net/mlx5e: Calculate SQ stop room in a robust way

Currently, different formulas are used to estimate the space that may be
taken by WQEs in the SQ during a single packet transmit. This space is
called stop room, and it's checked in the end of packet transmit to find
out if the next packet could overflow the SQ. If it could, the driver
tells the kernel to stop sending next packets.

Many factors affect the stop room:

1. Padding with NOPs to avoid WQEs spanning over page boundaries.

2. Enabled and disabled offloads (TLS, upcoming MPWQE).

3. The maximum size of a WQE.

The padding is performed before every WQE if it doesn't fit the current
page.

The current formula assumes that only one padding will be required per
packet, and it doesn't take into account that the WQEs posted during the
transmission of a single packet might exceed the page size in very rare
circumstances. For example, to hit this condition with 4096-byte pages,
TLS offload will have to interrupt an almost-full MPWQE session, be in
the resync flow and try to transmit a near to maximum amount of data.

To avoid SQ overflows in such rare cases after MPWQE is added, this
patch introduces a more robust formula to estimate the stop room. The
new formula uses the fact that a WQE of size X will not require more
than X-1 WQEBBs of padding. More exact estimations are possible, but
they result in much more complex and error-prone code for little gain.

Before this patch, the TLS stop room included space for both INNOVA and
ConnectX TLS offloads that couldn't run at the same time anyway, so this
patch accounts only for the active one.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

1 files changed, 21 insertions, 19 deletions

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h b/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
index dce2bbbf9109..bfd3e1161bc6 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
@@ -6,25 +6,6 @@
 
 #include "en.h"
 
-#define MLX5E_SQ_NOPS_ROOM (MLX5_SEND_WQE_MAX_WQEBBS - 1)
-#define MLX5E_SQ_STOP_ROOM (MLX5_SEND_WQE_MAX_WQEBBS +\
-			    MLX5E_SQ_NOPS_ROOM)
-
-#ifndef CONFIG_MLX5_EN_TLS
-#define MLX5E_SQ_TLS_ROOM (0)
-#else
-/* TLS offload requires additional stop_room for:
- *  - a resync SKB.
- * kTLS offload requires fixed additional stop_room for:
- * - a static params WQE, and a progress params WQE.
- * The additional MTU-depending room for the resync DUMP WQEs
- * will be calculated and added in runtime.
- */
-#define MLX5E_SQ_TLS_ROOM  \
-	(MLX5_SEND_WQE_MAX_WQEBBS + \
-	 MLX5E_KTLS_STATIC_WQEBBS + MLX5E_KTLS_PROGRESS_WQEBBS)
-#endif
-
 #define INL_HDR_START_SZ (sizeof(((struct mlx5_wqe_eth_seg *)NULL)->inline_hdr.start))
 
 enum mlx5e_icosq_wqe_type {
@@ -331,4 +312,25 @@ mlx5e_set_eseg_swp(struct sk_buff *skb, struct mlx5_wqe_eth_seg *eseg,
 	}
 }
 
+static inline u16 mlx5e_stop_room_for_wqe(u16 wqe_size)
+{
+	BUILD_BUG_ON(PAGE_SIZE / MLX5_SEND_WQE_BB < MLX5_SEND_WQE_MAX_WQEBBS);
+
+	/* A WQE must not cross the page boundary, hence two conditions:
+	 * 1. Its size must not exceed the page size.
+	 * 2. If the WQE size is X, and the space remaining in a page is less
+	 *    than X, this space needs to be padded with NOPs. So, one WQE of
+	 *    size X may require up to X-1 WQEBBs of padding, which makes the
+	 *    stop room of X-1 + X.
+	 * WQE size is also limited by the hardware limit.
+	 */
+
+	if (__builtin_constant_p(wqe_size))
+		BUILD_BUG_ON(wqe_size > MLX5_SEND_WQE_MAX_WQEBBS);
+	else
+		WARN_ON_ONCE(wqe_size > MLX5_SEND_WQE_MAX_WQEBBS);
+
+	return wqe_size * 2 - 1;
+}
+
 #endif