linux-kernel - [PATCH 1/1] dwc2: Delay resume until device connection is stable

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Message-Id: <20251215125317.85624-1-sunqian@senarytech.com>
Date: Mon, 15 Dec 2025 20:53:17 +0800
From: sunqian <sunqian@...arytech.com>
To: hminas@...opsys.com
Cc: gregkh@...uxfoundation.org,
	linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	sunqian@...arytech.com
Subject: [PATCH 1/1] dwc2: Delay resume until device connection is stable

When DWC2 is used as a host with a dynamically controlled VBUS supply,
VBUS may be turned off during system suspend and enabled again during
resume. Some USB devices (e.g. USB mass storage) require additional time
after VBUS is restored to power up and for the D+/D- line state to settle.

The resume sequence may proceed too early, observe HPRT0.CONNSTS as 0,
and treat an already-connected device as disconnected. This can lead to
a spurious USB disconnect followed by re-enumeration, making an
already-mounted filesystem unusable after resume. Even if the device
reconnects later, the resume does not behave like a true restoration of
the pre-suspend device state.

Poll HPRT0.CONNSTS for a short, bounded period after enabling the
external VBUS supply, allowing the controller to report a stable
connection state and prevent spurious disconnects during resume.

Without this change, resume often results in a disconnect and a new
device enumeration:

    dwc2_enable_host_interrupts()
    ClearPortFeature USB_PORT_FEAT_C_SUSPEND
    ClearPortFeature USB_PORT_FEAT_ENABLE
    usb 1-1: USB disconnect, device number 3
    ...
    usb 1-1: new high-speed USB device number 4 using dwc2

With this change applied, the controller reliably detects the device
after resume and restores the link without triggering a full disconnect
and re-enumeration cycle:

    dwc2_enable_host_interrupts()
    gintsts=05000021  gintmsk=f3000806
    Device connected after 9 retries
    ClearPortFeature USB_PORT_FEAT_C_CONNECTION
    ClearPortFeature USB_PORT_FEAT_C_SUSPEND
    ...
    usb 1-1: reset high-speed USB device number 4 using dwc2

As a side effect, when an OTG host adapter is connected but no USB
device is present, HPRT0.CONNSTS remains deasserted and the polling
reaches the timeout. In this case, system resume latency may increase
by the duration of the bounded wait, which is considered an acceptable
tradeoff to avoid spurious disconnects and filesystem corruption.

Tested on:
  - Kernel: v5.15.140
  - Suspend mode: suspend-to-RAM (STR)
  - dr_mode: OTG (dual-role), host mode via OTG adapter
  - Devices:
      * USB mass storage (Aigo, Kingston, SanDisk)
      * USB HID (mouse, keyboard)

Signed-off-by: sunqian <sunqian@...arytech.com>
---
 drivers/usb/dwc2/hcd.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/dwc2/hcd.c b/drivers/usb/dwc2/hcd.c
index 60ef8092259a..96345eeb9e2f 100644
--- a/drivers/usb/dwc2/hcd.c
+++ b/drivers/usb/dwc2/hcd.c
@@ -4404,11 +4404,15 @@ static int _dwc2_hcd_suspend(struct usb_hcd *hcd)
 	return ret;
 }

+#define CONNSTS_POLL_RETRIES       80
+#define CONNSTS_POLL_DELAY_US_MIN  3000
+#define CONNSTS_POLL_DELAY_US_MAX  5000
 static int _dwc2_hcd_resume(struct usb_hcd *hcd)
 {
 	struct dwc2_hsotg *hsotg = dwc2_hcd_to_hsotg(hcd);
 	unsigned long flags;
 	u32 hprt0;
+	int retry;
 	int ret = 0;

 	spin_lock_irqsave(&hsotg->lock, flags);
@@ -4501,8 +4505,26 @@ static int _dwc2_hcd_resume(struct usb_hcd *hcd)
 	spin_unlock_irqrestore(&hsotg->lock, flags);
 	dwc2_vbus_supply_init(hsotg);

-	/* Wait for controller to correctly update D+/D- level */
-	usleep_range(3000, 5000);
+	/*
+	 * Wait for device connection to stabilize after VBUS is restored.
+	 * Some externally powered devices may need time for D+/D- lines to settle.
+	 * This runs in the resume path where sleeping is allowed.
+	 */
+	for (retry = 0; retry < CONNSTS_POLL_RETRIES; retry++) {
+		spin_lock_irqsave(&hsotg->lock, flags);
+		hprt0 = dwc2_read_hprt0(hsotg);
+		spin_unlock_irqrestore(&hsotg->lock, flags);
+
+		if (hprt0 & HPRT0_CONNSTS) {
+			dev_dbg(hsotg->dev,
+				"Device connected after %d retries\n", retry);
+			break;
+		}
+
+		usleep_range(CONNSTS_POLL_DELAY_US_MIN,
+			     CONNSTS_POLL_DELAY_US_MAX);
+	}
+
 	spin_lock_irqsave(&hsotg->lock, flags);

 	/*
-- 
2.25.1