Re: [PATCH v4 3/5] usb: add apis for sideband uasge tracking

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Oct 09, 2024 at 05:42:57AM +0000, Guan-Yu Lin wrote:
> Introduce sb_usage_count and corresponding apis to track sideband usage
> on each usb_device. A sideband refers to the co-processor that accesses
> the usb_device via shared control on the same USB host controller. To
> optimize power usage, it's essential to monitor whether ther sideband is
> actively using the usb_device. 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@google.com>
> ---
>  drivers/usb/core/driver.c | 54 +++++++++++++++++++++++++++++++++++++++
>  include/linux/usb.h       | 13 ++++++++++
>  2 files changed, 67 insertions(+)
> 
> diff --git a/drivers/usb/core/driver.c b/drivers/usb/core/driver.c
> index 0c3f12daac79..c1ba5ed15214 100644
> --- a/drivers/usb/core/driver.c
> +++ b/drivers/usb/core/driver.c
> @@ -1673,6 +1673,60 @@ void usb_disable_autosuspend(struct usb_device *udev)
>  }
>  EXPORT_SYMBOL_GPL(usb_disable_autosuspend);
>  
> +/**
> + * usb_sideband_get - notify usb driver there's a new active sideband
> + * @udev: the usb_device which has an active sideband
> + *
> + * An active sideband indicates that another entity is currently using the usb
> + * device. Notify the usb device by increasing the sb_usage_count. This allows
> + * usb driver to adjust power management policy based on sideband activities.
> + */
> +void usb_sideband_get(struct usb_device *udev)
> +{
> +	struct usb_device *parent = udev;
> +
> +	do {
> +		atomic_inc(&parent->sb_usage_count);

As this is a reference count, use refcount_t please.

> +		parent = parent->parent;
> +	} while (parent);

Woah, walking up the device chain?  That should not be needed, or if so,
then each device's "usage count" is pointless.

> +}
> +EXPORT_SYMBOL_GPL(usb_sideband_get);
> +
> +/**
> + * usb_sideband_put - notify usb driver there's a sideband deactivated
> + * @udev: the usb_device which has a sideband deactivated
> + *
> + * The inverse operation of usb_sideband_get, which notifies the usb device by
> + * decreasing the sb_usage_count. This allows usb driver to adjust power
> + * management policy based on sideband activities.
> + */
> +void usb_sideband_put(struct usb_device *udev)
> +{
> +	struct usb_device *parent = udev;
> +
> +	do {
> +		atomic_dec(&parent->sb_usage_count);
> +		parent = parent->parent;
> +	} while (parent);
> +}
> +EXPORT_SYMBOL_GPL(usb_sideband_put);
> +
> +/**
> + * usb_sideband_check - check sideband activities on the host controller
> + * @udev: the usb_device which has a sideband deactivated
> + *
> + * Check if there are any sideband activity on the USB device right now. This
> + * information could be used for power management or other forms or resource
> + * management.
> + *
> + * Returns true on any active sideband existence, false otherwise
> + */
> +bool usb_sideband_check(struct usb_device *udev)
> +{
> +	return !!atomic_read(&udev->sb_usage_count);

And what happens if it changes right after you make this call?  This
feels racy and broken.

> +}
> +EXPORT_SYMBOL_GPL(usb_sideband_check);
> +
>  /**
>   * usb_autosuspend_device - delayed autosuspend of a USB device and its interfaces
>   * @udev: the usb_device to autosuspend
> diff --git a/include/linux/usb.h b/include/linux/usb.h
> index 672d8fc2abdb..5b9fea378960 100644
> --- a/include/linux/usb.h
> +++ b/include/linux/usb.h
> @@ -645,6 +645,7 @@ struct usb3_lpm_parameters {
>   *	parent->hub_delay + wHubDelay + tTPTransmissionDelay (40ns)
>   *	Will be used as wValue for SetIsochDelay requests.
>   * @use_generic_driver: ask driver core to reprobe using the generic driver.
> + * @sb_usage_count: number of active sideband accessing this usb device.
>   *
>   * Notes:
>   * Usbcore drivers should not set usbdev->state directly.  Instead use
> @@ -731,6 +732,8 @@ struct usb_device {
>  
>  	u16 hub_delay;
>  	unsigned use_generic_driver:1;
> +
> +	atomic_t sb_usage_count;

Why is this on the device and not the interface that is bound to the
driver that is doing this work?

thanks,

greg k-h