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

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Message-ID: <CAAsGZS6QuMJoXz8mmqZsgfTF2ykRz6KG_nCoPBBkTA4z3idoGA@mail.gmail.com>
Date:	Wed, 21 Mar 2012 17:50:34 -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 Wed, Mar 21, 2012 at 1:09 PM, Keller, Jacob E
<jacob.e.keller@...el.com> wrote:
>> -----Original Message-----
>> From: Richard Cochran [mailto:richardcochran@...il.com]
>> Sent: Wednesday, March 21, 2012 10:00 AM
>> To: chetan loke
>> Cc: netdev@...r.kernel.org; e1000-devel@...ts.sourceforge.net; Keller, Jacob
>> E; Kirsher, Jeffrey T; Ronciak, John; john.stultz@...aro.org;
>> tglx@...utronix.de
>> Subject: Re: [PATCH net V4 2/2] igb: offer a PTP Hardware Clock instead of the
>> timecompare method
>>
>> Can you please explain how using a seqlock could help here?
>>
>> Thanks,
>> Richard
>
> My understanding of the seqlock, is that it prevents starvation of the hwtstamp calls in the rx and tx routines if/when a user hammers the gettime ioctl due to bad software design where 100+ apps are wanting direct access to the PHC.
>

Richard - Intent is to make the readers(get_time) wait (or return last
read value if the seq_counter tripped because you know that this value
was recent) and let the tx/rx path continue. I haven't looked in more
details but as Jake mentioned you will also need to change the way you
read the values(by not using timecounter_read in get_time).

thanks
Chetan
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