[PATCH] net: apm: xgene: force XGene enet driver to re-balance IRQ usage

[Al Stone <ahs3@redhat.com>]

When using the user-space command 'tuned-adm profile network-latency',
the XGene enet driver will cause a hang trying to enable IRQs while
the system is trying to tune itself to be more responsive to network
traffic; dmesg will even tell us that the enables/disables are not
in balance.  With this fix, we force the driver to only enable_irq()
when there has been a previous disable_irq() -- i.e., force the number
of calls to each to be balanced.  This allows the kernel to continue
operating.

In an ideal world, the XGene enet driver would be restructured to avoid
this problem (it seems to be an artifact of when additional packets
arrive and differences of opinion in how the NIC responds under those
circumstances, some of which is controlled by firmware).  In the XGene2
driver, this is not an issue.

However, the XGene (aka Mustang) where this NIC is used is most likely
at the end of its useful life (APM which originally created the XGene
has completely morphed into a new company).  It is unlikely the driver
restructuring that is needed will ever be done.   There are, however,
a bunch of these machines out in the real world, and there are many
of us still using them daily (me, for example).  So, while this patch
is not the ideal way to repair the NIC driver, it does work and allows
us to continue using these boxes for a while longer.

Cc: Iyappan Subramanian <isubramanian@apm.com>
Cc: Keyur Chudgar <kchudgar@apm.com>
Cc: Quan Nguyen <qnguyen@apm.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Al Stone <ahs3@redhat.com>
---
 drivers/net/ethernet/apm/xgene/xgene_enet_main.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/apm/xgene/xgene_enet_main.c b/drivers/net/ethernet/apm/xgene/xgene_enet_main.c
index 3b889efddf78..90fb87f7e24e 100644
--- a/drivers/net/ethernet/apm/xgene/xgene_enet_main.c
+++ b/drivers/net/ethernet/apm/xgene/xgene_enet_main.c
@@ -866,8 +866,11 @@ static int xgene_enet_napi(struct napi_struct *napi, const int budget)
 	processed = xgene_enet_process_ring(ring, budget);
 
 	if (processed != budget) {
+		struct irq_desc *desc = irq_to_desc(ring->irq);
+
 		napi_complete_done(napi, processed);
-		enable_irq(ring->irq);
+		if (desc && desc->depth > 0)
+			enable_irq(ring->irq);
 	}
 
 	return processed;
-- 
2.17.1