linux-kernel - Re: [RFC PATCH] usb: core: correct usb_get

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Date:   Fri, 28 Oct 2016 13:05:46 +0800
From:   Peter Chen <hzpeterchen@...il.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Brian Norris <computersforpeace@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] usb: core: correct usb_get_dev() documentation

On Thu, Oct 27, 2016 at 04:49:18PM -0700, Dmitry Torokhov wrote:
> On Thu, Oct 27, 2016 at 03:02:30PM -0700, Brian Norris wrote:
> > In reading through a USB interface driver, I noticed that it called
> > usb_{get,put}_dev() in its probe() and disconnect() methods. This seemed
> > unnecessary, but a look at the comments here matched the usage.
> > 
> > USB interface devices seem to be well covered by the parent/child
> > relationship of the device model, and so it should be unnecessary for a
> > child device to grab a refcount on its parent device.
> > 
> > Signed-off-by: Brian Norris <computersforpeace@...il.com>
> 
> Yes, usb_device is parent of usb_interface and device core does "parent
> = get_device(dev->parent);" as part of device_add() when registering new
> interfaces.
> 
> Reviewed-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> 

Yes, current code seems a little messy for get{put}_device.
Eg, for USB device, it tries to call get_device again at usb_set_configuration
when create its child device (interface device).
For USB interface device, it handles get{put}_device at message.c for
common interface, it seems to be not necessary to call
usb_get{put}_dev again at individual interface driver.

Peter
> > ---
> > This reflects my understanding (and testing), as well as the majority of usage
> > -- there are *very* few interface drivers that actually call usb_get_dev(). If
> > I'm wrong, please feel free to tell me so! But I thought patching the
> > documentation would be the best way to solicit a response :)
> > 
> >  drivers/usb/core/usb.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
> > index 592151461017..0ba7e070f04e 100644
> > --- a/drivers/usb/core/usb.c
> > +++ b/drivers/usb/core/usb.c
> > @@ -539,9 +539,9 @@ EXPORT_SYMBOL_GPL(usb_alloc_dev);
> >   *
> >   * Each live reference to a device should be refcounted.
> >   *
> > - * Drivers for USB interfaces should normally record such references in
> > - * their probe() methods, when they bind to an interface, and release
> > - * them by calling usb_put_dev(), in their disconnect() methods.
> > + * The device driver core automatically handles this refcounting for USB
> > + * interface drivers, but this API can be used for non-parent/child
> > + * relationships.
> >   *
> >   * Return: A pointer to the device with the incremented reference counter.
> >   */
> > -- 
> > 2.8.0.rc3.226.g39d4020
> > 
> 
> -- 
> Dmitry
> --
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-- 

Best Regards,
Peter Chen