netdev - Re: [PATCH v4 1/4] Produce system time from correlated clocksource

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Date:	Wed, 14 Oct 2015 19:34:03 -0700
From:	"Christopher Hall" <christopher.s.hall@...el.com>
To:	"Thomas Gleixner" <tglx@...utronix.de>,
	"Richard Cochran" <richardcochran@...il.com>
Cc:	jeffrey.t.kirsher@...el.com, hpa@...or.com, mingo@...hat.com,
	john.stultz@...aro.org, peterz@...radead.org, x86@...nel.org,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kevin.b.stanton@...el.com
Subject: Re: [PATCH v4 1/4] Produce system time from correlated clocksource

Richard,

On Tue, 13 Oct 2015 14:12:24 -0700, Richard Cochran  
<richardcochran@...il.com> wrote:
> On Tue, Oct 13, 2015 at 09:15:51PM +0200, Thomas Gleixner wrote:

> Can we at least have a explanation of how the firmware operates?  How
> are (ART,sys) pairs are generated, and how they are supposed to get
> into the DSP?

I'll give it a try. The audio controller has a set of registers almost  
exactly like those on the network device. The e1000e patch adds the  
e1000e_phc_get_ts() function. It writes a register to start the  
cross-timestamp process and some time later the hardware sets a bit  
indicating that it's finished.

In the case of the network, the host polls for this bit to be set,  
indicating the cross-timestamp registers have valid data.  In the audio  
DSP case, it is the DSP that's doing the polling and it can only poll once  
per millisecond.

The transfers look like:

Host -PCI (write request) -> DSP

[Transaction started from host]

DSP -PCI (write to initiate)-> Audio controller

[Transaction started from DSP]

DSP <-PCI (read to poll status)- Audio Controller

[Transaction Complete from DSP perspective]

DSP <-PCI (read (ART,device) pair)- Audio Controller

DSP -PCI (write notification) -> Host

[Transaction complete from Host perspective]

Host <-PCI read (ART,device) pair- DSP


I hope this is helpful. Thanks.

Chris
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