[PATCH net-next 1/6] i40e: remove unnecessary memory writes of the next to clean pointer

[Tony Nguyen]

From: Cristian Dumitrescu <cristian.dumitre...@intel.com>

For performance reasons, avoid writing the ring next-to-clean pointer
value back to memory on every update, as it is not really necessary.
Instead, simply read it at initialization into a local copy, update
the local copy as necessary and write the local copy back to memory
after the last update.

Signed-off-by: Cristian Dumitrescu <cristian.dumitre...@intel.com>
Tested-by: Kiran Bhandare <kiranx.bhand...@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.ngu...@intel.com>
---
 drivers/net/ethernet/intel/i40e/i40e_xsk.c | 30 ++++++++--------------
 1 file changed, 11 insertions(+), 19 deletions(-)

diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c 
b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
index 492ce213208d..87d43407653c 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
@@ -261,18 +261,6 @@ static struct sk_buff *i40e_construct_skb_zc(struct 
i40e_ring *rx_ring,
        return skb;
 }
 
-/**
- * i40e_inc_ntc: Advance the next_to_clean index
- * @rx_ring: Rx ring
- **/
-static void i40e_inc_ntc(struct i40e_ring *rx_ring)
-{
-       u32 ntc = rx_ring->next_to_clean + 1;
-
-       ntc = (ntc < rx_ring->count) ? ntc : 0;
-       rx_ring->next_to_clean = ntc;
-}
-
 /**
  * i40e_clean_rx_irq_zc - Consumes Rx packets from the hardware ring
  * @rx_ring: Rx ring
@@ -284,6 +272,8 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int 
budget)
 {
        unsigned int total_rx_bytes = 0, total_rx_packets = 0;
        u16 cleaned_count = I40E_DESC_UNUSED(rx_ring);
+       u16 next_to_clean = rx_ring->next_to_clean;
+       u16 count_mask = rx_ring->count - 1;
        unsigned int xdp_res, xdp_xmit = 0;
        bool failure = false;
        struct sk_buff *skb;
@@ -294,7 +284,7 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int 
budget)
                unsigned int size;
                u64 qword;
 
-               rx_desc = I40E_RX_DESC(rx_ring, rx_ring->next_to_clean);
+               rx_desc = I40E_RX_DESC(rx_ring, next_to_clean);
                qword = le64_to_cpu(rx_desc->wb.qword1.status_error_len);
 
                /* This memory barrier is needed to keep us from reading
@@ -307,11 +297,11 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int 
budget)
                        i40e_clean_programming_status(rx_ring,
                                                      rx_desc->raw.qword[0],
                                                      qword);
-                       bi = i40e_rx_bi(rx_ring, rx_ring->next_to_clean);
+                       bi = i40e_rx_bi(rx_ring, next_to_clean);
                        xsk_buff_free(*bi);
                        *bi = NULL;
                        cleaned_count++;
-                       i40e_inc_ntc(rx_ring);
+                       next_to_clean = (next_to_clean + 1) & count_mask;
                        continue;
                }
 
@@ -320,7 +310,7 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int 
budget)
                if (!size)
                        break;
 
-               bi = i40e_rx_bi(rx_ring, rx_ring->next_to_clean);
+               bi = i40e_rx_bi(rx_ring, next_to_clean);
                (*bi)->data_end = (*bi)->data + size;
                xsk_buff_dma_sync_for_cpu(*bi, rx_ring->xsk_pool);
 
@@ -336,7 +326,7 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int 
budget)
                        total_rx_packets++;
 
                        cleaned_count++;
-                       i40e_inc_ntc(rx_ring);
+                       next_to_clean = (next_to_clean + 1) & count_mask;
                        continue;
                }
 
@@ -355,7 +345,7 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int 
budget)
 
                *bi = NULL;
                cleaned_count++;
-               i40e_inc_ntc(rx_ring);
+               next_to_clean = (next_to_clean + 1) & count_mask;
 
                if (eth_skb_pad(skb))
                        continue;
@@ -367,6 +357,8 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int 
budget)
                napi_gro_receive(&rx_ring->q_vector->napi, skb);
        }
 
+       rx_ring->next_to_clean = next_to_clean;
+
        if (cleaned_count >= I40E_RX_BUFFER_WRITE)
                failure = !i40e_alloc_rx_buffers_zc(rx_ring, cleaned_count);
 
@@ -374,7 +366,7 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int 
budget)
        i40e_update_rx_stats(rx_ring, total_rx_bytes, total_rx_packets);
 
        if (xsk_uses_need_wakeup(rx_ring->xsk_pool)) {
-               if (failure || rx_ring->next_to_clean == rx_ring->next_to_use)
+               if (failure || next_to_clean == rx_ring->next_to_use)
                        xsk_set_rx_need_wakeup(rx_ring->xsk_pool);
                else
                        xsk_clear_rx_need_wakeup(rx_ring->xsk_pool);
-- 
2.26.2