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

[Russell King - ARM Linux]

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.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.