[PATCH v13 4/4] usb: host: enable USB offload during system sleep

Guan-Yu Lin
2025-05-28T09:09:27Z

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.
- not suspending the xhci controller. This is done by skipping the
suspend/resume callbacks in the xhci platform driver.

Signed-off-by: Guan-Yu Lin <guanyulin@google.com>
---
drivers/usb/core/driver.c | 50 +++++++++++++++++++++++++++++++-----
drivers/usb/host/xhci-plat.c | 19 ++++++++++++++
drivers/usb/host/xhci-plat.h | 1 +
3 files changed, 64 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/core/driver.c b/drivers/usb/core/driver.c
index 0690619454fe..9670c10f4ade 100644
--- a/drivers/usb/core/driver.c
+++ b/drivers/usb/core/driver.c
@@ -1420,11 +1420,28 @@ static int usb_suspend_both(struct usb_device *udev, pm_message_t msg)
udev->state == USB_STATE_SUSPENDED)
goto done;

+ if (msg.event == PM_EVENT_SUSPEND && usb_offload_check(udev)) {
+ dev_dbg(&udev->dev, "device offloaded, skip suspend.\n");
+ udev->offload_at_suspend = 1;
+ }
+
/* 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];
+ /*
+ * Don't suspend interfaces with remote wakeup while
+ * the controller is active. This preserves pending
+ * interrupt urbs, allowing interrupt events to be
+ * handled during system suspend.
+ */
+ if (udev->offload_at_suspend &&
+ intf->needs_remote_wakeup) {
+ dev_dbg(&intf->dev,
+ "device offloaded, skip suspend.\n");
+ continue;
+ }
status = usb_suspend_interface(udev, intf, msg);

/* Ignore errors during system sleep transitions */
@@ -1435,7 +1452,8 @@ static int usb_suspend_both(struct usb_device *udev, pm_message_t msg)
}
}
if (status == 0) {
- status = usb_suspend_device(udev, msg);
+ if (!udev->offload_at_suspend)
+ status = usb_suspend_device(udev, msg);

/*
* Ignore errors from non-root-hub devices during
@@ -1480,9 +1498,11 @@ 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 (!udev->offload_at_suspend) {
+ for (i = 0; i < 16; ++i) {
+ usb_hcd_flush_endpoint(udev, udev->ep_out[i]);
+ usb_hcd_flush_endpoint(udev, udev->ep_in[i]);
+ }
}
}

@@ -1524,17 +1544,35 @@ static int usb_resume_both(struct usb_device *udev, pm_message_t msg)
udev->can_submit = 1;

/* Resume the device */
- if (udev->state == USB_STATE_SUSPENDED || udev->reset_resume)
- status = usb_resume_device(udev, msg);
+ if (udev->state == USB_STATE_SUSPENDED || udev->reset_resume) {
+ if (!udev->offload_at_suspend)
+ status = usb_resume_device(udev, msg);
+ else
+ dev_dbg(&udev->dev,
+ "device offloaded, skip resume.\n");
+ }

/* Resume the interfaces */
if (status == 0 && udev->actconfig) {
for (i = 0; i < udev->actconfig->desc.bNumInterfaces; i++) {
intf = udev->actconfig->interface[i];
+ /*
+ * Interfaces with remote wakeup aren't suspended
+ * while the controller is active. This preserves
+ * pending interrupt urbs, allowing interrupt events
+ * to be handled during system suspend.
+ */
+ if (udev->offload_at_suspend &&
+ intf->needs_remote_wakeup) {
+ dev_dbg(&intf->dev,
+ "device offloaded, skip resume.\n");
+ continue;
+ }
usb_resume_interface(udev, intf, msg,
udev->reset_resume);
}
}
+ udev->offload_at_suspend = 0;
usb_mark_last_busy(udev);

done:
diff --git a/drivers/usb/host/xhci-plat.c b/drivers/usb/host/xhci-plat.c
index 9843d3ad5cf4..598f1b974347 100644
--- a/drivers/usb/host/xhci-plat.c
+++ b/drivers/usb/host/xhci-plat.c
@@ -20,6 +20,7 @@
#include <linux/acpi.h>
#include <linux/usb/of.h>
#include <linux/reset.h>
+#include <linux/usb/xhci-sideband.h>

#include "xhci.h"
#include "xhci-plat.h"
@@ -483,6 +484,15 @@ static int xhci_plat_suspend_common(struct device *dev)

static int xhci_plat_suspend(struct device *dev)
{
+ struct usb_hcd *hcd = dev_get_drvdata(dev);
+ struct xhci_plat_priv *priv = hcd_to_xhci_priv(hcd);
+
+ if (xhci_sideband_check(hcd)) {
+ priv->sideband_at_suspend = 1;
+ dev_dbg(dev, "sideband instance active, skip suspend.\n");
+ return 0;
+ }
+
return xhci_plat_suspend_common(dev);
}

@@ -535,6 +545,15 @@ static int xhci_plat_resume_common(struct device *dev, bool power_lost)

static int xhci_plat_resume(struct device *dev)
{
+ struct usb_hcd *hcd = dev_get_drvdata(dev);
+ struct xhci_plat_priv *priv = hcd_to_xhci_priv(hcd);
+
+ if (priv->sideband_at_suspend) {
+ priv->sideband_at_suspend = 0;
+ dev_dbg(dev, "sideband instance active, skip resume.\n");
+ return 0;
+ }
+
return xhci_plat_resume_common(dev, false);
}

diff --git a/drivers/usb/host/xhci-plat.h b/drivers/usb/host/xhci-plat.h
index fe4f95e690fa..2b32a93d2b76 100644
--- a/drivers/usb/host/xhci-plat.h
+++ b/drivers/usb/host/xhci-plat.h
@@ -16,6 +16,7 @@ struct xhci_plat_priv {
const char *firmware_name;
unsigned long long quirks;
bool power_lost;
+ unsigned sideband_at_suspend:1;
void (*plat_start)(struct usb_hcd *);
int (*init_quirk)(struct usb_hcd *);
int (*suspend_quirk)(struct usb_hcd *);
--
2.49.0.1164.gab81da1b16-goog