[PATCH v2] usb: dwc3: gadget: Prevent irq storm when TH re-executes

[Badhri Jagan Sridharan <badhri@google.com>]

While commit d325a1de49d6 ("usb: dwc3: gadget: Prevent losing events in
event cache") makes sure that top half(TH) does not end up overwriting the
cached events before processing them when the TH gets invoked more than one
time, returning IRQ_HANDLED results in occasional irq storm where the TH
hogs the CPU. The irq storm can be prevented by clearing the flag before
event handler busy is cleared. Default enable interrupt moderation in all
versions which support them.

ftrace event stub during dwc3 irq storm:
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000866: irq_handler_exit: irq=14 ret=handled
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000872: irq_handler_entry: irq=504 name=dwc3
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000874: irq_handler_exit: irq=504 ret=handled
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000881: irq_handler_entry: irq=504 name=dwc3
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000883: irq_handler_exit: irq=504 ret=handled
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000889: irq_handler_entry: irq=504 name=dwc3
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000892: irq_handler_exit: irq=504 ret=handled
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000898: irq_handler_entry: irq=504 name=dwc3
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000901: irq_handler_exit: irq=504 ret=handled
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000907: irq_handler_entry: irq=504 name=dwc3
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000909: irq_handler_exit: irq=504 ret=handled
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000915: irq_handler_entry: irq=504 name=dwc3
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000918: irq_handler_exit: irq=504 ret=handled
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000924: irq_handler_entry: irq=504 name=dwc3
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000927: irq_handler_exit: irq=504 ret=handled
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000933: irq_handler_entry: irq=504 name=dwc3
    irq/504_dwc3-1111  ( 1111) [000] .... 70.000935: irq_handler_exit: irq=504 ret=handled
    ....

Cc: stable@kernel.org
Suggested-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Fixes: d325a1de49d6 ("usb: dwc3: gadget: Prevent losing events in event cache")
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
---
 drivers/usb/dwc3/core.c   |  2 +-
 drivers/usb/dwc3/gadget.c | 10 +++++++---
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
index dfa1b5fe48dc..6df971ef7285 100644
--- a/drivers/usb/dwc3/core.c
+++ b/drivers/usb/dwc3/core.c
@@ -1835,7 +1835,7 @@ static void dwc3_get_properties(struct dwc3 *dwc)
 	dwc->tx_thr_num_pkt_prd = tx_thr_num_pkt_prd;
 	dwc->tx_max_burst_prd = tx_max_burst_prd;
 
-	dwc->imod_interval = 0;
+	dwc->imod_interval = 1;
 
 	dwc->tx_fifo_resize_max_num = tx_fifo_resize_max_num;
 }
diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index d27af65eb08a..fad115113d28 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -4467,14 +4467,18 @@ static irqreturn_t dwc3_process_event_buf(struct dwc3_event_buffer *evt)
 	dwc3_writel(dwc->regs, DWC3_GEVNTSIZ(0),
 		    DWC3_GEVNTSIZ_SIZE(evt->length));
 
+	evt->flags &= ~DWC3_EVENT_PENDING;
+	/*
+	 * Add an explicit write memory barrier to make sure that the update of
+	 * clearing DWC3_EVENT_PENDING is observed in dwc3_check_event_buf()
+	 */
+	wmb();
+
 	if (dwc->imod_interval) {
 		dwc3_writel(dwc->regs, DWC3_GEVNTCOUNT(0), DWC3_GEVNTCOUNT_EHB);
 		dwc3_writel(dwc->regs, DWC3_DEV_IMOD(0), dwc->imod_interval);
 	}
 
-	/* Keep the clearing of DWC3_EVENT_PENDING at the end */
-	evt->flags &= ~DWC3_EVENT_PENDING;
-
 	return ret;
 }
 

base-commit: 9682c35ff6ecd76d9462d4749b8b413d3e8e605e
-- 
2.48.1.502.g6dc24dfdaf-goog