v4l2-subdev.h: Add min and max enum

[Sakari Ailus]

Hi Laurent,

On Thu, Apr 30, 2020 at 02:10:14PM +0300, Laurent Pinchart wrote:
> Hello,
> 
> On Thu, Apr 30, 2020 at 12:42:33PM +0300, Sakari Ailus wrote:
> > Hi Daniel,
> > 
> > Thanks for the patchset.
> > 
> > I'm also cc'ing Laurent who I think could be interested in this one.
> > 
> > On Tue, Apr 14, 2020 at 10:01:49PM +0200, Daniel Gomez wrote:
> > > Add min and max structures to the v4l2-subdev callback in order to allow
> > > the subdev to return a range of valid frame intervals.
> > > 
> > > This would operate similar to the struct v4l2_subdev_frame_size_enum and
> > > its max and min values for the width and the height. In this case, the
> > > possibility to return a frame interval range is added to the v4l2-subdev level
> > > whenever the v4l2 device operates in step-wise or continuous mode.
> > 
> > The current API only allows providing a list of enumerated values. That is
> > limiting indeed, especially on register list based sensor drivers where
> > vertical blanking is configurable.
> > 
> > I guess this could be extended to cover what V4L2, more or less. If we tell
> > it's a range, is it assumed to be contiguous? We don't have try operation
> > for the frame interval, but I guess set is good enough. The fraction is
> > probably best for TV standards but it's not what camera sensors natively
> > use. (But for a register list based driver, the established practice
> > remains to use frame interval.)
> > 
> > I'm also wondering the effect on existing user space; if a driver gives a
> > range, how will the existing programs work with such a driver?
> > 
> > I'd add an anonymous union with the interval field, the other field being
> > min_interval. Then the current applications would get the minimum interval
> > and still continue to function. I guess compilers are modern enough these
> > days we can have an anonymous union in the uAPI?
> 
> We can discuss all this, but given patch 3/3 in this series, I think
> this isn't the right API :-) The sensor driver should not expose the
> frame interval enumeration API. It should instead expose control of the
> frame rate through V4L2_CID_PIXEL_RATE, V4L2_CID_HBLANK and
> V4L2_CID_VBLANK.
> 

That would require also exposing the size of the pixel array (and the
analogue crop), in order to provide all the necessary information to
calculate the frame rate. No objections there; this is a new driver.

There are however existing drivers that implement s_frame_interval subdev
ioctl; those might benefit from this one. Or would you implement the pixel
rate based control as well, and effectively deprecate the s_frame_interval
on those?

> > > Signed-off-by: Daniel Gomez <daniel@xxxxxxxx>
> > > ---
> > >  include/uapi/linux/v4l2-subdev.h | 6 +++++-
> > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/include/uapi/linux/v4l2-subdev.h b/include/uapi/linux/v4l2-subdev.h
> > > index 03970ce30741..ee15393c58cd 100644
> > > --- a/include/uapi/linux/v4l2-subdev.h
> > > +++ b/include/uapi/linux/v4l2-subdev.h
> > > @@ -117,6 +117,8 @@ struct v4l2_subdev_frame_interval {
> > >   * @code: format code (MEDIA_BUS_FMT_ definitions)
> > >   * @width: frame width in pixels
> > >   * @height: frame height in pixels
> > > + * @min_interval: min frame interval in seconds
> > > + * @max_interval: max frame interval in seconds
> > >   * @interval: frame interval in seconds
> > >   * @which: format type (from enum v4l2_subdev_format_whence)
> > >   */
> > > @@ -126,9 +128,11 @@ struct v4l2_subdev_frame_interval_enum {
> > >  	__u32 code;
> > >  	__u32 width;
> > >  	__u32 height;
> > > +	struct v4l2_fract min_interval;
> > > +	struct v4l2_fract max_interval;
> > 
> > This changes the memory layout of the struct and breaks the ABI. Any new
> > fields in the struct may only replace reserved fields while the rest must
> > stay unchanged.
> > 
> > >  	struct v4l2_fract interval;
> > >  	__u32 which;
> > > -	__u32 reserved[8];
> > > +	__u32 reserved[4];
> > >  };
> > >  
> > >  /**
> 
-- 
Regards,

Sakari Ailus