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

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Message-ID: <20200430111014.GD5856@pendragon.ideasonboard.com>
Date:   Thu, 30 Apr 2020 14:10:14 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     Daniel Gomez <daniel@...c.com>, mchehab@...nel.org,
        hverkuil-cisco@...all.nl, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/3] media: v4l2-subdev.h: Add min and max enum

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.

> > Signed-off-by: Daniel Gomez <daniel@...c.com>
> > ---
> >  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,

Laurent Pinchart