[PATCH] usbcore, remove urb->next

Given the discussions of last week, this removes urb->next from
the USB core API.  This change simplifies the driver API by getting
rid of a superfluous feature (and related new-developer confusion),
gets rid of a hidden failure mode (drivers can now see resubmit
failures), lets us get rid of a HCD feature that isn't consistently
implemented, and so on.

This will break some code.  There are ISO drivers that don't use
urb->next (like audio), but most video drivers do.  My patch #2
fixes one such driver.  My patch #3 fixes host controller drivers,
most of which were already converted.

drivers/usb/core/devio.c

@@ -892,7 +892,6 @@ static int proc_submiturb(struct dev_state *ps, void *arg)
 		free_async(as);
 		return -ENOMEM;
 	}
-        as->urb->next = NULL;
         as->urb->dev = ps->dev;
         as->urb->pipe = (uurb.type << 30) | __create_pipe(ps->dev, uurb.endpoint & 0xf) | (uurb.endpoint & USB_DIR_IN);
         as->urb->transfer_flags = uurb.flags;

drivers/usb/core/hcd.c

@@ -1407,11 +1407,6 @@ static int hcd_submit_urb (struct urb *urb, int mem_flags)
 	if (urb->transfer_buffer_length < 0)
 		return -EINVAL;
 
-	if (urb->next) {
-		warn ("use explicit queuing not urb->next");
-		return -EINVAL;
-	}
-
 #ifdef DEBUG
 	/* stuff that drivers shouldn't do, but which shouldn't
 	 * cause problems in HCDs if they get it wrong.
@@ -1785,10 +1780,6 @@ static void hcd_irq (int irq, void *__hcd, struct pt_regs * r)
  * HCDs must not use this for periodic URBs that are still scheduled
  * and will be reissued.  They should just call their completion handlers
  * until the urb is returned to the device driver by unlinking.
- *
- * NOTE that no urb->next processing is done, even for isochronous URBs.
- * ISO streaming functionality can be achieved by having completion handlers
- * re-queue URBs.  Such explicit queuing doesn't discard error reports.
  */
 void usb_hcd_giveback_urb (struct usb_hcd *hcd, struct urb *urb)
 {

drivers/usb/core/usb-debug.c

@@ -184,7 +184,6 @@ void usb_show_string(struct usb_device *dev, char *id, int index)
 void usb_dump_urb (struct urb *urb)
 {
 	printk ("urb                   :%p\n", urb);
-	printk ("next                  :%p\n", urb->next);
 	printk ("dev                   :%p\n", urb->dev);
 	printk ("pipe                  :%08X\n", urb->pipe);
 	printk ("status                :%d\n", urb->status);

include/linux/usb.h

@@ -767,7 +767,6 @@ typedef void (*usb_complete_t)(struct urb *);
 /**
  * struct urb - USB Request Block
  * @urb_list: For use by current owner of the URB.
- * @next: Used to link ISO requests into rings.
  * @pipe: Holds endpoint number, direction, type, and max packet size.
  *	Create these values with the eight macros available;
  *	usb_{snd,rcv}TYPEpipe(dev,endpoint), where the type is "ctrl"
@@ -827,7 +826,7 @@ typedef void (*usb_complete_t)(struct urb *);
  *
  * Initialization:
  *
- * All URBs submitted must initialize dev, pipe, next (may be null),
+ * All URBs submitted must initialize dev, pipe,
  * transfer_flags (may be zero), complete, timeout (may be zero).
  * The USB_ASYNC_UNLINK transfer flag affects later invocations of
  * the usb_unlink_urb() routine.
@@ -873,7 +872,9 @@ typedef void (*usb_complete_t)(struct urb *);
  * the quality of service is only "best effort".  Callers provide specially
  * allocated URBs, with number_of_packets worth of iso_frame_desc structures
  * at the end.  Each such packet is an individual ISO transfer.  Isochronous
- * URBs are normally submitted with urb->next fields set up as a ring, so
+ * URBs are normally queued (no flag like USB_BULK_QUEUE is needed) so that
+ * transfers are at least double buffered, and then explicitly resubmitted
+ * in completion handlers, so
  * that data (such as audio or video) streams at as constant a rate as the
  * host controller scheduler can support.
  *
@@ -891,14 +892,15 @@ typedef void (*usb_complete_t)(struct urb *);
  * When completion callback is invoked for non-isochronous URBs, the
  * actual_length field tells how many bytes were transferred.
  *
- * For interrupt and isochronous URBs, the URB provided to the callback
+ * For interrupt URBs, the URB provided to the callback
  * function is still "owned" by the USB core subsystem unless the status
  * indicates that the URB has been unlinked.  Completion handlers should
  * not modify such URBs until they have been unlinked.
  *
  * ISO transfer status is reported in the status and actual_length fields
  * of the iso_frame_desc array, and the number of errors is reported in
- * error_count.
+ * error_count.  Completion callbacks for ISO transfers will normally
+ * (re)submit URBs to ensure a constant transfer rate.
  */
 struct urb
 {
@@ -906,7 +908,6 @@ struct urb
 	atomic_t count;			/* reference count of the URB */
 	void *hcpriv;			/* private data for host controller */
 	struct list_head urb_list;	/* list pointer to all active urbs */
-	struct urb *next; 		/* (in) pointer to next URB */
 	struct usb_device *dev; 	/* (in) pointer to associated device */
 	unsigned int pipe;		/* (in) pipe information */
 	int status;			/* (return) non-ISO status */