[RFC] e1000e: Add delays after writing to registers

[Jonathan David <jonathan.david@ni.com>]

There is a noticeable impact on determinism when a large number of
writes are flushed. Writes to the hardware registers are sent across
the PCI bus and take a significant amount of time to complete after
a flush, which causes high priority tasks (including interrupts) to
be delayed.

Adding a delay after long series of writes gives them time to
complete, and for higher priority tasks to run unimpeded.

Signed-off-by: Jonathan David <jonathan.david@ni.com>
---
 drivers/net/ethernet/intel/Kconfig            |  9 +++++++++
 drivers/net/ethernet/intel/e1000/e1000.h      |  3 +++
 drivers/net/ethernet/intel/e1000/e1000_main.c |  5 +++++
 drivers/net/ethernet/intel/e1000e/82571.c     |  3 +++
 drivers/net/ethernet/intel/e1000e/e1000.h     |  4 ++++
 drivers/net/ethernet/intel/e1000e/mac.c       |  3 +++
 drivers/net/ethernet/intel/e1000e/netdev.c    | 18 +++++++++++++++++-
 7 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/Kconfig b/drivers/net/ethernet/intel/Kconfig
index f4ff465..4c1fba2 100644
--- a/drivers/net/ethernet/intel/Kconfig
+++ b/drivers/net/ethernet/intel/Kconfig
@@ -85,6 +85,15 @@ config E1000E
 	  To compile this driver as a module, choose M here. The module
 	  will be called e1000e.
 
+config E1000_DELAY
+	bool "Add delays to e1000x drivers"
+	default n
+	depends on (E1000E || E1000)
+	---help---
+	  Enable delays after large numbers of MMIO writes to registers.
+	  The delays aid in preventing noticeable impact on real-time
+	  performance when a connection is interrupted.
+
 config IGB
 	tristate "Intel(R) 82575/82576 PCI-Express Gigabit Ethernet support"
 	depends on PCI
diff --git a/drivers/net/ethernet/intel/e1000/e1000.h b/drivers/net/ethernet/intel/e1000/e1000.h
index 6970710..ea405f3 100644
--- a/drivers/net/ethernet/intel/e1000/e1000.h
+++ b/drivers/net/ethernet/intel/e1000/e1000.h
@@ -223,6 +223,9 @@ struct e1000_rx_ring {
 #define E1000_TX_DESC(R, i)		E1000_GET_DESC(R, i, e1000_tx_desc)
 #define E1000_CONTEXT_DESC(R, i)	E1000_GET_DESC(R, i, e1000_context_desc)
 
+/* Time to wait after writing large amount of data to registers */
+#define E1000_WR_DELAY_RNG(val)         usleep_range(val*2, val*4)
+
 /* board specific private data structure */
 
 struct e1000_adapter {
diff --git a/drivers/net/ethernet/intel/e1000/e1000_main.c b/drivers/net/ethernet/intel/e1000/e1000_main.c
index 983eb4e..eb57148 100644
--- a/drivers/net/ethernet/intel/e1000/e1000_main.c
+++ b/drivers/net/ethernet/intel/e1000/e1000_main.c
@@ -2331,6 +2331,11 @@ static void e1000_set_rx_mode(struct net_device *netdev)
 		 */
 		E1000_WRITE_REG_ARRAY(hw, MTA, i, mcarray[i]);
 	}
+
+#ifdef CONFIG_E1000_DELAY
+	E1000_WR_DELAY_RNG(mta_reg_count);
+#endif
+
 	E1000_WRITE_FLUSH();
 
 	if (hw->mac_type == e1000_82542_rev2_0)
diff --git a/drivers/net/ethernet/intel/e1000e/82571.c b/drivers/net/ethernet/intel/e1000e/82571.c
index 32e7775..3e381fe 100644
--- a/drivers/net/ethernet/intel/e1000e/82571.c
+++ b/drivers/net/ethernet/intel/e1000e/82571.c
@@ -998,6 +998,9 @@ static s32 e1000_reset_hw_82571(struct e1000_hw *hw)
 
 	e_dbg("Issuing a global reset to MAC\n");
 	ew32(CTRL, ctrl | E1000_CTRL_RST);
+#ifdef CONFIG_E1000_DELAY
+	E1000_WR_DELAY();
+#endif
 
