[PATCH 0/2] Multi-segment Event Ring support for XHCI

[PATCH 0/2] Multi-segment Event Ring support for XHCI

[Lukas Wunner]

Enlarge the XHCI Event Ring to cope with high load situations
by allowing more than one segment (patch [2/2]).  The patch is
lifted from the Raspberry Pi kernel, it has been in use there
for 4 years.  I've taken the liberty to slightly edit the commit
message and the patch itself for upstream.

As a prerequisite for the patch, ensure that the DESI bits in the
ERDP register are set correctly (patch [1/2]).  Incorrect DESI bits
cause an interrupt storm on Renesas uPD720201 host controllers.

Jonathan Bell (1):
  xhci: Use more than one Event Ring segment

Lukas Wunner (1):
  xhci: Set DESI bits in ERDP register correctly

 drivers/usb/host/xhci-mem.c  | 35 ++++++++++++++++++-----------------
 drivers/usb/host/xhci-ring.c |  2 +-
 drivers/usb/host/xhci.h      |  6 ++++--
 3 files changed, 23 insertions(+), 20 deletions(-)

-- 
2.39.2

[PATCH 1/2] xhci: Set DESI bits in ERDP register correctly

[Lukas Wunner]

When using more than one Event Ring segment (ERSTSZ > 1), software shall
set the DESI bits in the ERDP register to the number of the segment to
which the upper ERDP bits are pointing.  The xHC may use the DESI bits
as a shortcut to determine whether it needs to check for an Event Ring
Full condition:  If it's enqueueing events in a different segment, it
need not compare its internal Enqueue Pointer with the Dequeue Pointer
in the upper bits of the ERDP register (sec 5.5.2.3.3).

Not setting the DESI bits correctly can result in the xHC enqueueing
events past the Dequeue Pointer.  On Renesas uPD720201 host controllers,
incorrect DESI bits cause an interrupt storm.  For comparison, VIA VL805
host controllers do not exhibit such problems.  Perhaps they do not take
advantage of the optimization afforded by the DESI bits.

To fix the issue, assign the segment number to each struct xhci_segment
in xhci_segment_alloc().  When advancing the Dequeue Pointer in
xhci_update_erst_dequeue(), write the segment number to the DESI bits.

On driver probe, set the DESI bits to zero in xhci_set_hc_event_deq() as
processing starts in segment 0.  Likewise on driver teardown, clear the
DESI bits to zero in xhci_free_interrupter() when clearing the upper
bits of the ERDP register.  Previously those functions (incorrectly)
treated the DESI bits as if they're declared RsvdP.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
---
 drivers/usb/host/xhci-mem.c  | 25 +++++++++++--------------
 drivers/usb/host/xhci-ring.c |  2 +-
 drivers/usb/host/xhci.h      |  1 +
 3 files changed, 13 insertions(+), 15 deletions(-)

diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
index 19a4021..c265425 100644
--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -29,6 +29,7 @@
 static struct xhci_segment *xhci_segment_alloc(struct xhci_hcd *xhci,
 					       unsigned int cycle_state,
 					       unsigned int max_packet,
+					       unsigned int num,
 					       gfp_t flags)
 {
 	struct xhci_segment *seg;
@@ -60,6 +61,7 @@ static struct xhci_segment *xhci_segment_alloc(struct xhci_hcd *xhci,
 		for (i = 0; i < TRBS_PER_SEGMENT; i++)
 			seg->trbs[i].link.control = cpu_to_le32(TRB_CYCLE);
 	}
+	seg->num = num;
 	seg->dma = dma;
 	seg->next = NULL;
 
@@ -325,22 +327,24 @@ static int xhci_alloc_segments_for_ring(struct xhci_hcd *xhci,
 {
 	struct xhci_segment *prev;
 	bool chain_links;
+	int num = 0;
 
 	/* Set chain bit for 0.95 hosts, and for isoc rings on AMD 0.96 host */
 	chain_links = !!(xhci_link_trb_quirk(xhci) ||
 			 (type == TYPE_ISOC &&
 			  (xhci->quirks & XHCI_AMD_0x96_HOST)));
 
-	prev = xhci_segment_alloc(xhci, cycle_state, max_packet, flags);
+	prev = xhci_segment_alloc(xhci, cycle_state, max_packet, num, flags);
 	if (!prev)
 		return -ENOMEM;
-	num_segs--;
+	num++;
 
 	*first = prev;
-	while (num_segs > 0) {
+	while (num < num_segs) {
 		struct xhci_segment	*next;
 
