linux-kernel - Re: [PATCH v10 2/4] usb: add apis for offload usage tracking

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Message-ID: <fd54ab58-1357-462a-8e3f-a785f7b56813@rowland.harvard.edu>
Date: Tue, 1 Apr 2025 10:24:41 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Guan-Yu Lin <guanyulin@...gle.com>
Cc: gregkh@...uxfoundation.org, mathias.nyman@...el.com,
	sumit.garg@...nel.org, kekrby@...il.com,
	jeff.johnson@....qualcomm.com, elder@...nel.org,
	quic_zijuhu@...cinc.com, ben@...adent.org.uk,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 2/4] usb: add apis for offload usage tracking

On Tue, Apr 01, 2025 at 06:22:40AM +0000, Guan-Yu Lin wrote:
> Introduce offload_usage and corresponding apis to track offload usage
> on each USB device. Offload denotes that there is another co-processor
> accessing the USB device via the same USB host controller. To optimize
> power usage, it's essential to monitor whether the USB device is
> actively used by other co-processor. This information is vital when
> determining if a USB device can be safely suspended during system power
> state transitions.
> 
> Signed-off-by: Guan-Yu Lin <guanyulin@...gle.com>
> ---

> --- a/drivers/usb/core/driver.c
> +++ b/drivers/usb/core/driver.c
> @@ -2036,6 +2036,114 @@ int usb_disable_usb2_hardware_lpm(struct usb_device *udev)
>  
>  #endif /* CONFIG_PM */
>  
> +#if IS_ENABLED(CONFIG_USB_XHCI_SIDEBAND_SUSPEND)
> +
> +/**
> + * usb_offload_get - increment the offload_usage of a USB device
> + * @udev: the USB device to increment its offload_usage
> + *
> + * Incrementing the offload_usage of a usb_device indicates that offload is
> + * enabled on this usb_device; that is, another entity is actively handling USB
> + * transfers. This information allows the USB driver to adjust its power
> + * management policy based on offload activity.
> + *
> + * The caller must hold @udev's device lock.
> + *
> + * Return: 0 on success. A negative error code otherwise.
> + */
> +int usb_offload_get(struct usb_device *udev)
> +{
> +	int ret;
> +
> +	if (udev->state == USB_STATE_NOTATTACHED ||
> +			udev->state == USB_STATE_SUSPENDED)
> +		return -EAGAIN;
> +
> +	/*
> +	 * offload_usage could only be modified when the device is active, since
> +	 * it will alter the suspend flow of the device.
> +	 */
> +	ret = pm_runtime_resume_and_get(&udev->dev);
> +
> +	if (ret < 0)
> +		return ret;
> +
> +	refcount_inc(&udev->offload_usage);
> +	pm_runtime_put_sync(&udev->dev);

The USB subsystem has wrappers: usb_autoresume_device() and 
usb_autosuspend_device().  You should use them instead of calling the 
runtime PM routines directly.

The same advice applies to usb_offload_put().

Alan Stern