[PATCH] USB: update urb documentation

On Wed, May 04, 2005 at 01:37:30PM -0700, David Brownell wrote:
> On Wednesday 04 May 2005 12:19 pm, Roman Kagan wrote:
> > struct urb {
> > 	/* private, usb core and host controller only fields in the urb */
> > 	...
> > 	struct list_head urb_list;	/* list pointer to all active urbs */
> > 	...
> > };
> >
> > Is it safe to use it for driver's purposes when the driver owns the urb,
> > that is, starting from the completion routine until the urb is submitted
> > with usb_submit_urb()?
>
> Right now, it should be.

Great!  FWIW I've briefly tested a modified version of usbatm using
the list head in struct urb instead of creating a wrapper struct, and I
haven't seen any failures yet.  So I tend to believe that your "should
be" actually means "is" :)

> > If it is, can it be guaranteed in future, e.g.
> > by moving the list head into the public section of struct urb?
>
> In fact I'm not sure why it ever got called "private" to usbcore/hcds.
> I thought the idea was that it should be like urb->status, reserved for
> whoever controls the URB.

OK then how about the following (essentially documentation) patch?
Signed-off-by: Roman Kagan <rkagan@mail.ru>
Acked-by: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/usb.h

@@ -796,6 +796,10 @@ typedef void (*usb_complete_t)(struct urb *, struct pt_regs *);
  * of the iso_frame_desc array, and the number of errors is reported in
  * error_count.  Completion callbacks for ISO transfers will normally
  * (re)submit URBs to ensure a constant transfer rate.
+ *
+ * Note that even fields marked "public" should not be touched by the driver
+ * when the urb is owned by the hcd, that is, since the call to
+ * usb_submit_urb() till the entry into the completion routine.
  */
 struct urb
 {
@@ -803,12 +807,12 @@ struct urb
 	struct kref kref;		/* reference count of the URB */
 	spinlock_t lock;		/* lock for the URB */
 	void *hcpriv;			/* private data for host controller */
-	struct list_head urb_list;	/* list pointer to all active urbs */
 	int bandwidth;			/* bandwidth for INT/ISO request */
 	atomic_t use_count;		/* concurrent submissions counter */
 	u8 reject;			/* submissions will fail */
 
 	/* public, documented fields in the urb that can be used by drivers */
+	struct list_head urb_list;	/* list head for use by the urb owner */
 	struct usb_device *dev; 	/* (in) pointer to associated device */
 	unsigned int pipe;		/* (in) pipe information */
 	int status;			/* (return) non-ISO status */