linux-kernel - Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross timestamps infrastructure

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Date:   Fri, 20 Jan 2017 14:36:40 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Radim Krcmar <rkrcmar@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross
 timestamps infrastructure



On 20/01/2017 14:07, Marcelo Tosatti wrote:
> On Fri, Jan 20, 2017 at 01:55:27PM +0100, Paolo Bonzini wrote:
>>
>>
>> On 20/01/2017 13:20, Marcelo Tosatti wrote:
>>>  kernel/time/timekeeping.c        |   79 +++++++++++++++++++++++++++++++++++++++
>>
>> Why not leave this in drivers/ptp/ptp_chardev.c?
> 
> timekeeper_lock

Why does emulate_ptp_sys_offset need it, if the current PTP_SYS_OFFSET
code doesn't?  Is the latency acceptable (considering this is a raw spin
lock) or is there a seqlock that we can use instead (such as tk_core.seq
like in get_device_system_crosststamp)?

>>> +		if (ptp->info->emulate_ptp_sys_offset_mean) {
>>> +			err = emulate_ptp_sys_offset(ptp->info, sysoff, arg);
>>> +			break;
>>> +		}
>>
>> I think this should be simply "if (!ptp->info->gettime64)" and,
>> likewise, there should be an emulation based getcrosststamp in
>> ptp_clock_gettime.
>>
>> Paolo
> 
> gettime64 is called directly via ptp_clock_gettime.

Yes, but ptp_clock_gettime can be taught to use getcrosststamp instead.

Paolo