netdev - RE: Improving accuracy of PHC readings

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Date:   Fri, 19 Oct 2018 16:52:13 +0000
From:   "Keller, Jacob E" <jacob.e.keller@...el.com>
To:     Miroslav Lichvar <mlichvar@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Richard Cochran <richardcochran@...il.com>
Subject: RE: Improving accuracy of PHC readings

> -----Original Message-----
> From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org] On
> Behalf Of Miroslav Lichvar
> Sent: Friday, October 19, 2018 2:52 AM
> To: netdev@...r.kernel.org
> Cc: Richard Cochran <richardcochran@...il.com>; Keller, Jacob E
> <jacob.e.keller@...el.com>
> Subject: Improving accuracy of PHC readings
> 
> I think there might be a way how we could significantly improve
> accuracy of synchronization between the system clock and a PTP
> hardware clock, at least with some network drivers.
> 
> Currently, the PTP_SYS_OFFSET ioctl reads the system clock, reads the
> PHC using the gettime64 function of the driver, and reads the system
> clock again. The ioctl can repeat this to provide multiple readings to
> the user space.
> 
> phc2sys (or another program synchronizing the system clock to the PHC)
> assumes the PHC timestamps were captured in the middle between the two
> closest system clock timestamps.
> 
> The trouble is that gettime64 typically reads multiple (2-3) registers
> and the timestamp is latched on the first one, so the assumption about
> middle point is wrong. There is an asymmetry, even if the delays on
> the PCIe bus are perfectly symmetric.
> 

Right! I feel like this is obvious now that you said it, so I'm surprised no one thought of it before...

> A solution to this would be a new driver function that wraps the
> latching register read with readings of the system clock and return
> three timestamps instead of one. For example:
> 
>         ktime_get_real_ts64(&sys_ts1);
> 	IXGBE_READ_REG(hw, IXGBE_SYSTIMR);
> 	ktime_get_real_ts64(&sys_ts2);
> 	phc_ts.tv_nsec = IXGBE_READ_REG(hw, IXGBE_SYSTIML);
> 	phc_ts.tv_sec = IXGBE_READ_REG(hw, IXGBE_SYSTIMH);
> 
> The extra timestamp doesn't fit the API of the PTP_SYS_OFFSET ioctl,
> so it would need to shift the timestamp it returns by the missing
> intervals (assuming the frequency offset between the PHC and system
> clock is small), or a new ioctl could be introduced that would return
> all timestamps in an array looking like this:
> 
> 	[sys, phc, sys, sys, phc, sys, ...]
> 

I think the new ioctl is probably the better solution.

> This should significantly improve the accuracy of the synchronization,
> reduce the uncertainty in the readings to less than a half or third,
> and also reduce the jitter as there are fewer register reads sensitive
> to the PCIe delay.
> 
> What do you think?
> 

Nice! I think this is good. I'd love to see some data to back it up, but it makes sense to me.

Thanks,
Jake

> --
> Miroslav Lichvar