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

[Russell King - ARM Linux]

On Mon, Mar 13, 2017 at 11:02:34AM +0100, Hans Verkuil wrote:
> On 03/11/2017 07:14 PM, Steve Longerbeam wrote:
> > The event must be user visible, otherwise the user has no indication
> > the error, and can't correct it by stream restart.
> 
> In that case the driver can detect this and call vb2_queue_error. It's
> what it is there for.
> 
> The event doesn't help you since only this driver has this issue. So nobody
> will watch this event, unless it is sw specifically written for this SoC.
> 
> Much better to call vb2_queue_error to signal a fatal error (which this
> apparently is) since there are more drivers that do this, and vivid supports
> triggering this condition as well.

So today, I can fiddle around with the IMX219 registers to help gain
an understanding of how this sensor works.  Several of the registers
(such as the PLL setup [*]) require me to disable streaming on the
sensor while changing them.

This is something I've done many times while testing various ideas,
and is my primary way of figuring out and testing such things.

Whenever I resume streaming (provided I've let the sensor stop
streaming at a frame boundary) it resumes as if nothing happened.  If I
stop the sensor mid-frame, then I get the rolling issue that Steve
reports, but once the top of the frame becomes aligned with the top of
the capture, everything then becomes stable again as if nothing happened.

The side effect of what you're proposing is that when I disable streaming
at the sensor by poking at its registers, rather than the capture just
stopping, an error is going to be delivered to gstreamer, and gstreamer
is going to exit, taking the entire capture process down.

This severely restricts the ability to be able to develop and test
sensor drivers.

So, I strongly disagree with you.

Loss of capture frames is not necessarily a fatal error - as I have been
saying repeatedly.  In Steve's case, there's some unknown interaction
between the source and iMX6 hardware that is causing the instability,
but that is simply not true of other sources, and I oppose any idea that
we should cripple the iMX6 side of the capture based upon just one
hardware combination where this is a problem.

Steve suggested that the problem could be in the iMX6 CSI block - and I
note comparing Steve's code with the code in FSL's repository that there
are some changes that are missing in Steve's code to do with the CCIR656
sync code setup, particularly for >8 bit.  The progressive CCIR656 8-bit
setup looks pretty similar though - but I think what needs to be asked
is whether the same problem is visible using the FSL/NXP vendor kernel.


* - the PLL setup is something that requires research at the moment.
Sony's official position (even to their customers) is that they do not
supply the necessary information, instead they expect customers to tell
them the capture settings they want, and Sony will throw the values into
a spreadsheet, and they'll supply the register settings back to the
customer.  Hence, the only way to proceed with a generic driver for
this sensor is to experiment, and experimenting requires the ability to
pause the stream at the sensor while making changes.  Take this away,
and we're stuck with the tables-of-register-settings-for-set-of-fixed-
capture-settings approach.  I've made a lot of progress away from this
which is all down to the flexibility afforded by _not_ killing the
capture process.

-- 
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