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

Christopher Hall
2015-10-15T02:34:07Z

Richard,

On Tue, 13 Oct 2015 14:12:24 -0700, Richard Cochran
<richardcochran@gmail.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