Re: [PATCH 0/7] TC-ETF support PTP clocks series

[Geva, Erez]

On 02/10/2020 21:01, Vinicius Costa Gomes wrote:
> Hi Erez,
>
> Erez Geva <erez.geva.ext@xxxxxxxxxxx> writes:
>
>> Add support for using PTP clock with
>>   Traffic control Earliest TxTime First (ETF) Qdisc.
>>
>> Why do we need ETF to use PTP clock?
>> Current ETF requires to synchronization the system clock
>>   to the PTP Hardware clock (PHC) we want to send through.
>> But there are cases that we can not synchronize the system clock with
>>   the desire NIC PHC.
>> 1. We use several NICs with several PTP domains that our device
>>      is not allowed to be PTP master.
>>     And we are not allowed to synchronize these PTP domains.
>> 2. We are using another clock source which we need for our system.
>>     Yet our device is not allowed to be PTP master.
>> Regardless of the exact topology, as the Linux tradition is to allow
>>   the user the freedom to choose, we propose a patch that will allow
>>   the user to configure the TC-ETF with a PTP clock as well as
>>   using the system clock.
>> * NOTE: we do encourage the users to synchronize the system clock with
>>    a PTP clock.
>>   As the ETF watchdog uses the system clock.
>>   Synchronizing the system clock with a PTP clock will probably
>>    reduce the frequency different of the PHC and the system clock.
>>   As sequence, the user might be able to reduce the ETF delta time
>>    and the packets latency cross the network.
>>
>> Follow the decision to derive a dynamic clock ID from the file description
>>   of an opened PTP clock device file.
>> We propose a simple way to use the dynamic clock ID with the ETF Qdisc.
>> We will submit a patch to the "tc" tool from the iproute2 project
>>   once this patch is accepted.
>>
>
> In addition to what Thomas said, I would like to add some thoughts
> (mostly re-wording some of Thomas' comments :-)).
>
> I think that there's an underlying problem/limitation that is the cause
> of the issue (or at least a step in the right direction) you are trying
> to solve: the issue is that PTP clocks can't be used as hrtimers.
>
> I didn't spend a lot of time thinking about how to solve this (the only
> thing that comes to mind is having a timecounter, or similar, "software
> view" over the PHC clock).
>
> Anyway, my feeling is that until this is solved, we would only be
> working around the problem, and creating even more hard to handle corner
> cases.
>
>
> Cheers,
>

You are right.

Thanks for the insight.

Erez