Re: [PATCH v3] imu: inv_mpu6050: interpolate missing timestamps

[Martin Kelly <mkelly@xevo.com>]

On 04/06/2018 09:33 AM, Martin Kelly wrote:
> On 04/06/2018 08:41 AM, Jonathan Cameron wrote:
>>
>>
>> On 6 April 2018 16:21:05 BST, Jean-Baptiste Maneyrol 
>> <JManeyrol@invensense.com> wrote:
>>> Hello,
>>>
>>>
>>> there is just a problem if I'm understanding well.
>>>
>>>
>>> Reading FIFO count register under hard irq handler (when taking the
>>> timestamp) is not possible since i2c access is using a mutex. That's
>>> why we are using an irq thread for reading FIFO content.
>>
>> Good point. Need more sleep or caffeine!
>>
>>
> 
> I was about to reply with the same, as I started coding it up :). Too 
> bad, it was such a great plan!
> 
> I have a little update: When switching to level triggered interrupts, 
> the problem goes away for me, as do the bus errors I get at high 
> frequencies. I'm working on a patch to support other interrupt types 
> than rising edge, which is almost done.
> 
> I also intend to look again at the bus errors for edge driven 
> interrupts. Since they happen only at high frequency and go away with 
> level driven interrupts (which must be acked and therefore prevent 
> reentrancy), I suspect there's a concurrency bug.
> 
> That said, I think the question remains: Since we can't get the FIFO 
> count from the hard IRQ handler, and since it could be some time before 
> the bottom half thread is scheduled, I don't see any way to accurately 
> handle forward interpolation.
> 
> Though we can't do forward interpolation, I think at least having 
> backward interpolation is better than filling in blank timestamps, 
> especially as we haven't seen an actual case of forward interpolation 
> being needed, while we have real use cases that require backward 
> interpolation (and can be easily verified in a logic analyzer).
> 
> However, that's only my opinion. Jonathan, Jean-Baptiste, and others, 
> what do you think?
> 

Hi,

What can I do to help get closure on this? Is there any data I could 
gather that would help make a decision?

In cases of a troubled system, I think the interpolation is very useful, 
and it's awkward to do in userspace, as it involves keeping a history of 
past records, which can be inconvenient and not always accurate (e.g. if 
userspace doesn't read fast enough and we miss records). However, I 
certainly understand the concern about interpolation. Should we consider 
making the interpolation configurable via sysfs or device-tree?

I'd be happy to hear other ideas too about better handling the case of 
missed interrupts.