[RFC PATCH] usb: dwc3: core: resume xHCI child on runtime resume in host mode

[Junzhong Pan <panjunzhong@linux.spacemit.com>]

USB controller runtime suspend halts the controller on disconnection.
It relies on platforms with custom connection notification to start
the controller again.

The host branch of dwc3_runtime_resume() does nothing currently. There
are two paths that can trigger a runtime resume of the dwc3 core in
host mode:

(a) Wake lands on the xHCI child (DT platforms, e.g. Qualcomm, i.MX8MP)
    A platform wakeup IRQ fires and the glue handler calls
    pm_runtime_resume(&xhci->dev). Runtime PM walks upward, resumes
    the dwc3 parent first, then resumes xHCI. Both wake correctly.

(b) Wake lands on the dwc3 parent (ACPI platforms)
    When an ACPI Notify(ACPI_NOTIFY_DEVICE_WAKE) targets the dwc3
    parent device -- for example via a Generic Event Device on a
    hardware-reduced platform where dwc3 is enumerated as an ACPI
    device without a glue driver -- the ACPI core turns this into
    pm_request_resume(dwc3_dev). Runtime PM does not propagate resume
    downward, so the xHCI child stays suspended. Root hub polling is
    never re-armed, and a device plugged in while suspended is silently
    dropped.

This patch fills in the host branch to cover case (b) by requesting an
async resume of the xHCI child so xhci_resume() can restore the port
state machine and re-arm root hub polling. Async is required to avoid
deadlocking on the parent's own rpm_resume() re-entry.

Signed-off-by: Junzhong Pan <panjunzhong@linux.spacemit.com>
---
base-commit: d58772d8520c7ef247c4b95c9bd76d3a25da9ff5
 drivers/usb/dwc3/core.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
index ceb49f2f8004..bce56765ad90 100644
--- a/drivers/usb/dwc3/core.c
+++ b/drivers/usb/dwc3/core.c
@@ -2668,6 +2668,18 @@ int dwc3_runtime_resume(struct dwc3 *dwc)
 		}
 		break;
 	case DWC3_GCTL_PRTCAP_HOST:
+		/*
+		 * Only the xHCI child's resume re-arms root hub polling, which
+		 * is what rediscovers a device plugged in while suspended.
+		 * Runtime PM never resumes children on its own, so request it
+		 * here. This has to be asynchronous: resuming the child
+		 * synchronously would deadlock because rpm_resume() tries to
+		 * resume the parent before the child, and here the parent's
+		 * own callback has not returned yet.
+		 */
+		if (dwc->xhci)
+			pm_request_resume(&dwc->xhci->dev);
+		break;
 	default:
 		/* do nothing */
 		break;
--
2.25.1