Re: [PATCH v2] PCI: dwc: Add suspend_poweroff flag for platforms with RC power loss

On Thu, Jul 30, 2026 at 07:09:13AM -0500, Bjorn Helgaas wrote:
> On Thu, Jul 30, 2026 at 08:16:36AM +0000, Hongxing Zhu (OSS) wrote:
> > > -----Original Message-----
> > > From: Bjorn Helgaas <[email protected]>
> > > Sent: Thursday, July 30, 2026 6:30 AM
> > ...
> 
> > > On Fri, Jul 17, 2026 at 03:41:21PM +0800, [email protected] wrote:
> > > > From: Richard Zhu <[email protected]>
> > > >
> > > > Some platforms like i.MX power off their PCIe RC controllers during
> > > > system suspend, requiring full re-initialization on resume. These
> > > > platforms need to enter L2 state to properly notify endpoints before
> > > > power loss.
> > > >
> > > > According to PCIe base spec r7.0, sec 5.2, the system software should
> > > > transition the device into D3Hot before broadcasting the PME_Turn_Off
> > > > message to initiate L2 entry. However, some endpoint devices fail the
> > > > D3cold capability check in pci_host_common_d3cold_possible(), which
> > > > would normally prevent L2 entry.
> > > 
> > > Wakeup devices that don't support PME from D3cold will fail the
> > > D3cold capability check, but I don't think those are the problem
> > > you're solving.
> > > 
> > You're right.
> > 
> > > This appears to handle devices that are not in D3hot, and that's
> > > not a property of the endpoint; it's a property of its driver.  Is
> > > the problem here that some driver didn't put its device in D3hot?
> > > 
> > I observed some endpoint devices failing the D3cold capability
> > check, but I haven't identified the root cause.
> 
> Seems like some instrumentation in pci_host_common_d3cold_possible()
> could tell us which devices/drivers are relevant.
> 
> > > > For platforms where the RC loses power during suspend, L2 entry
> > > > is essential regardless of D3cold support, as the link will be
> > > > lost anyway.  Add a suspend_poweroff flag to force L2 entry in
> > > > such cases, and enable it for i.MX PCIe controllers.
> > > >
> > > > Note: This violates the spec requirement that devices be in
> > > > D3Hot before PME_Turn_Off, but is necessary for proper operation
> > > > on platforms with RC power loss during suspend.
> > > 
> > > If the device isn't in D3hot, it may still be active, and I think
> > > the PME_Turn_Off will abort any DMAs in progress, which doesn't
> > > sound like proper operation of the endpoint.
> >
> > You're correct - this isn't ideal for endpoint operation. However,
> > on i.MX platforms, the RC controller will lose power during system
> > suspend regardless. Without sending PME_Turn_Off, endpoints would
> > have no notification before the link abruptly goes down.
> 
> The PME_Turn_Off is some notification, but I don't think the endpoint
> can do DMA or MSI, so its driver won't be involved.  It may be able to
> do things internally, e.g., write caches to an SSD or transmit packets
> from its internal buffers.  But I think it might still lose power in
> the middle of operations like that, so it doesn't sound reliable.
> 
> > This patch adds a flag to force L2 entry for such platforms,
> > choosing to notify endpoints (even if not ideal) rather than having
> > the link drop unexpectedly. I'm appreciated for suggestions if
> > there's a better approach to handle this scenario.
> 
> Mani recently added pci_suspend_retains_context(), and I think it
> returns "true" on i.MX.  But if the RC is powered off and all the
> devices need full re-initialization on resume, it sounds like it
> *should* return "false".

Right. pci_suspend_retains_context() was added just for that purpose of telling
client drivers to prepare for context loss. So if this API returns 'false', then
they should not hold the device in D0.

- Mani

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