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SubjectRe: [PATCH v3 2/2] USB: misc: Add onboard_usb_hub driver
On Wed, Sep 23, 2020 at 06:10:12PM -0700, Matthias Kaehlcke wrote:
> The main issue this driver addresses is that a USB hub needs to be
> powered before it can be discovered. For discrete onboard hubs (an
> example for such a hub is the Realtek RTS5411) this is often solved
> by supplying the hub with an 'always-on' regulator, which is kind
> of a hack. Some onboard hubs may require further initialization
> steps, like changing the state of a GPIO or enabling a clock, which
> requires even more hacks. This driver creates a platform device
> representing the hub which performs the necessary initialization.
> Currently it only supports switching on a single regulator, support
> for multiple regulators or other actions can be added as needed.
> Different initialization sequences can be supported based on the
> compatible string.
>
> Besides performing the initialization the driver can be configured
> to power the hub off during system suspend. This can help to extend
> battery life on battery powered devices which have no requirements
> to keep the hub powered during suspend. The driver can also be
> configured to leave the hub powered when a wakeup capable USB device
> is connected when suspending, and power it off otherwise.
>
> Technically the driver consists of two drivers, the platform driver
> described above and a very thin USB driver that subclasses the
> generic driver. The purpose of this driver is to provide the platform
> driver with the USB devices corresponding to the hub(s) (a hub
> controller may provide multiple 'logical' hubs, e.g. one to support
> USB 2.0 and another for USB 3.x).
>
> Co-developed-by: Ravi Chandra Sadineni <ravisadineni@chromium.org>
> Signed-off-by: Ravi Chandra Sadineni <ravisadineni@chromium.org>
> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> ---

> --- a/drivers/usb/misc/Kconfig
> +++ b/drivers/usb/misc/Kconfig
> @@ -275,3 +275,19 @@ config USB_CHAOSKEY
>
> To compile this driver as a module, choose M here: the
> module will be called chaoskey.
> +
> +config USB_ONBOARD_HUB
> + tristate "Onboard USB hub support"
> + depends on OF || COMPILE_TEST
> + help
> + Say Y here if you want to support discrete onboard USB hubs that
> + don't require an additional control bus for initialization (an

... but does require nontrivial form of initialization, such as
enabling a power regulator.


> +static void onboard_hub_remove_usbdev(struct onboard_hub *hub, struct usb_device *udev)
> +{
> + struct udev_node *node;
> +
> + smp_rmb();
> + if (hub->going_away) {
> + /*
> + * We are most likely being called as a result of unbinding a USB device from
> + * onboard_hub_remove(). This function also holds the lock and iterates over
> + * 'udev_list'. Skip deleting the node in this case to avoid a self deadlock,
> + * keeping the node in the list isn't a problem, since the device is about to go
> + * away.
> + */
> + return;
> + }

This part has a suspicious look. For one thing, there's no comment
explaining the purpose of the smp_rmb(). For another, that barrier
doesn't seem to pair with any other memory barrier in the driver.

I get that you want to avoid self-deadlock here. But there must be a
better way. See below.

> +static int onboard_hub_remove(struct platform_device *pdev)
> +{
> + struct onboard_hub *hub = dev_get_drvdata(&pdev->dev);
> + struct udev_node *node;
> +
> + hub->going_away = true;
> +
> + mutex_lock(&hub->lock);
> +
> + /* unbind the USB devices to avoid dangling references to this device */
> + list_for_each_entry(node, &hub->udev_list, list)
> + device_release_driver(&node->udev->dev);
> +
> + mutex_unlock(&hub->lock);

Alternative approach:

/* unbind the USB devices to avoid dangling references to this device */
mutex_lock(&hub->lock);
while (!list_empty(&hub->udev_list)) {
node = list_first_entry(&hub->udev_list, struct udev_node, list);
udev = node->udev;

/*
* Unbinding the driver will call onboard_hub_remove_usbdev(),
* which acquires hub->lock. We must release the lock first.
*/
usb_get_device(udev);
mutex_unlock(&hub->lock);
device_release_driver(&udev->dev);
usb_put_device(udev);
mutex_lock(&hub->lock);
}
mutex_unlock(&hub->lock);

> +static int onboard_hub_usbdev_probe(struct usb_device *udev)
> +{
> + struct device *dev = &udev->dev;
> + struct onboard_hub *hub;
> +
> + /* ignore supported hubs without device tree node */
> + if (!dev->of_node)
> + return -ENODEV;
> +
> + hub = _find_onboard_hub(dev);
> + if (IS_ERR(hub))
> + return PTR_ERR(dev);

hub, not dev.

Alan Stern

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