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Message-ID: <e0935a7cc42ad34e71c17f3b5ada4a16a8d1f539.camel@infradead.org>
Subject: Re: [RFC PATCH v3 5/7] virtio_rtc: Add PTP clocks
From: David Woodhouse <dwmw2@infradead.org>
To: Peter Hilber <peter.hilber@opensynergy.com>,
 linux-kernel@vger.kernel.org,  virtualization@lists.linux.dev,
 virtio-dev@lists.oasis-open.org
Cc: "Michael S. Tsirkin" <mst@redhat.com>, Jason Wang <jasowang@redhat.com>,
  Xuan Zhuo <xuanzhuo@linux.alibaba.com>, Richard Cochran
 <richardcochran@gmail.com>,  netdev@vger.kernel.org, Marc Zyngier
 <maz@kernel.org>, Mark Rutland <mark.rutland@arm.com>, Daniel Lezcano
 <daniel.lezcano@linaro.org>, Thomas Gleixner <tglx@linutronix.de>,
 linux-arm-kernel@lists.infradead.org
Date: Sat, 15 Jun 2024 09:01:31 +0100
In-Reply-To: <20231218073849.35294-6-peter.hilber@opensynergy.com>
References: <20231218073849.35294-1-peter.hilber@opensynergy.com>
	 <20231218073849.35294-6-peter.hilber@opensynergy.com>
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On Mon, 2023-12-18 at 08:38 +0100, Peter Hilber wrote:
>=20
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0ret =3D viortc_hw_xtstamp_para=
ms(&hw_counter, &cs_id);
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (ret)
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0return ret;
> +
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0ktime_get_snapshot(&history_be=
gin);
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (history_begin.cs_id !=3D c=
s_id)
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0return -EOPNOTSUPP;

I think you have to call ktime_get_snapshot() anyway to get a snapshot
from before your crosststamp? But I still don't much like the fact that
you need to use it to work out which cs_id is being used.

Shouldn't get_device_system_crosststamp() pass that to its get_time_fn
as a hint?

On x86, you are likely to find that history_begin.cs_id is the KVM
clock, so this will return -EOPNOTSUPP and userspace will have to fall
back to PTP_SYS_OFFSET. I note the KVM PTP clock actually *converts* a
TSC-based crosststamp to kvmclock =C2=B5s for itself, so that it can report=
=C2=A0
*cs_id =3D=C2=A0CSID_X86_KVM_CLK. Not sure how I feel about that though. I'=
m
inclined to suggest that it shouldn't, as anyone who wants accurate
timekeeping shouldn't be using the KVM clock anyway.

But we should at least be relatively consistent about it.

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