linux-kernel - Re: [PATCH v4 29/36] media: imx: mipi-csi2: enable setting and getting of frame rates

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Date:   Sat, 18 Feb 2017 18:08:44 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Steve Longerbeam <slongerbeam@...il.com>
Cc:     robh+dt@...nel.org, mark.rutland@....com, shawnguo@...nel.org,
        kernel@...gutronix.de, fabio.estevam@....com, mchehab@...nel.org,
        hverkuil@...all.nl, nick@...anahar.org, markus.heiser@...marIT.de,
        p.zabel@...gutronix.de, laurent.pinchart+renesas@...asonboard.com,
        bparrot@...com, geert@...ux-m68k.org, arnd@...db.de,
        sudipm.mukherjee@...il.com, minghsiu.tsai@...iatek.com,
        tiffany.lin@...iatek.com, jean-christophe.trotin@...com,
        horms+renesas@...ge.net.au, niklas.soderlund+renesas@...natech.se,
        robert.jarzmik@...e.fr, songjun.wu@...rochip.com,
        andrew-ct.chen@...iatek.com, gregkh@...uxfoundation.org,
        shuah@...nel.org, sakari.ailus@...ux.intel.com, pavel@....cz,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        devel@...verdev.osuosl.org
Subject: Re: [PATCH v4 29/36] media: imx: mipi-csi2: enable setting and
 getting of frame rates

On Sat, Feb 18, 2017 at 09:29:17AM -0800, Steve Longerbeam wrote:
> On 02/18/2017 01:23 AM, Russell King - ARM Linux wrote:
> >On Fri, Feb 17, 2017 at 05:12:44PM -0800, Steve Longerbeam wrote:
> >>Hi Russell,
> >>
> >>I signed-off on this but after more review I'm not sure this is right.
> >>
> >>The CSI-2 receiver really has no control over frame rate. It's output
> >>frame rate is the same as the rate that is delivered to it.
> >>
> >>So this subdev should either not implement these ops, or it should
> >>refer them to the attached source subdev.
> >
> >Where in the V4L2 documentation does it say that is permissible?
> >
> 
> https://www.linuxtv.org/downloads/v4l-dvb-apis-old/vidioc-subdev-g-frame-interval.html
> 
> "The frame interval only makes sense for sub-devices that can control the
> frame period on their own. This includes, for instance, image sensors and TV
> tuners. Sub-devices that don't support frame intervals must not implement
> these ioctls."

That sounds clear - but the TV tuner example seems odd - the frame rate
is determined at transmission time, not reception time.  Yes, it's
possible to skip frames (which would be scaling) but you can't
_control_ the frame rate per se.

> >If you don't implement these, media-ctl fails to propagate _anything_
> >to the next sink pad if you specify a frame rate, because media-ctl
> >throws an error and exits immediately.
> >
> 
> But I agree with you here. I think our only option is to ignore that
> quoted requirement above and propagate [gs]_frame_interval all the way
> to the CSI (which can control the frame rate via frame skipping).

Sounds like something to tackle the media maintainers over - the
documentation vs media-ctl seem to have different ideas on this
point.

-- 
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