[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>

1 files changed, 5 insertions, 1 deletions

diff --git a/include/linux/usb.h b/include/linux/usb.h
index 41d1a644c9d4..2d1ac5058534 100644
--- a/include/linux/usb.h
+++ b/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 */