Subject: Re: [PATCH v5 15/39] [media] v4l2: add a frame interval error event
To: Russell King - ARM Linux <linux@armlinux.org.uk>,
        Nicolas Dufresne <nicolas@ndufresne.ca>
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From: Steve Longerbeam <slongerbeam@gmail.com>
Message-ID: <8461e333-6867-eecc-db7a-9bf44a2baf18@gmail.com>
Date: Tue, 14 Mar 2017 09:50:14 -0700
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On 03/14/2017 09:47 AM, Russell King - ARM Linux wrote:
> On Tue, Mar 14, 2017 at 12:21:31PM -0400, Nicolas Dufresne wrote:
>> My main concern here based on what I'm reading, is that this driver is
>> not even able to notice immediately that a produced frame was corrupted
>> (because it's out of sync). From usability perspective, this is really
>> bad. Can't the driver derive a clock from some irq and calculate for
>> each frame if the timing was correct ? And if not mark the buffer with
>> V4L2_BUF_FLAG_ERROR ?
> One of the issues of measuring timing with IRQs is the fact that the
> IRQ subsystem only allows one IRQ to run at a time.  If an IRQ takes
> a relatively long time to process, then it throws the timing of other
> IRQs out.
>
> If you're going to decide that a buffer should be marked in error on
> the basis of an interrupt arriving late, this can trigger spuriously.
>
> It wasn't that long ago that USB HID was regularly eating something
> like 20ms of interrupt time... that's been solved, but that doesn't
> mean all cases are solved - there are still interrupt handlers in the
> kernel that are on the order of milliseconds to complete.
>
> Given the quality I observe of some USB serial devices (eg, running at
> 115200 baud, but feeling like they deliver characters to userspace at
> 9600 baud) I wouldn't be surprised if some USB serial drivers eat a lot
> of IRQ time... and if so, all it'll take is to plug such a device in
> to disrupt capture.
>
> That sounds way too fragile to me.

exactly, hence the imx6 timer input capture support.

Steve