Subject: Re: [PATCH v5 15/39] [media] v4l2: add a frame interval error event
To: Russell King - ARM Linux <linux@armlinux.org.uk>
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Cc: Hans Verkuil <hverkuil@xs4all.nl>, robh+dt@kernel.org,
        mark.rutland@arm.com, shawnguo@kernel.org, kernel@pengutronix.de,
        fabio.estevam@nxp.com, mchehab@kernel.org, nick@shmanahar.org,
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        songjun.wu@microchip.com, andrew-ct.chen@mediatek.com,
        gregkh@linuxfoundation.org, shuah@kernel.org,
        sakari.ailus@linux.intel.com, pavel@ucw.cz, devel@driverdev.osuosl.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-media@vger.kernel.org
From: Steve Longerbeam <slongerbeam@gmail.com>
Message-ID: <1d3b4d81-587f-38c1-753f-4e7775dd4bca@gmail.com>
Date: Sat, 11 Mar 2017 11:00:53 -0800
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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