 	/* Must release MDIO ownership and mutex after MAC reset. */
 	switch (hw->mac.type) {
diff --git a/drivers/net/ethernet/intel/e1000e/e1000.h b/drivers/net/ethernet/intel/e1000e/e1000.h
index 0abc942..48db3df 100644
--- a/drivers/net/ethernet/intel/e1000e/e1000.h
+++ b/drivers/net/ethernet/intel/e1000e/e1000.h
@@ -89,6 +89,10 @@ struct e1000_info;
 /* Time to wait before putting the device into D3 if there's no link (in ms). */
 #define LINK_TIMEOUT		100
 
+/* Time to wait after writing large amount of data to registers */
+#define E1000_WR_DELAY()		usleep_range(100, 150)
+#define E1000_WR_DELAY_RNG(val)		usleep_range(val*2, val*4)
+
 /* Count for polling __E1000_RESET condition every 10-20msec.
  * Experimentation has shown the reset can take approximately 210msec.
  */
diff --git a/drivers/net/ethernet/intel/e1000e/mac.c b/drivers/net/ethernet/intel/e1000e/mac.c
index 30b74d5..d5ec122 100644
--- a/drivers/net/ethernet/intel/e1000e/mac.c
+++ b/drivers/net/ethernet/intel/e1000e/mac.c
@@ -353,6 +353,9 @@ void e1000e_update_mc_addr_list_generic(struct e1000_hw *hw,
 	/* replace the entire MTA table */
 	for (i = hw->mac.mta_reg_count - 1; i >= 0; i--)
 		E1000_WRITE_REG_ARRAY(hw, E1000_MTA, i, hw->mac.mta_shadow[i]);
+#ifdef CONFIG_E1000_DELAY
+	E1000_WR_DELAY_RNG(hw->mac.mta_reg_count);
+#endif
 	e1e_flush();
 }
 
diff --git a/drivers/net/ethernet/intel/e1000e/netdev.c b/drivers/net/ethernet/intel/e1000e/netdev.c
index 68913d1..18112ef 100644
--- a/drivers/net/ethernet/intel/e1000e/netdev.c
+++ b/drivers/net/ethernet/intel/e1000e/netdev.c
@@ -3360,6 +3360,9 @@ static int e1000e_write_uc_addr_list(struct net_device *netdev)
 	struct e1000_hw *hw = &adapter->hw;
 	unsigned int rar_entries;
 	int count = 0;
+#ifdef CONFIG_E1000_DELAY
+	unsigned int rar_count;
+#endif
 
 	rar_entries = hw->mac.ops.rar_get_count(hw);
 
@@ -3391,12 +3394,22 @@ static int e1000e_write_uc_addr_list(struct net_device *netdev)
 			count++;
 		}
 	}
-
+	
+	/* preserve number of remaining RAR entries for delay
+	 * function in order to prevent latency issues caused by
+	 * MMIO writes */
+#ifdef CONFIG_E1000_DELAY
+	rar_count = rar_entries;
+#endif
+	
 	/* zero out the remaining RAR entries not used above */
 	for (; rar_entries > 0; rar_entries--) {
 		ew32(RAH(rar_entries), 0);
 		ew32(RAL(rar_entries), 0);
 	}
+#ifdef CONFIG_E1000_DELAY
+	E1000_WR_DELAY_RNG(rar_count);
+#endif
 	e1e_flush();
 
 	return count;
@@ -3476,6 +3489,9 @@ static void e1000e_setup_rss_hash(struct e1000_adapter *adapter)
 	/* Direct all traffic to queue 0 */
 	for (i = 0; i < 32; i++)
 		ew32(RETA(i), 0);
+#ifdef CONFIG_E1000_DELAY
+	E1000_WR_DELAY();
+#endif
 
 	/* Disable raw packet checksumming so that RSS hash is placed in
 	 * descriptor on writeback.
-- 
1.9.1