Message-ID: <7151d5dc-9bf8-7aa2-dc58-75d09aa9fa96@gmail.com>
Date:   Mon, 13 Mar 2017 14:47:36 -0700
From:   Steve Longerbeam <slongerbeam@...il.com>
To:     Hans Verkuil <hverkuil@...all.nl>,
        Russell King - ARM Linux <linux@...linux.org.uk>
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        sudipm.mukherjee@...il.com, minghsiu.tsai@...iatek.com,
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Subject: Re: [PATCH v5 15/39] [media] v4l2: add a frame interval error event



On 03/13/2017 10:10 AM, Hans Verkuil wrote:
> On 03/13/2017 06:06 PM, Steve Longerbeam wrote:
>>
>>
>> On 03/13/2017 03:53 AM, Hans Verkuil wrote:
>>> On 03/13/2017 11:45 AM, Russell King - ARM Linux wrote:
>>>> 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.
>>>>
>>>
>>> In other words: Steve should either find a proper fix for this, or only
>>> call vb2_queue_error in this specific case. Sending an event that nobody
>>> will know how to handle or what to do with is pretty pointless IMHO.
>>>
>>> Let's just give him time to try and figure out the real issue here.
>>
>>
>> This is a long-standing issue, I've traveled to Hildesheim working with
>> our customer to try and get to the bottom of it. I can go into a lot of
>> details from those trips, we probed the bt.656 bus with a logic analyzer
>> and I can share those results with anyone who asks. But the results of
>> those investigations indicate the CSI is not handling the SAV/EAV sync
>> codes correctly - if there is a shift in the line position at which
>> those codes occur, the CSI/IPU does not abort the frame capture DMA
>> and start from the new sync code position, it just continues to capture
>> lines until the programmed number of lines are transferred, hence you
>> get these split images. Freescale also informed us of a mechanism in the
>> IPU that will add lines if it detects these short frames, until the
>> programmed number of lines are reached. Apparently that is what creates
>> the rolling effect, but this rolling can last for up to a full minute,
>> which is completely unacceptable, it must be corrected as soon as
>> possible.
>>
>> So the only thing we could come up with was to monitor frame intervals,
>> this is purely empirical, but we observed that frame intervals drop
>> by approx. one line time (~60 usec) when these short frames are
>> received. I don't really have any explanation for that but we take
>> advantage of that observation by sending this event to userspace so
>> the problem can be corrected immediately with a stream restart.
>>
>> As I've said, the ADV718x does not provide _any_ indication via status
>> when the shift in the sync code position occurs. And the IPU is also
>> severely lacking in DMA completion status as well (no short packet
>> status like in USB for example). So the only way to detect this event
>> is by monitoring the frame intervals.
>>
>> I will review differences in the CCIR code register setup from FSL's
>> repo that Russell pointed out, but I'm fairly sure those code registers
>> are setup correctly, there's not much room for variability in those
>> values. They only define the values of the sync codes, so the CSI can
>> detect them, those values are defined by the bt.656 spec.
>>
>> Anyway, perhaps for now I can remove the event, but keep the FI
>> monitoring, and for now just report a kernel error message on
>> a detected bad FI.
>
> Is it possible to detect this specific situation? Apparently this issue
> is not present (or at least resolves itself very quickly) on the imx219.

The imx219 is not a BT.656 interface, it is MIPI CSI-2, so this is
not an issue for imx219. It is an issue for any sensor with a
parallel BT.656 interface.

>
> If it is not possible, then a private event would be acceptable, but
> needs to be carefully documented. After all, it is a workaround for a bug,
> since otherwise there would be no need for this event.

Ok, sounds like a plan, I'll keep the event as a private event and make
sure it is documented well. Yes it is a workaround, for a silicon bug,
although Freescale/NXP has not issued an errata for it yet AFAIK. THat
might be because they claim to handle it via this adding lines
mechanism, but that mechanism doesn't work well (long duration rolling),
or not at all (permanent split images).


Steve

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