netdev - Re: [net-next 03/15] net/mlx5: Add adjphase function to support hardware-only offset control

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Date:   Thu, 19 Jan 2023 20:03:43 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Rahul Rameshbabu <rrameshbabu@...dia.com>
Cc:     Jacob Keller <jacob.e.keller@...el.com>,
        Saeed Mahameed <saeed@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Saeed Mahameed <saeedm@...dia.com>, <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Gal Pressman <gal@...dia.com>,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [net-next 03/15] net/mlx5: Add adjphase function to support
 hardware-only offset control

On Thu, 19 Jan 2023 19:56:24 -0800 Rahul Rameshbabu wrote:
> >> Makes sense. Once you've verified that the delta is within the accepted
> >> range you can just re-use the existing adjtime function.  
> >
> > Seems like we should add a "max_time_adj" to struct ptp_clock_info
> > and let the core call adjphase if the offset is small enough to fit.
> > Instead of having all drivers redirect the calls internally.  
> 
> With guidance from Saeed on this topic, I have a patch in the works for
> advertising the max phase adjustment supported by a driver through the
> use of the PTP_CLOCK_GETCAPS ioctl. This is how the ptp stack handles
> advertising the max frequency supported by a driver today. In linuxptp,
> this ioctl is wrapped in a function call for getting the max frequency
> adjustment supported by a device before ptp is actually run. I believe a
> similar logic should occur for phase (time) adjustments. This patch
> would introduce a "max_phase_adj" in ptp_clock_info that would be
> handled in ptp_clock_adjtime in the ptp core stack.

Nice, can we make the core also call ->adjtime automatically if driver
doesn't define ->adjphase and the abs(delta) < .max_phase_adj ?

The other question is about the exact semantics of ->adjphase
- do all timecounter based clock implementations support it
by definition?