iwlwifi: disable the buggy chain extension feature in HW

commit d012d04e upstream.

This feature has been reported to be buggy and enabled by
default. We therefore need to disable it manually.
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -1128,6 +1128,11 @@ 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),