iwlwifi: disable the buggy chain extension feature in HW

This feature has been reported to be buggy and enabled by
default. We therefore need to disable it manually.

Cc: stable@vger.kernel.org
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-prph.h

@@ -224,6 +224,7 @@
 #define SCD_TXFACT		(SCD_BASE + 0x10)
 #define SCD_ACTIVE		(SCD_BASE + 0x14)
 #define SCD_QUEUECHAIN_SEL	(SCD_BASE + 0xe8)
+#define SCD_CHAINEXT_EN		(SCD_BASE + 0x244)
 #define SCD_AGGR_SEL		(SCD_BASE + 0x248)
 #define SCD_INTERRUPT_MASK	(SCD_BASE + 0x108)
 

drivers/net/wireless/iwlwifi/iwl-trans-pcie.c

@@ -1058,6 +1058,11 @@ static void iwl_tx_start(struct iwl_trans *trans)
 	iwl_write_prph(trans, SCD_DRAM_BASE_ADDR,
 		       trans_pcie->scd_bc_tbls.dma >> 10);
 
+	/* The chain extension of the SCD doesn't work well. This feature is
+	 * enabled by default by the HW, so we need to disable it manually.
+	 */
+	iwl_write_prph(trans, SCD_CHAINEXT_EN, 0);
+
 	/* Enable DMA channel */
 	for (chan = 0; chan < FH_TCSR_CHNL_NUM ; chan++)
 		iwl_write_direct32(trans, FH_TCSR_CHNL_TX_CONFIG_REG(chan),