-		next = xhci_segment_alloc(xhci, cycle_state, max_packet, flags);
+		next = xhci_segment_alloc(xhci, cycle_state, max_packet, num,
+					  flags);
 		if (!next) {
 			prev = *first;
 			while (prev) {
@@ -353,7 +357,7 @@ static int xhci_alloc_segments_for_ring(struct xhci_hcd *xhci,
 		xhci_link_segments(prev, next, type, chain_links);
 
 		prev = next;
-		num_segs--;
+		num++;
 	}
 	xhci_link_segments(prev, *first, type, chain_links);
 	*last = prev;
@@ -1804,7 +1808,6 @@ int xhci_alloc_erst(struct xhci_hcd *xhci,
 {
 	struct device *dev = xhci_to_hcd(xhci)->self.sysdev;
 	size_t erst_size;
-	u64 tmp64;
 	u32 tmp;
 
 	if (!ir)
@@ -1827,9 +1830,7 @@ int xhci_alloc_erst(struct xhci_hcd *xhci,
 		tmp &= ERST_SIZE_MASK;
 		writel(tmp, &ir->ir_set->erst_size);
 
-		tmp64 = xhci_read_64(xhci, &ir->ir_set->erst_dequeue);
-		tmp64 &= (u64) ERST_PTR_MASK;
-		xhci_write_64(xhci, tmp64, &ir->ir_set->erst_dequeue);
+		xhci_write_64(xhci, ERST_EHB, &ir->ir_set->erst_dequeue);
 	}
 
 	/* free interrrupter event ring */
@@ -1936,7 +1937,6 @@ void xhci_mem_cleanup(struct xhci_hcd *xhci)
 
 static void xhci_set_hc_event_deq(struct xhci_hcd *xhci, struct xhci_interrupter *ir)
 {
-	u64 temp;
 	dma_addr_t deq;
 
 	deq = xhci_trb_virt_to_dma(ir->event_ring->deq_seg,
@@ -1944,15 +1944,12 @@ static void xhci_set_hc_event_deq(struct xhci_hcd *xhci, struct xhci_interrupter
 	if (!deq)
 		xhci_warn(xhci, "WARN something wrong with SW event ring dequeue ptr.\n");
 	/* Update HC event ring dequeue pointer */
-	temp = xhci_read_64(xhci, &ir->ir_set->erst_dequeue);
-	temp &= ERST_PTR_MASK;
 	/* Don't clear the EHB bit (which is RW1C) because
 	 * there might be more events to service.
 	 */
-	temp &= ~ERST_EHB;
 	xhci_dbg_trace(xhci, trace_xhci_dbg_init,
 		       "// Write event ring dequeue pointer, preserving EHB bit");
-	xhci_write_64(xhci, ((u64) deq & (u64) ~ERST_PTR_MASK) | temp,
+	xhci_write_64(xhci, ((u64) deq & (u64) ~ERST_PTR_MASK),
 			&ir->ir_set->erst_dequeue);
 }
 
diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index bc6280b..ca8b848 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -3018,7 +3018,7 @@ static void xhci_update_erst_dequeue(struct xhci_hcd *xhci,
 			return;
 
 		/* Update HC event ring dequeue pointer */
-		temp_64 &= ERST_PTR_MASK;
+		temp_64 = ir->event_ring->deq_seg->num & ERST_DESI_MASK;
 		temp_64 |= ((u64) deq & (u64) ~ERST_PTR_MASK);
 	}
 
diff --git a/drivers/usb/host/xhci.h b/drivers/usb/host/xhci.h
index 7e282b4..45c9177 100644
--- a/drivers/usb/host/xhci.h
+++ b/drivers/usb/host/xhci.h
@@ -1545,6 +1545,7 @@ struct xhci_segment {
 	union xhci_trb		*trbs;
 	/* private to HCD */
 	struct xhci_segment	*next;
+	unsigned int		num;
 	dma_addr_t		dma;
 	/* Max packet sized bounce buffer for td-fragmant alignment */
 	dma_addr_t		bounce_dma;
-- 
2.39.2

Re: [PATCH 1/2] xhci: Set DESI bits in ERDP register correctly

[Mathias Nyman]

On 15.8.2023 15.40, Lukas Wunner wrote:
> When using more than one Event Ring segment (ERSTSZ > 1), software shall
> set the DESI bits in the ERDP register to the number of the segment to
> which the upper ERDP bits are pointing.  The xHC may use the DESI bits
> as a shortcut to determine whether it needs to check for an Event Ring
> Full condition:  If it's enqueueing events in a different segment, it
> need not compare its internal Enqueue Pointer with the Dequeue Pointer
> in the upper bits of the ERDP register (sec 5.5.2.3.3).
> 
> Not setting the DESI bits correctly can result in the xHC enqueueing
> events past the Dequeue Pointer.  On Renesas uPD720201 host controllers,
> incorrect DESI bits cause an interrupt storm.  For comparison, VIA VL805
> host controllers do not exhibit such problems.  Perhaps they do not take
> advantage of the optimization afforded by the DESI bits.
> 
> To fix the issue, assign the segment number to each struct xhci_segment
> in xhci_segment_alloc().  When advancing the Dequeue Pointer in
> xhci_update_erst_dequeue(), write the segment number to the DESI bits.
> 
> On driver probe, set the DESI bits to zero in xhci_set_hc_event_deq() as
> processing starts in segment 0.  Likewise on driver teardown, clear the
> DESI bits to zero in xhci_free_interrupter() when clearing the upper
> bits of the ERDP register.  Previously those functions (incorrectly)
> treated the DESI bits as if they're declared RsvdP.
> 
> Signed-off-by: Lukas Wunner <lukas@wunner.de>

Thanks for working on this, sorting out the DESI bits

The segment numbers might become useful for transfer rings as well, but with
current transfer ring expansion support those numbers need to be adjusted
after expansion.

