netdev - Re: [EXTERNAL] [PATCH v8 net-next 5/5] net: ena: Add PHC documentation

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Message-ID: <65d766e4a06bf85b9141452039f10a1d59545f76.camel@infradead.org>
Date: Wed, 05 Mar 2025 12:59:39 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: "Arinzon, David" <darinzon@...zon.com>, Andrew Lunn <andrew@...n.ch>
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>, Simon Horman
 <horms@...nel.org>, Richard Cochran <richardcochran@...il.com>, "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>, "Allen, Neil" <shayagr@...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>, Vadim Fedorenko
 <vadim.fedorenko@...ux.dev>
Subject: Re: [EXTERNAL] [PATCH v8 net-next 5/5] net: ena: Add PHC
 documentation

On Wed, 2025-03-05 at 12:50 +0000, Arinzon, David wrote:
>  
> > Why is not opening /dev/ptpX sufficient to enable the PHC?
> > 
> >      Andrew
> 
> Hi Andrew,
> 
> The reasoning for the enablement option of PHC was explained in patch 3 in the series
> https://lore.kernel.org/netdev/20250304190504.3743-4-darinzon@amazon.com/

Not really. That says,

"The PHC feature is disabled by default, although its
footprint is small, most customers do not require such
high-precision timestamping. Enabling PHC unnecessarily
in environments that do not need it might introduce a
minor but unnecessary overhead."

Disable by default, sure. But it gives no explanation for why we need a
separate knob in sysfs to enable/disable it, and we can't just enable
it automatically when /dev/ptpX is opened as Andrew suggested.

If you read the actual code in that patch, there's a hint though.
Because the actual process in ena_phc_enable_set() does the following:

+	ena_destroy_device(adapter, false);
+	rc = ena_restore_device(adapter);

Is that actually tearing down the whole netdev and recreating it when
the PHC enable is flipped? Does it really have to? That doesn't seem
ideal even if it *is* a separate knob in sysfs.

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