Sharing a USB controller with another entity via xhci-sideband driver
creates power management complexities. To prevent the USB controller
from being inadvertently deactivated while in use by the other entity, a
usage-count based mechanism is implemented. This allows the system to
manage power effectively, ensuring the controller remains available
whenever needed.
In order to maintain full functionality of an offloaded USB devices,
several changes are made within the suspend flow of such devices:
- skip usb_suspend_device() so that the port/hub are still active for
  USB transfers via offloaded path.
- not suspending the endpoints which are used by USB interfaces marked
  with needs_remote_wakeup. Namely, skip usb_suspend_interface() and
  usb_hcd_flush_endpoint() on associated USB interfaces. This reserves a
  pending interrupt urb during system suspend for handling the interrupt
  transfer, which is necessary since remote wakeup doesn't apply in the
  offloaded USB devices when controller is still active.
- not flushing the endpoints of actively offloaded USB devices. Given
  that the USB devices is used by another entity, unilaterally flush the
  endpoint might lead to unexpected behavior on another entity.

Signed-off-by: Guan-Yu Lin <guanyulin@google.com>
---
 drivers/usb/core/driver.c         | 23 +++++++++++++++++++----
 drivers/usb/core/endpoint.c       |  8 --------
 drivers/usb/dwc3/core.c           | 28 ++++++++++++++++++++++++++++
 drivers/usb/dwc3/core.h           |  1 +
 drivers/usb/host/xhci-plat.c      | 14 ++++++++++++++
 include/linux/usb.h               |  8 +++++++-
 include/linux/usb/hcd.h           |  7 +++++++
 sound/usb/qcom/qc_audio_offload.c |  3 +++
 8 files changed, 79 insertions(+), 13 deletions(-)

diff --git a/drivers/usb/core/driver.c b/drivers/usb/core/driver.c
index 1bbf9592724f..f8d3b50c188a 100644
--- a/drivers/usb/core/driver.c
+++ b/drivers/usb/core/driver.c
@@ -1415,17 +1415,29 @@ static int usb_suspend_both(struct usb_device *udev, pm_message_t msg)
 {
 	int			status = 0;
 	int			i = 0, n = 0;
+	bool			offload = false;
 	struct usb_interface	*intf;
 
 	if (udev->state == USB_STATE_NOTATTACHED ||
 			udev->state == USB_STATE_SUSPENDED)
 		goto done;
 
+#ifdef CONFIG_USB_XHCI_SIDEBAND
+	if (msg.event == PM_EVENT_SUSPEND && usb_offload_check(udev)) {
+		dev_dbg(&udev->dev, "device offload active.\n");
+		offload = true;
+	}
+#endif
+
 	/* Suspend all the interfaces and then udev itself */
 	if (udev->actconfig) {
 		n = udev->actconfig->desc.bNumInterfaces;
 		for (i = n - 1; i >= 0; --i) {
 			intf = udev->actconfig->interface[i];
+			if (offload && intf->needs_remote_wakeup) {
+				dev_dbg(&intf->dev, "interface stays active on an offloaded device\n");
+				continue;
+			}
 			status = usb_suspend_interface(udev, intf, msg);
 
 			/* Ignore errors during system sleep transitions */
@@ -1436,7 +1448,8 @@ static int usb_suspend_both(struct usb_device *udev, pm_message_t msg)
 		}
 	}
 	if (status == 0) {
-		status = usb_suspend_device(udev, msg);
+		if (!offload)
+			status = usb_suspend_device(udev, msg);
 
 		/*
 		 * Ignore errors from non-root-hub devices during
@@ -1481,9 +1494,11 @@ static int usb_suspend_both(struct usb_device *udev, pm_message_t msg)
 	 */
 	} else {
 		udev->can_submit = 0;
-		for (i = 0; i < 16; ++i) {
-			usb_hcd_flush_endpoint(udev, udev->ep_out[i]);
-			usb_hcd_flush_endpoint(udev, udev->ep_in[i]);
+		if (!offload) {
+			for (i = 0; i < 16; ++i) {
+				usb_hcd_flush_endpoint(udev, udev->ep_out[i]);
+				usb_hcd_flush_endpoint(udev, udev->ep_in[i]);
+			}
 		}
 	}
 
diff --git a/drivers/usb/core/endpoint.c b/drivers/usb/core/endpoint.c
index e48399401608..658ef39ebcd1 100644
--- a/drivers/usb/core/endpoint.c
+++ b/drivers/usb/core/endpoint.c
@@ -18,14 +18,6 @@
 #include <linux/usb.h>
 #include "usb.h"
 
