Messages in this thread | | | Subject | Re: [PATCH v4 1/4] Produce system time from correlated clocksource | Date | Wed, 14 Oct 2015 19:34:03 -0700 | From | "Christopher Hall" <> |
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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
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