Re: [RFC PATCH 0/3] PCI: Use Configuration RRS to wait for device ready

[Mario Limonciello]

On 8/28/2024 16:42, Bjorn Helgaas wrote:
> On Wed, Aug 28, 2024 at 04:24:01PM -0500, Mario Limonciello wrote:
> > On 8/27/2024 18:48, Bjorn Helgaas wrote:
> > > From: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> > >
> > > After a device reset, pci_dev_wait() waits for a device to become
> > > completely ready by polling the PCI_COMMAND register.  The spec envisions
> > > that software would instead poll for the device to stop responding to
> > > config reads with Completions with Request Retry Status (RRS).
> > >
> > > Polling PCI_COMMAND leads to hardware retries that are invisible to
> > > software and the backoff between software retries doesn't work correctly.
> > >
> > > Root Ports are not required to support the Configuration RRS Software
> > > Visibility feature that prevents hardware retries and makes the RRS
> > > Completions visible to software, so this series only uses it when available
> > > and falls back to PCI_COMMAND polling when it's not.
> > >
> > > This is completely untested and posted for comments.
> > >
> > > Bjorn Helgaas (3):
> > >     PCI: Wait for device readiness with Configuration RRS
> > >     PCI: aardvark: Correct Configuration RRS checking
> > >     PCI: Rename CRS Completion Status to RRS
> > >
> > >    drivers/bcma/driver_pci_host.c             | 10 ++--
> > >    drivers/pci/controller/dwc/pcie-tegra194.c | 18 +++---
> > >    drivers/pci/controller/pci-aardvark.c      | 64 +++++++++++-----------
> > >    drivers/pci/controller/pci-xgene.c         |  6 +-
> > >    drivers/pci/controller/pcie-iproc.c        | 18 +++---
> > >    drivers/pci/pci-bridge-emul.c              |  4 +-
> > >    drivers/pci/pci.c                          | 41 +++++++++-----
> > >    drivers/pci/pci.h                          | 11 +++-
> > >    drivers/pci/probe.c                        | 33 +++++------
> > >    include/linux/bcma/bcma_driver_pci.h       |  2 +-
> > >    include/linux/pci.h                        |  1 +
> > >    include/uapi/linux/pci_regs.h              |  6 +-
> > >    12 files changed, 117 insertions(+), 97 deletions(-)
> >
> > Although this looks like a useful series, I'm sorry to say but this doesn't
> > solve the issue that Gary and I raised.  We double checked today and found
> > that reading the vendor ID works just fine at this time.
>
> Thanks for testing that.

Sure.

> > I think that we're still better off polling PCI_PM_CTRL to "wait" for D0
> > after the state change from D3cold.
>
> Is there some spec justification for polling PCI_PM_CTRL?  I'm dubious
> about doing that just because "it works" in this situation, unless we
> have some better understanding about *why* it works and whether all
> devices are supposed to work that way.

I mentioned this a little bit in my patch 3/5 in my submission.  The 
issue isn't "normal" D3cold exit that is fully settled down.
That takes ~6ms from measurements.  The issue is how long it takes for 
D3cold *entry* followed by *exit*.

When this issue occurs it's tied with a tight loop of runtime PM entry 
and exit happening in that short window.
That's why it can be tripped by unplugging a dock, waiting until 
~approximately autosuspend delay and plugging it back in.  If you catch 
the right timing then the USB4 router is still on it's way down to D3cold.

In terms of a way to match this problem to the spec, the closest I could 
think is PCI-PM spec.

But I do see in the PCI-PM spec that the delay for D0->D3hot should be 
10ms. In the Linux kernel implementation __pci_set_power_state() when 
called with D3cold calls pci_set_low_power_state() which does wait 10ms 
followed by using the platform to remove power.

I can't find any timing requirements for D3hot->D3cold transition though.

I would hypothesize that if we injected a longer delay on the "other 
end" for the D3cold transition entry it would solve this issue as well 
though.