xhci: Use more event ring segment table entries

Users have reported log spam created by "Event Ring Full" xHC event
TRBs. These are caused by interrupt latency in conjunction with a very
busy set of devices on the bus. The errors are benign, but throughput
will suffer as the xHC will pause processing of transfers until the
event ring is drained by the kernel. Expand the number of event TRB slots
available by increasing the number of event ring segments in the ERST.

Controllers have a hardware-defined limit as to the number of ERST
entries they can process, so make the actual number in use
min(ERST_MAX_SEGS, hw_max).

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.org>

drivers/usb/host/xhci-mem.c

@@ -2499,9 +2499,11 @@ int xhci_mem_init(struct xhci_hcd *xhci, gfp_t flags)
 	 * Event ring setup: Allocate a normal ring, but also setup
 	 * the event ring segment table (ERST).  Section 4.9.3.
 	 */
+	val2 = 1 << HCS_ERST_MAX(xhci->hcs_params2);
+	val2 = min_t(unsigned int, ERST_MAX_SEGS, val2);
 	xhci_dbg_trace(xhci, trace_xhci_dbg_init, "// Allocating event ring");
-	xhci->event_ring = xhci_ring_alloc(xhci, ERST_NUM_SEGS, 1, TYPE_EVENT,
-					0, flags);
+	xhci->event_ring = xhci_ring_alloc(xhci, val2, 1, TYPE_EVENT,
+					   0, flags);
 	if (!xhci->event_ring)
 		goto fail;
 	if (xhci_check_trb_in_td_math(xhci) < 0)
@@ -2514,7 +2516,7 @@ int xhci_mem_init(struct xhci_hcd *xhci, gfp_t flags)
 	/* set ERST count with the number of entries in the segment table */
 	val = readl(&xhci->ir_set->erst_size);
 	val &= ERST_SIZE_MASK;
-	val |= ERST_NUM_SEGS;
+	val |= val2;
 	xhci_dbg_trace(xhci, trace_xhci_dbg_init,
 			"// Write ERST size = %i to ir_set 0 (some bits preserved)",
 			val);

drivers/usb/host/xhci.h

@@ -1673,8 +1673,8 @@ struct urb_priv {
  * Each segment table entry is 4*32bits long.  1K seems like an ok size:
  * (1K bytes * 8bytes/bit) / (4*32 bits) = 64 segment entries in the table,
  * meaning 64 ring segments.
- * Initial allocated size of the ERST, in number of entries */
-#define	ERST_NUM_SEGS	1
+ * Maximum number of segments in the ERST */
+#define	ERST_MAX_SEGS	8
 /* Poll every 60 seconds */
 #define	POLL_TIMEOUT	60
 /* Stop endpoint command timeout (secs) for URB cancellation watchdog timer */