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

[Bjorn Helgaas]

On Thu, Jul 30, 2026 at 08:16:36AM +0000, Hongxing Zhu (OSS) wrote:
> > -----Original Message-----
> > From: Bjorn Helgaas <helgaas@xxxxxxxxxx>
> > Sent: Thursday, July 30, 2026 6:30 AM
> ...

> > On Fri, Jul 17, 2026 at 03:41:21PM +0800, hongxing.zhu@xxxxxxxxxxx wrote:
> > > From: Richard Zhu <hongxing.zhu@xxxxxxx>
> > >
> > > 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".