ath9k: implement IO serialization

All 802.11n PCI devices (Cardbus, PCI, mini-PCI) require
serialization of IO when on non-uniprocessor systems. PCI
express devices not not require this.

This should fix our only last standing open ath9k kernel.org
bugzilla bug report:

http://bugzilla.kernel.org/show_bug.cgi?id=12110

A port is probably required to older kernels and I can work on
that.

Cc: stable@kernel.org
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/drivers/net/wireless/ath9k/hw.c b/drivers/net/wireless/ath9k/hw.c
index 34474edefc97..5c870436fc5f 100644
--- a/drivers/net/wireless/ath9k/hw.c
+++ b/drivers/net/wireless/ath9k/hw.c
@@ -437,6 +437,25 @@ static void ath9k_hw_set_defaults(struct ath_hal *ah)
         }
 
         ah->ah_config.intr_mitigation = 1;
+
+        /*
+         * We need this for PCI devices only (Cardbus, PCI, miniPCI)
+         * _and_ if on non-uniprocessor systems (Multiprocessor/HT).
+         * This means we use it for all AR5416 devices, and the few
+         * minor PCI AR9280 devices out there.
+         *
+         * Serialization is required because these devices do not handle
+         * well the case of two concurrent reads/writes due to the latency
+         * involved. During one read/write another read/write can be issued
+         * on another CPU while the previous read/write may still be working
+         * on our hardware, if we hit this case the hardware poops in a loop.
+         * We prevent this by serializing reads and writes.
+         *
+         * This issue is not present on PCI-Express devices or pre-AR5416
+         * devices (legacy, 802.11abg).
+         */
+        if (num_possible_cpus() > 1)
+                ah->ah_config.serialize_regmode = SER_REG_MODE_AUTO;
 }
 
 static struct ath_hal_5416 *ath9k_hw_newstate(u16 devid,