[PATCH v6 05/21] net/txgbe: fix inaccuracy in Tx rate limiting

[Zaiyu Wang <zaiyuwang@trustnetic.com>]

Amber-lite NIC's TX rate limiting has large deviations for small
packets. To address this, the following changes are made:

1. Set TDM_RL_ADJ (0x1820c) to 21B (includes 7B Ethernet preamble,
   1B SFD, 1B EFD, and 12B IPG).
2) Remove the rate offset in the driver (e.g., 105 / 100, a rough
   compensation value from Linux kernel driver tests).

After these changes, accuracy deviation for 64B packets is within
~5%, while large packets show lower deviation.

Fixes: a309ab43acf3 ("net/txgbe: support Tx queue rate limiting for Amber-Lite")
Cc: stable@dpdk.org

Signed-off-by: Zaiyu Wang <zaiyuwang@trustnetic.com>
---
 drivers/net/txgbe/base/txgbe_regs.h | 3 +++
 drivers/net/txgbe/txgbe_ethdev.c    | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/txgbe/base/txgbe_regs.h b/drivers/net/txgbe/base/txgbe_regs.h
index 95c585a025..3d1bc88430 100644
--- a/drivers/net/txgbe/base/txgbe_regs.h
+++ b/drivers/net/txgbe/base/txgbe_regs.h
@@ -1670,6 +1670,9 @@ enum txgbe_5tuple_protocol {
 #define   TXGBE_TDM_FACTOR_INT_SHIFT    16
 #define   TXGBE_TDM_FACTOR_FRA_SHIFT    2
 
+#define TXGBE_TDM_RL_ADJ                0x1820C
+ /* Ethernet framing overhead: 7B Ethernet preamble + 1B SFD + 1B EFD + 12B IPG */
+#define   TXGBE_FRAME_OVERHEAD          21
 #define TXGBE_TDM_RL_VM_IDX             0x018218
 #define TXGBE_TDM_RL_VM_CFG             0x01821C
 #define TXGBE_TDM_RL_CFG                0x018400
diff --git a/drivers/net/txgbe/txgbe_ethdev.c b/drivers/net/txgbe/txgbe_ethdev.c
index 779874aac9..099341b5ab 100644
--- a/drivers/net/txgbe/txgbe_ethdev.c
+++ b/drivers/net/txgbe/txgbe_ethdev.c
@@ -4314,7 +4314,6 @@ txgbe_set_queue_rate_limit(struct rte_eth_dev *dev,
 			u16 frac;
 
 			link_speed = dev->data->dev_link.link_speed;
-			tx_rate  = tx_rate * 105 / 100;
 			/* Calculate the rate factor values to set */
 			factor_int = link_speed / tx_rate;
 			frac = (link_speed % tx_rate) * 10000 / tx_rate;
@@ -4324,6 +4323,7 @@ txgbe_set_queue_rate_limit(struct rte_eth_dev *dev,
 				factor_fra = 0;
 			}
 
+			wr32(hw, TXGBE_TDM_RL_ADJ, TXGBE_FRAME_OVERHEAD);
 			wr32(hw, TXGBE_TDM_RL_QUEUE_IDX, queue_idx);
 			wr32m(hw, TXGBE_TDM_RL_QUEUE_CFG,
 			      TXGBE_TDM_FACTOR_INT_MASK, factor_int << TXGBE_TDM_FACTOR_INT_SHIFT);
-- 
2.21.0.windows.1