Thanks much for your help. I'll review the design. Meanwhile I have a couple
more questions if you don't mind.
(1) Could you point me in the code where ptp4l slave clock sets for the first
time the kernel CLOCK_REALTIME for SW time stamping? And I guess it does that
right after transitioning into Slave state from Uncalibrated. I'm thinking the
FPGA clock time should be stepped at that moment.
(2) I tried to revert to sendto() and sk_receive() and ran into the following
error messages:
timed out while polling for tx timestamp
increasing tx_timestamp_timeout may correct this issue, but it is likely
caused by a driver bug
port <portN>: send delay request failed
Does that mean sendto() of the original program needs customization?
Daniel
-----Original Message-----
From: Richard Cochran [mailto:[email protected]]
Sent: Thursday, April 02, 2015 12:19 PM
To: Daniel Le
Cc: [email protected]
Subject: Re: [Linuxptp-users] SLAVE to UNCALIBRATED on SYNCHRONIZATION_FAULT
On Thu, Apr 02, 2015 at 03:38:38PM +0000, Daniel Le wrote:
> [DL] The FPGA has its own clock and a proprietary slewing mechanism to sync
> to a time source. It does not use phc2sys
> because my embedded system doesn't have 3.x Linux kernel.
>
> [DL] In the case of PTP time source, the FPGA engine on the NIC periodically
> reads the kernel system time
> (do_gettimeofday) in order to step/slew to the system time which is
> synchronized to PTP grandmaster time.
(IOW, software time stamping)
> [DL] The ptp4l program is run with -S option, however, for example when
> sending/receiving packets via IPv4 transport
> in udp_send() and udp_recv(), a timestamping pipe is used to get the
> FPGA hardware timestamps of the packets,
> instead of the functions sendto() and sk_receive().
(IOW, hardware time stamping)
This design seems wrong to me. Why not let the FPGA have it own clock, and
then synchronize the Linux system time to it? That is how all the other
devices do it.
In any case, you have some elaborate custom kernel and ptp4l modifications. I
really can't help you with those, sorry.
> [DL] Could you further elaborate this clockcheck_sample functionality (such
> as uncorrected frequency)?
Please take a look at the code. It is not all that complicated.
> Is my understating of the following correct?
> - The synchronized clock is the PTP clock and is maintained by PTP
> packet TX/RX timestamps per 1588 standard.
Yes.
> - The system monotonic clock (CLOCK_MONOTONIC) is the Linux kernel
> system clock.
No. See 'man 3 clock_gettime'.
> [DL] What is the threshold to determine that clock jumped forward/backward
> too much?
The code does this:
Check the sanity of the synchronized clock by comparing its uncorrected
frequency with the system monotonic clock. When the measured frequency
offset is larger than the value of the sanity_freq_limit option (20% by
default), a warning message will be printed and the servo will be reset.
Setting the option to zero disables the check.
This is useful to detect when the clock is broken or adjusted by another
program.
> [DL]Upon a system boot-up or restart, how does PTP slave clock sets the
> system clock initially? Is CLOCK_REALTIME involved?
First of all, the ptp4l program will not start at boot-up or restart unless you
arrange for that to happen. Secondly, the ptp4l program sets its target clock
(either the PHC in the case of HW time stamping, or CLOCK_REALTIME for SW time
stamping) to match that of the remote master, according to the
'first_step_threshold' configuration option.
HTH,
Richard
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