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SubjectRe: [PATCH v3 2/2] USB: misc: Add onboard_usb_hub driver
Hi Alan,

On Thu, Sep 24, 2020 at 11:27:58AM -0400, Alan Stern wrote:
> 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.

ok, I'll add that

> > +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.

IIUC the mutex_lock() in onboard_hub_remove() is an implicit barrier, but
it is indeed not obvious from looking at the code.

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

I wasn't super happy about this either ...

> > +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);
>

Thanks, that should work. I also thought about unlocking the mutex before
calling device_release_driver(), but that wouldn't be the right thing when
using list_for_each_entry(_safe). The alternative loop style allows for it.

> > +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.

ugh, yes

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