Date:   Thu, 30 Apr 2020 17:15:52 +0300
From:   Sakari Ailus <sakari.ailus@....fi>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
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

Hi Laurent,

On Thu, Apr 30, 2020 at 04:59:04PM +0300, Laurent Pinchart wrote:
> Hi Sakari,
> 
> 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?

-- 
Regards,

Sakari Ailus

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