On 06/13/2016 06:18, Bjorn Helgaas wrote:
I'm still trying to understand what PTM should look like from the
driver's perspective. I know the PCIe spec doesn't define any way to
initiate PTM dialogs or read the results. But I don't know what the
intended usage model is and how the device, driver, and PCI core
pieces should fit together.
- Do we expect endpoints to notice that PTM is enabled and
automatically start using it, without the driver doing anything?
Would driver changes be needed, e.g., to tell the device to add
timestamps to network packet DMAs?
As far as I understand, it is a flag to tell the device that it may
begin using PTM to synchronize the on-board clock with PTM. From the
text in the specification (6.22.3.1 PTM Requester Role):
PTM Requesters are permitted to request PTM Master Time only when PTM is
enabled. The mechanism for directing a PTM Requester to issue such a
request is implementation specific.
If any, there won't be a generic way to trigger a PTM conversation.
- Should there be a pci_enable_ptm() interface for a driver to
enable PTM for its device? If PTM isn't useful without driver
changes, e.g., to tell the device to add timestamps, we probably
should have such an interface so we don't enable PTM when it won't be
useful.
- If the PCI core instead enables PTM automatically whenever
possible (as in the current patch), what performance impact do we
expect? I know you probably can't measure it yet, but can we at
least calculate the worst-case bandwidth usage, based on the message
size and frequency? I previously assumed it would be small, but I
hate to give up *any* performance unless there is some benefit.
If the driver is already utilizing timestamps from the device, it would
be more precise and compensated for link delays. According to the
Implementation Note part in the specs, it says it can be used to
approximate the round trip message transit time and from there, measure
the link delay times, assuming upstream/downstream delays are symmetrical.
- The PTM benefit is mostly for endpoints, and not so much for root
ports or switches themselves. If the PCI core enabled PTM
automatically only on non-endpoints, would there be any overhead?
If it has any local clocks (noted with a requester bit, I have not seen
such a switch), it may start sending synchronization requests, but from
the specs...
Here's my line of thought: If an endpoint never issued a PTM
request, obviously there would never be a PTM dialog on the link
between the last switch and the endpoint. What about on links
farther upstream? Would the switch ever issue a PTM request itself,
without having received a request from the endpoint? If not, the PCI
core could enable PTM on all non-endpoint devices, and there should
be no performance effect at all. This would be nice because a driver
call to enable PTM would only need to touch the endpoint; it wouldn't
need to touch any upstream devices.
From the wording (6.22.2 PTM Link Protocol):
The Upstream Port, on behalf of the PTM Requester, initiates the PTM
dialog by transmitting a PTM Request message.
The Downstream Port, on behalf of the PTM Responder, has knowledge of or
access (directly or indirectly) to the PTM Master Time.
My naive interpretation tells me switches will only act on behalf of
requester/responder, never for itself.
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