-struct ep_device {
-	struct usb_endpoint_descriptor *desc;
-	struct usb_device *udev;
-	struct device dev;
-};
-#define to_ep_device(_dev) \
-	container_of(_dev, struct ep_device, dev)
-
 struct ep_attribute {
 	struct attribute attr;
 	ssize_t (*show)(struct usb_device *,
diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
index 0735881d4650..793fbc030fc4 100644
--- a/drivers/usb/dwc3/core.c
+++ b/drivers/usb/dwc3/core.c
@@ -2602,8 +2602,22 @@ static int dwc3_runtime_idle(struct device *dev)
 static int dwc3_suspend(struct device *dev)
 {
 	struct dwc3	*dwc = dev_get_drvdata(dev);
+#ifdef CONFIG_USB_XHCI_SIDEBAND
+	struct platform_device *xhci = dwc->xhci;
+	struct usb_hcd  *hcd;
+#endif
 	int		ret;
 
+#ifdef CONFIG_USB_XHCI_SIDEBAND
+	if (xhci) {
+		hcd = dev_get_drvdata(&xhci->dev);
+		if (xhci_sideband_check(hcd)) {
+			dev_dbg(dev, "sideband instance active.\n");
+			return 0;
+		}
+	}
+#endif
+
 	ret = dwc3_suspend_common(dwc, PMSG_SUSPEND);
 	if (ret)
 		return ret;
@@ -2616,8 +2630,22 @@ static int dwc3_suspend(struct device *dev)
 static int dwc3_resume(struct device *dev)
 {
 	struct dwc3	*dwc = dev_get_drvdata(dev);
+#ifdef CONFIG_USB_XHCI_SIDEBAND
+	struct platform_device *xhci = dwc->xhci;
+	struct usb_hcd  *hcd;
+#endif
 	int		ret = 0;
 
+#ifdef CONFIG_USB_XHCI_SIDEBAND
+	if (xhci) {
+		hcd = dev_get_drvdata(&xhci->dev);
+		if (xhci_sideband_check(hcd)) {
+			dev_dbg(dev, "sideband instance active.\n");
+			return 0;
+		}
+	}
+#endif
+
 	pinctrl_pm_select_default_state(dev);
 
 	pm_runtime_disable(dev);
diff --git a/drivers/usb/dwc3/core.h b/drivers/usb/dwc3/core.h
index 0b6a07202609..57c3e768cdac 100644
--- a/drivers/usb/dwc3/core.h
+++ b/drivers/usb/dwc3/core.h
@@ -26,6 +26,7 @@
 #include <linux/usb/ch9.h>
 #include <linux/usb/gadget.h>
 #include <linux/usb/otg.h>
+#include <linux/usb/hcd.h>
 #include <linux/usb/role.h>
 #include <linux/ulpi/interface.h>
 
diff --git a/drivers/usb/host/xhci-plat.c b/drivers/usb/host/xhci-plat.c
index b676d4dbcec1..9e01450328d7 100644
--- a/drivers/usb/host/xhci-plat.c
+++ b/drivers/usb/host/xhci-plat.c
@@ -456,6 +456,13 @@ static int xhci_plat_suspend_common(struct device *dev, struct pm_message pmsg)
 	struct xhci_hcd	*xhci = hcd_to_xhci(hcd);
 	int ret;
 
+#ifdef CONFIG_USB_XHCI_SIDEBAND
+	if (pmsg.event == PM_EVENT_SUSPEND && xhci_sideband_check(hcd)) {
+		dev_dbg(dev, "sideband instance active.\n");
+		return 0;
+	}
+#endif
+
 	if (pm_runtime_suspended(dev))
 		pm_runtime_resume(dev);
 
@@ -499,6 +506,13 @@ static int xhci_plat_resume_common(struct device *dev, struct pm_message pmsg)
 	struct xhci_hcd	*xhci = hcd_to_xhci(hcd);
 	int ret;
 
+#ifdef CONFIG_USB_XHCI_SIDEBAND
+	if (pmsg.event == PM_EVENT_RESUME && xhci_sideband_check(hcd)) {
+		dev_dbg(dev, "sideband instance active.\n");
+		return 0;
+	}
+#endif
+
 	if (!device_may_wakeup(dev) && (xhci->quirks & XHCI_SUSPEND_RESUME_CLKS)) {
 		ret = clk_prepare_enable(xhci->clk);
 		if (ret)
diff --git a/include/linux/usb.h b/include/linux/usb.h
index 9b3f630e763e..c4ff11ad14d5 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -44,7 +44,13 @@ struct usb_driver;
  * Devices may also have class-specific or vendor-specific descriptors.
  */
 
