linux-kernel - Re: [PATCH v5 15/39] [media] v4l2: add a frame interval error event

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Message-ID: <1d3b4d81-587f-38c1-753f-4e7775dd4bca@gmail.com>
Date:   Sat, 11 Mar 2017 11:00:53 -0800
From:   Steve Longerbeam <slongerbeam@...il.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Hans Verkuil <hverkuil@...all.nl>, robh+dt@...nel.org,
        mark.rutland@....com, shawnguo@...nel.org, kernel@...gutronix.de,
        fabio.estevam@....com, mchehab@...nel.org, 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,
        devel@...verdev.osuosl.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-media@...r.kernel.org
Subject: Re: [PATCH v5 15/39] [media] v4l2: add a frame interval error event



On 03/11/2017 10:51 AM, Russell King - ARM Linux wrote:
> On Sat, Mar 11, 2017 at 10:14:49AM -0800, Steve Longerbeam wrote:
>> On 03/11/2017 03:39 AM, Hans Verkuil wrote:
>>> It's fine to use an internal event as long as the end-user doesn't
>>> see it. But if you lose vsyncs, then you never capture another frame,
>>> right?
>>
>> No, that's not correct. By loss of vertical sync, I mean the IPU
>> captures portions of two different frames, resulting in a permanent
>> "split image", with one frame containing portions of two consecutive
>> images. Or, the video rolls continuously, if you remember the old CRT
>> television sets of yore, it's the same rolling effect.
>
> I have seen that rolling effect, but the iMX6 regains correct sync
> within one complete "roll" just fine here with IMX219.  However, it
> has always recovered.
>
> So, I don't think there's a problem with the iMX6 part of the
> processing, and so I don't think we should cripple the iMX6 capture
> drivers for this problem.

And there is no crippling going on. Measuring the frame intervals
adds very little overhead.

Steve