linux-kernel - Re: [PATCH v7 3/5] usb: add apis for offload usage tracking

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Message-ID: <69a4ae28-dbdc-48eb-b77b-147f58739913@rowland.harvard.edu>
Date: Thu, 9 Jan 2025 10:04:34 -0500
From: Alan Stern <stern@...land.harvard.edu>
To: Guan-Yu Lin <guanyulin@...gle.com>
Cc: gregkh@...uxfoundation.org, Thinh.Nguyen@...opsys.com,
	mathias.nyman@...el.com, perex@...ex.cz, tiwai@...e.com,
	sumit.garg@...aro.org, kekrby@...il.com, oneukum@...e.com,
	ricardo@...liere.net, lijiayi@...inos.cn, quic_jjohnson@...cinc.com,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-sound@...r.kernel.org
Subject: Re: [PATCH v7 3/5] usb: add apis for offload usage tracking

On Thu, Jan 09, 2025 at 03:55:07AM +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>
> ---
>  drivers/usb/core/driver.c | 108 ++++++++++++++++++++++++++++++++++++++
>  drivers/usb/core/usb.c    |   4 ++
>  include/linux/usb.h       |  19 +++++++
>  3 files changed, 131 insertions(+)
> 
> diff --git a/drivers/usb/core/driver.c b/drivers/usb/core/driver.c
> index f203fdbfb6f6..01f6287cf73f 100644
> --- a/drivers/usb/core/driver.c
> +++ b/drivers/usb/core/driver.c
> @@ -2037,6 +2037,114 @@ int usb_disable_usb2_hardware_lpm(struct usb_device *udev)
>  
>  #endif /* CONFIG_PM */
>  
> +#ifdef CONFIG_USB_XHCI_SIDEBAND
> +
> +/**
> + * 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_get_sync(&udev->dev);

Note that this function will increment the runtime PM reference count 
even if it returns an error.  You probably want to call 
pm_runtime_resume_and_get() instead.  Likewise in usb_offload_put().

> +
> +	if (ret < 0)
> +		return ret;
> +
> +	refcount_inc(&udev->offload_usage);
> +	pm_runtime_put_sync(&udev->dev);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(usb_offload_get);

Alan Stern