-struct ep_device;
+struct ep_device {
+	struct usb_endpoint_descriptor *desc;
+	struct usb_device *udev;
+	struct device dev;
+};
+#define to_ep_device(_dev) \
+	container_of(_dev, struct ep_device, dev)
 
 /**
  * struct usb_host_endpoint - host-side endpoint descriptor and queue
diff --git a/include/linux/usb/hcd.h b/include/linux/usb/hcd.h
index ac95e7c89df5..a9577da6ecff 100644
--- a/include/linux/usb/hcd.h
+++ b/include/linux/usb/hcd.h
@@ -766,6 +766,13 @@ extern struct rw_semaphore ehci_cf_port_reset_rwsem;
 #define USB_EHCI_LOADED		2
 extern unsigned long usb_hcds_loaded;
 
+#if IS_ENABLED(CONFIG_USB_XHCI_SIDEBAND)
+extern bool xhci_sideband_check(struct usb_hcd *hcd);
+#else
+static inline bool xhci_sideband_check(struct usb_hcd *hcd)
+{ return false; }
+#endif
+
 #endif /* __KERNEL__ */
 
 #endif /* __USB_CORE_HCD_H */
diff --git a/sound/usb/qcom/qc_audio_offload.c b/sound/usb/qcom/qc_audio_offload.c
index 7dd7e51109df..d201b1ad33b1 100644
--- a/sound/usb/qcom/qc_audio_offload.c
+++ b/sound/usb/qcom/qc_audio_offload.c
@@ -1619,8 +1619,11 @@ static void handle_uaudio_stream_req(struct qmi_handle *handle,
 			mutex_lock(&chip->mutex);
 			subs->opened = 0;
 			mutex_unlock(&chip->mutex);
+		} else {
+			xhci_sideband_get(uadev[pcm_card_num].si);
 		}
 	} else {
+		xhci_sideband_put(uadev[pcm_card_num].si);
 		info = &uadev[pcm_card_num].info[info_idx];
 		if (info->data_ep_pipe) {
 			ep = usb_pipe_endpoint(uadev[pcm_card_num].udev,
-- 
2.48.0.rc2.279.g1de40edade-goog

Hi,

> - not flushing the endpoints of actively offloaded USB devices. Given
>   that the USB devices is used by another entity, unilaterally flush
>   the endpoint might lead to unexpected behavior on another entity.

This doesn't seem right, because flushing applies to URBs managed by
the kernel, so it should have no effect on offloaded endpoints.

As far as I understand from your earlier discussion with Alan Stern,
the real reason is that it disrupted operation of class drivers, in
particular causing kernel-managed interrupt endpoints not to be polled
during suspend and some events were being lost.

Or maybe the real problem was that if the INT IN endpoint isn't being
polled, device events don't trigger xHCI IRQs that wake up the CPU?


And by the way, usb_hcd_flush_endpoint() doc states that no new URBs
may be submitted during this call. I wonder if this can be guaranteed
if the interface has not been suspended first? Perhaps this alone is
good reason not to flush.

Regards,
Michal

On Thu, Feb 6, 2025 at 8:13 AM Michał Pecio <michal.pecio@gmail.com> wrote:
>
> Hi,
>
> > - not flushing the endpoints of actively offloaded USB devices. Given
> >   that the USB devices is used by another entity, unilaterally flush
> >   the endpoint might lead to unexpected behavior on another entity.
>
> This doesn't seem right, because flushing applies to URBs managed by
> the kernel, so it should have no effect on offloaded endpoints.
>
> As far as I understand from your earlier discussion with Alan Stern,
> the real reason is that it disrupted operation of class drivers, in
> particular causing kernel-managed interrupt endpoints not to be polled
> during suspend and some events were being lost.
>
> Or maybe the real problem was that if the INT IN endpoint isn't being
> polled, device events don't trigger xHCI IRQs that wake up the CPU?
>

The main reason is as you described above. Without polling the INT IN
endpoint, some functions (e.g. hid, hub) failed during system suspend.

>
> And by the way, usb_hcd_flush_endpoint() doc states that no new URBs
> may be submitted during this call. I wonder if this can be guaranteed
> if the interface has not been suspended first? Perhaps this alone is
> good reason not to flush.
>
> Regards,
> Michal

To my understanding if the interface creates another URB, then the
interface might be able to submit it. Is there a reason to not
flushing all endpoints on a USB device when we have offloaded transfer
happening during system suspend? If there is, we'll identify what kinds
of endpoints are influenced and not flush them independently.
Otherwise, not flushing all of them might maintain a simpler code
logic.

Regards,
Guan-Yu