But I can make a small separate fix for that on top of this.

-Mathias

[PATCH 2/2] xhci: Use more than one Event Ring segment

[Lukas Wunner]

From: Jonathan Bell <jonathan@raspberrypi.com>

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.

Commit dc0ffbea5729 ("usb: host: xhci: update event ring dequeue pointer
on purpose") mitigated the issue by advancing the Event Ring Dequeue
Pointer already after half a segment has been processed.  Nevertheless,
providing a larger Event Ring would be useful to cope with load peaks.

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, but with up to 32k it can be excessively high
(sec 5.3.4).  So cap the actual number at 8 (configurable through the
ERST_MAX_SEGS macro), which seems like a reasonable quantity.

An alternative to increasing the number of Event Ring segments would be
an increase of the segment size.  But that requires allocating multiple
contiguous pages, which may be impossible if memory is fragmented.

Link: https://forums.raspberrypi.com/viewtopic.php?t=246263
Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
---
 drivers/usb/host/xhci-mem.c | 10 +++++++---
 drivers/usb/host/xhci.h     |  5 +++--
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
index c265425..cb50bf8 100644
--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -2238,14 +2238,18 @@ static int xhci_setup_port_arrays(struct xhci_hcd *xhci, gfp_t flags)
 {
 	struct device *dev = xhci_to_hcd(xhci)->self.sysdev;
 	struct xhci_interrupter *ir;
+	unsigned int num_segs;
 	int ret;
 
 	ir = kzalloc_node(sizeof(*ir), flags, dev_to_node(dev));
 	if (!ir)
 		return NULL;
 
-	ir->event_ring = xhci_ring_alloc(xhci, ERST_NUM_SEGS, 1, TYPE_EVENT,
-					0, flags);
+	num_segs = min_t(unsigned int, 1 << HCS_ERST_MAX(xhci->hcs_params2),
+			 ERST_MAX_SEGS);
+
+	ir->event_ring = xhci_ring_alloc(xhci, num_segs, 1, TYPE_EVENT, 0,
+					 flags);
 	if (!ir->event_ring) {
 		xhci_warn(xhci, "Failed to allocate interrupter event ring\n");
 		kfree(ir);
@@ -2281,7 +2285,7 @@ static int xhci_setup_port_arrays(struct xhci_hcd *xhci, gfp_t flags)
 	/* set ERST count with the number of entries in the segment table */
 	erst_size = readl(&ir->ir_set->erst_size);
 	erst_size &= ERST_SIZE_MASK;
-	erst_size |= ERST_NUM_SEGS;
+	erst_size |= ir->event_ring->num_segs;
 	writel(erst_size, &ir->ir_set->erst_size);
 
 	erst_base = xhci_read_64(xhci, &ir->ir_set->erst_base);
diff --git a/drivers/usb/host/xhci.h b/drivers/usb/host/xhci.h
index 45c9177..0948d51 100644
--- a/drivers/usb/host/xhci.h
+++ b/drivers/usb/host/xhci.h
@@ -1674,8 +1674,9 @@ 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
+ *
+ * Reasonable limit for number of Event Ring segments (spec allows 32k) */
+#define	ERST_MAX_SEGS	8
 /* Poll every 60 seconds */
 #define	POLL_TIMEOUT	60
 /* Stop endpoint command timeout (secs) for URB cancellation watchdog timer */
-- 
2.39.2

Re: [PATCH 2/2] xhci: Use more than one Event Ring segment

[Mathias Nyman]

On 15.8.2023 15.40, Lukas Wunner wrote:
> From: Jonathan Bell <jonathan@raspberrypi.com>
> 
> 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.
> 
> Commit dc0ffbea5729 ("usb: host: xhci: update event ring dequeue pointer
> on purpose") mitigated the issue by advancing the Event Ring Dequeue
> Pointer already after half a segment has been processed.  Nevertheless,
> providing a larger Event Ring would be useful to cope with load peaks.
> 
> 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, but with up to 32k it can be excessively high
> (sec 5.3.4).  So cap the actual number at 8 (configurable through the
> ERST_MAX_SEGS macro), which seems like a reasonable quantity.

Making the new event ring default size 8 times bigger seems a bit over the top,
especially when most systems worked fine with just one segment.

How about doubling the current size as a default, and then think about
adding more segments dynamically if we get "Event Ring Full" events?

Thanks
Mathias