netdev - RE: [PATCH v4 net-next 3/3] net: ena: Add PHC documentation

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Message-ID: <a86eb32a374d4853a409c02777e71501@amazon.com>
Date: Tue, 19 Nov 2024 08:45:52 +0000
From: "Arinzon, David" <darinzon@...zon.com>
To: Richard Cochran <richardcochran@...il.com>
CC: David Miller <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, Eric Dumazet
	<edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, "Woodhouse, David"
	<dwmw@...zon.co.uk>, "Machulsky, Zorik" <zorik@...zon.com>, "Matushevsky,
 Alexander" <matua@...zon.com>, "Bshara, Saeed" <saeedb@...zon.com>, "Wilson,
 Matt" <msw@...zon.com>, "Liguori, Anthony" <aliguori@...zon.com>, "Bshara,
 Nafea" <nafea@...zon.com>, "Schmeilin, Evgeny" <evgenys@...zon.com>,
	"Belgazal, Netanel" <netanel@...zon.com>, "Saidi, Ali" <alisaidi@...zon.com>,
	"Herrenschmidt, Benjamin" <benh@...zon.com>, "Kiyanovski, Arthur"
	<akiyano@...zon.com>, "Dagan, Noam" <ndagan@...zon.com>, "Bernstein, Amit"
	<amitbern@...zon.com>, "Agroskin, Shay" <shayagr@...zon.com>, "Abboud, Osama"
	<osamaabb@...zon.com>, "Ostrovsky, Evgeny" <evostrov@...zon.com>, "Tabachnik,
 Ofir" <ofirt@...zon.com>, "Machnikowski, Maciek" <maciek@...hnikowski.net>,
	Rahul Rameshbabu <rrameshbabu@...dia.com>, Gal Pressman <gal@...dia.com>
Subject: RE: [PATCH v4 net-next 3/3] net: ena: Add PHC documentation

> On Sat, Nov 16, 2024 at 05:53:26PM -0800, Richard Cochran wrote:
> > On Thu, Nov 14, 2024 at 11:59:30AM +0200, David Arinzon wrote:
> >
> > > +**phc_skp**         Number of skipped get time attempts (during block
> period).
> > > +**phc_err**         Number of failed get time attempts (entering into
> block state).
> >
> > Just curious...  I understand that the HW can't support a very high
> > rate of gettime calls and that the driver will throttle them.
> >
> > But why did you feel the need to document the throttling behavior in
> > such a overt way?  Are there user space programs out there calling
> > gettime excessively?
> 
> Answering my own question (maybe)
> 
> I see that your PHC only supports gettime(), and so I guess you must have
> some atypical system setup in mind.
> 
> I didn't see any comments in the cover letter or in the patch about why the
> PHC isn't adjustable or how offering gettime() only is useful?
> 
> Thanks,
> Richard

Hi Richard
	
Thank you for the queries.

Our device limits the number of requests per client (VM) through throttling.
To avoid reaching our device throttling limit and ensure a fail-fast mechanism,
the driver preemptively applies throttling.
In addition, AWS cannot be adjusted or controlled by the OS/User to correct any time deviations,
instead, AWS manages the synchronization and accuracy of the clock internally and provides a
pre-disciplined clock that already adheres to Coordinated Universal Time (UTC) standards.
An upcoming patch, which is a continuation of https://lore.kernel.org/netdev/4c2e99b4-b19e-41f5-a048-3bcc8c33a51c@lunn.ch/T/#m58cf9b05b34046b3952ae8bf4cc7d7f2c8b011d7,
will introduce the error bound in ENA PHC, providing internal AWS accuracy error measurements (in nanoseconds).

Thanks,
David