netdev - Re: [PATCH net V4 2/2] igb: offer a PTP Hardware Clock instead of the timecompare method

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Message-ID: <CAAsGZS4Jqe5ZcZyGZjurmjBCgzj=+a0cgHAEwB+esXNoQ3P7NQ@mail.gmail.com>
Date:	Mon, 26 Mar 2012 15:32:08 -0400
From:	chetan loke <loke.chetan@...il.com>
To:	"Keller, Jacob E" <jacob.e.keller@...el.com>
Cc:	Richard Cochran <richardcochran@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"e1000-devel@...ts.sourceforge.net" 
	<e1000-devel@...ts.sourceforge.net>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"Ronciak, John" <john.ronciak@...el.com>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH net V4 2/2] igb: offer a PTP Hardware Clock instead of the
 timecompare method

On Mon, Mar 26, 2012 at 2:56 PM, Keller, Jacob E
<jacob.e.keller@...el.com> wrote:

> I am curious how you see a kernel thread resolving the Tx/Rx issue? or is the
> kernel thread being used by gettime? I don't believe we can wait for the Tx/Rx

tx, rx path and kernel thread will use seq_lock_isave(tmreg_lock).

A new 'u64 cached_ns' will be introduced.

tx and rx path can update 'cached_ns' when they read the NIC counter.

A kernel-thread should be scheduled periodically, will read the NIC
counter and update cached_ns. It will use a _trylock variant. If the
lock fails then its a hint that 'cached_ns' is getting updated
somewhere, so just refresh the timer. Periodic update is needed to
handle idle/bursty link conditions because the Rx/tx path may not run
that often.

gettime uses read_seq_lock and reads 'cached_ns'. I mean we could also
do a atomic_read(?).

> path, because if we take too long the software sees it as a dropped timestamp.

What code-path is this?

> How would we only allow one app? Any app with permissions could call the ioctls.
> I do agree that having too many ioctls is a problem. Even in cases where PTP is

We don't. So this is what I'm thinking. Ideally speaking only 1 app
should be adjusting the host-clock and 1 app per NIC if you adjust
NIC's clock. Ignore the host-clock app. Once you enforce the
rate-limit, driver will return -EBUSY if you exceed it. Too many
EBUSYs will provide a hint on user-side.

Chetan
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