[PATCH net 3/6] i40e: do not round up result of dbuff calculation for xsk pool

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

When programming dbuff on rx queue context, avoid division round up as
it causes to actually corrupt the tailroom for AF_XDP ZC. Below is an
example based on 4k chunk size when xsk pool pointer is valid on given
rx ring:

chunk_size = 4096
headroom = 256
tailroom = 320

ring->rx_buf_len = 4096 - 256 - 320 = 3520

rx_ctx.dbuff = DIV_ROUND_UP(3520, 128) ->
3520 / 128 = 27.5 -> round up results in 28

dbuff programming unit is 128. If we give 128 * 28 = 3584. So HW will
corrupt 64 bytes from tailroom. Doing plain division will floor the
result and would not exceed reserved space at the end.

Fixes: 1c9ba9c14658 ("i40e: xsk: add RX multi-buffer support")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
---
 drivers/net/ethernet/intel/i40e/i40e_main.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c b/drivers/net/ethernet/intel/i40e/i40e_main.c
index 926d001b2150..f37cf6ab4461 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_main.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
@@ -3621,9 +3621,11 @@ static int i40e_configure_rx_ring(struct i40e_ring *ring)
 skip:
 	xdp_init_buff(&ring->xdp, xdp_frame_sz, &ring->xdp_rxq);
 
-	rx_ctx.dbuff = DIV_ROUND_UP(ring->rx_buf_len,
-				    BIT_ULL(I40E_RXQ_CTX_DBUFF_SHIFT));
-
+	if (ring->xsk_pool)
+		rx_ctx.dbuff = ring->rx_buf_len / BIT_ULL(I40E_RXQ_CTX_DBUFF_SHIFT);
+	else
+		rx_ctx.dbuff = DIV_ROUND_UP(ring->rx_buf_len,
+					    BIT_ULL(I40E_RXQ_CTX_DBUFF_SHIFT));
 	rx_ctx.base = (ring->dma / 128);
 	rx_ctx.qlen = ring->count;
 
-- 
2.43.0