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Date:   Thu, 30 Apr 2020 17:17:53 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Sakari Ailus <sakari.ailus@iki.fi>
Cc:     Daniel Gomez <daniel@qtec.com>, mchehab@kernel.org,
        hverkuil-cisco@xs4all.nl, linux-media@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 1/3] media: v4l2-subdev.h: Add min and max enum
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Hi Sakari,

On Thu, Apr 30, 2020 at 05:15:52PM +0300, Sakari Ailus wrote:
> On Thu, Apr 30, 2020 at 04:59:04PM +0300, Laurent Pinchart wrote:
> > On Thu, Apr 30, 2020 at 04:31:25PM +0300, Sakari Ailus wrote:
> > > On Thu, Apr 30, 2020 at 02:10:14PM +0300, Laurent Pinchart wrote:
> > > > On Thu, Apr 30, 2020 at 12:42:33PM +0300, Sakari Ailus wrote:
> > > >> 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?
> > 
> > That's what I would recommend, yes. I would only keep
> > .s_frame_interval() for sensors that expose that concept at the hardware
> > level (for instance with an integrated ISP whose firmware exposes a
> > frame interval or frame rate control).
> 
> Sounds good to me.
> 
> Jacopo's set exposing read-only subdevs completes the puzzle so the user
> space should have all it needs, right?

Until we run into the next missing piece :-)

-- 
Regards,

Laurent Pinchart