[PATCH RT] ehea: make receive irq handler non-threaded (IRQF_NODELAY)

[Darren Hart]

The underlying hardware is edge triggered but presented by XICS as level
triggered. The edge triggered interrupts are not reissued after masking. This
is not a problem in mainline which does not mask the interrupt (relying on the
EOI mechanism instead). The threaded interrupts in PREEMPT_RT do mask the
interrupt, and can lose interrupts that occurred while masked, resulting in a
hung ethernet interface.

The receive handler simply calls napi_schedule(), as such, there is no
significant additional overhead in making this non-threaded, since we either
wakeup the threaded irq handler to call napi_schedule(), or just call
napi_schedule() directly to wakeup the softirqs.  As the receive handler is
lockless, there is no need to convert any of the ehea spinlock_t's to
atomic_spinlock_t's.

Without this patch, a simple scp file copy loop would fail quickly (usually
seconds). We have over two hours of sustained scp activity with the patch
applied.

Credit goes to Will Schmidt for lots of instrumentation and tracing which
clarified the scenario and to Thomas Gleixner for the incredibly simple
solution.

Signed-off-by: Darren Hart <dvhltc@xxxxxxxxxx>
Acked-by: Will Schmidt <will_schmidt@xxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxxx>
Cc: Jan-Bernd Themann <themann@xxxxxxxxxx>
Cc: Nivedita Singhvi <niv@xxxxxxxxxx>
Cc: Brian King <bjking1@xxxxxxxxxx>
Cc: Michael Ellerman <ellerman@xxxxxxxxxxx>
Cc: Doug Maxey <doug.maxey@xxxxxxxxxx>
---
 drivers/net/ehea/ehea_main.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/net/ehea/ehea_main.c b/drivers/net/ehea/ehea_main.c
index 977c3d3..2c53df2 100644
--- a/drivers/net/ehea/ehea_main.c
+++ b/drivers/net/ehea/ehea_main.c
@@ -1263,7 +1263,7 @@ static int ehea_reg_interrupts(struct net_device *dev)
 			 "%s-queue%d", dev->name, i);
 		ret = ibmebus_request_irq(pr->eq->attr.ist1,
 					  ehea_recv_irq_handler,
-					  IRQF_DISABLED, pr->int_send_name,
+					  IRQF_DISABLED | IRQF_NODELAY, pr->int_send_name,
 					  pr);
 		if (ret) {
 			ehea_error("failed registering irq for ehea_queue "
-- 
1.5.5.6
-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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