RE: [RFC PATCH v7 4/7] time: Add mechanism to recognize clocksource in time_get_snapshot

[Jianyong Wu (Arm Technology China)]

Hi tglx,

> -----Original Message-----
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Sent: Thursday, November 14, 2019 10:19 PM
> To: Jianyong Wu (Arm Technology China) <Jianyong.Wu@xxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx; yangbo.lu@xxxxxxx; john.stultz@xxxxxxxxxx;
> pbonzini@xxxxxxxxxx; sean.j.christopherson@xxxxxxxxx; maz@xxxxxxxxxx;
> richardcochran@xxxxxxxxx; Mark Rutland <Mark.Rutland@xxxxxxx>;
> will@xxxxxxxxxx; Suzuki Poulose <Suzuki.Poulose@xxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> kvmarm@xxxxxxxxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; Steve Capper
> <Steve.Capper@xxxxxxx>; Kaly Xin (Arm Technology China)
> <Kaly.Xin@xxxxxxx>; Justin He (Arm Technology China)
> <Justin.He@xxxxxxx>; nd <nd@xxxxxxx>
> Subject: Re: [RFC PATCH v7 4/7] time: Add mechanism to recognize
> clocksource in time_get_snapshot
> 
> On Thu, 14 Nov 2019, Jianyong Wu wrote:
> > From: Thomas Gleixner <tglx@xxxxxxxxxxxxx> In some scenario like
> > return device time to ptp_kvm guest, we need identify the current
> > clocksource outside core time code.
> > A mechanism added to recognize the current clocksource by export
> > clocksource id in time_get_snapshot.
> 
> Can you please replace that with the following:
> 
>  System time snapshots are not conveying information about the current
> clocksource which was used, but callers like the PTP KVM guest
> implementation have the requirement to evaluate the clocksource type to
> select the appropriate mechanism.
> 
>  Introduce a clocksource id field in struct clocksource which is by default  set
> to CSID_GENERIC (0). Clocksource implementations can set that field to  a
> value which allows to identify the clocksource.
> 
>  Store the clocksource id of the current clocksource in the
> system_time_snapshot so callers can evaluate which clocksource was used
> to  take the snapshot and act accordingly.
> 

Ok, really better.

> > diff --git a/include/linux/clocksource_ids.h
> > b/include/linux/clocksource_ids.h new file mode 100644 index
> > 000000000000..93bec8426c44
> > --- /dev/null
> > +++ b/include/linux/clocksource_ids.h
> > @@ -0,0 +1,13 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */ #ifndef
> > +_LINUX_CLOCKSOURCE_IDS_H #define _LINUX_CLOCKSOURCE_IDS_H
> > +
> > +/* Enum to give clocksources a unique identifier */ enum
> > +clocksource_ids {
> > +	CSID_GENERIC		= 0,
> > +	CSID_ARM_ARCH_COUNTER,
> 
> This should only add the infrastructure with just CSID_GENERIC in place.
> 
> The ARM_ARCH variant needs to come in a seperate patch which adds the
> enum and uses it in the corresponding driver. Seperate means a patch doing
> only that and nothing else, i.e. not hidden in some other patch which actually
> makes use of it.
> 

Yeah, this patch should be arch independent and "CSID_ARM_ARCH_COUNTER" should be in an
Separate patch.

Thanks
Jianyong

Thanks
> Thanks,
> 
> 	tglx