[Intel-wired-lan] [PATCH RFC net-next] e1000e: Fix real-time violations on link up

[Gerhard Engleder <gerhard@engleder-embedded.com>]

From: Gerhard Engleder <eg@keba.com>

Link down and up triggers update of MTA table. This update executes many
PCIe writes and a final flush. Thus, PCIe will be blocked until all writes
are flushed. As a result, DMA transfers of other targets suffer from delay
in the range of 50us. The result are timing violations on real-time
systems during link down and up of e1000e.

Execute a flush after every single write. This prevents overloading the
interconnect with posted writes. As this also increases the time spent for
MTA table update considerable this change is limited to PREEMPT_RT.

Signed-off-by: Gerhard Engleder <eg@keba.com>
---
 drivers/net/ethernet/intel/e1000e/mac.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/e1000e/mac.c b/drivers/net/ethernet/intel/e1000e/mac.c
index d7df2a0ed629..f4693d355886 100644
--- a/drivers/net/ethernet/intel/e1000e/mac.c
+++ b/drivers/net/ethernet/intel/e1000e/mac.c
@@ -331,9 +331,15 @@ 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--)
+	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_PREEMPT_RT
+		e1e_flush();
+#endif
+	}
+#ifndef CONFIG_PREEMPT_RT
 	e1e_flush();
+#endif
 }
 
 /**
-- 
2.39.2