USB: HCD: support giveback of URB in tasklet context

This patch implements the mechanism of giveback of URB in
tasklet context, so that hardware interrupt handling time for
usb host controller can be saved much, and HCD interrupt handling
can be simplified.

Motivations:

1), on some arch(such as ARM), DMA mapping/unmapping is a bit
time-consuming, for example: when accessing usb mass storage
via EHCI on pandaboard, the common length of transfer buffer is 120KB,
the time consumed on DMA unmapping may reach hundreds of microseconds;
even on A15 based box, the time is still about scores of microseconds

2), on some arch, reading DMA coherent memoery is very time-consuming,
the most common example is usb video class driver[1]

3), driver's complete() callback may do much things which is driver
specific, so the time is consumed unnecessarily in hardware irq context.

4), running driver's complete() callback in hardware irq context causes
that host controller driver has to release its lock in interrupt handler,
so reacquiring the lock after return may busy wait a while and increase
interrupt handling time. More seriously, releasing the HCD lock makes
HCD becoming quite complicated to deal with introduced races.

So the patch proposes to run giveback of URB in tasklet context, then
time consumed in HCD irq handling doesn't depend on drivers' complete and
DMA mapping/unmapping any more, also we can simplify HCD since the HCD
lock isn't needed to be released during irq handling.

The patch should be reasonable and doable:

1), for drivers, they don't care if the complete() is called in hard irq
context or softirq context

2), the biggest change is the situation in which usb_submit_urb() is called
in complete() callback, so the introduced tasklet schedule delay might be a
con, but it shouldn't be a big deal:

	- control/bulk asynchronous transfer isn't sensitive to schedule
	  delay

	- the patch schedules giveback of periodic URBs using
	  tasklet_hi_schedule, so the introduced delay should be very
	  small

	- for ISOC transfer, generally, drivers submit several URBs
	  concurrently to avoid interrupt delay, so it is OK with the
	  little schedule delay.

	- for interrupt transfer, generally, drivers only submit one URB
	  at the same time, but interrupt transfer is often used in event
	  report, polling, ... situations, and a little delay should be OK.

Considered that HCDs may optimize on submitting URB in complete(), the
patch may cause the optimization not working, so introduces one flag to mark
if the HCD supports to run giveback URB in tasklet context. When all HCDs
are ready, the flag can be removed.

[1], http://marc.info/?t=136438111600010&r=1&w=2

Cc: Oliver Neukum <oliver@neukum.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/include/linux/usb/hcd.h b/include/linux/usb/hcd.h
index 1e88377e22f4..a9c7d44bd2ed 100644
--- a/include/linux/usb/hcd.h
+++ b/include/linux/usb/hcd.h
@@ -22,6 +22,7 @@
 #ifdef __KERNEL__
 
 #include <linux/rwsem.h>
+#include <linux/interrupt.h>
 
 #define MAX_TOPO_LEVEL		6
 
@@ -67,6 +68,13 @@
 
 /*-------------------------------------------------------------------------*/
 
+struct giveback_urb_bh {
+	bool running;
+	spinlock_t lock;
+	struct list_head  head;
+	struct tasklet_struct bh;
+};
+
 struct usb_hcd {
 
 	/*
@@ -139,6 +147,9 @@ struct usb_hcd {
 	resource_size_t		rsrc_len;	/* memory/io resource length */
 	unsigned		power_budget;	/* in mA, 0 = no limit */
 
+	struct giveback_urb_bh  high_prio_bh;
+	struct giveback_urb_bh  low_prio_bh;
+
 	/* bandwidth_mutex should be taken before adding or removing
 	 * any new bus bandwidth constraints:
 	 *   1. Before adding a configuration for a new device.
@@ -221,6 +232,7 @@ struct hc_driver {
 #define	HCD_USB25	0x0030		/* Wireless USB 1.0 (USB 2.5)*/
 #define	HCD_USB3	0x0040		/* USB 3.0 */
 #define	HCD_MASK	0x0070
+#define	HCD_BH		0x0100		/* URB complete in BH context */
 
 	/* called to init HCD and root hub */
 	int	(*reset) (struct usb_hcd *hcd);
@@ -361,6 +373,11 @@ struct hc_driver {
 	int	(*find_raw_port_number)(struct usb_hcd *, int);
 };
 
+static inline int hcd_giveback_urb_in_bh(struct usb_hcd *hcd)
+{
+	return hcd->driver->flags & HCD_BH;
+}
+
 extern int usb_hcd_link_urb_to_ep(struct usb_hcd *hcd, struct urb *urb);
 extern int usb_hcd_check_unlink_urb(struct usb_hcd *hcd, struct urb *urb,
 		int status);