nfp: avoid buffer leak when FW communication fails

After device is stopped we reset the rings by moving all free buffers
to positions [0, cnt - 2], and clear the position cnt - 1 in the ring.
We then proceed to clear the read/write pointers.  This means that if
we try to reset the ring again the code will assume that the next to
fill buffer is at position 0 and swap it with cnt - 1.  Since we
previously cleared position cnt - 1 it will lead to leaking the first
buffer and leaving ring in a bad state.

This scenario can only happen if FW communication fails, in which case
the ring will never be used again, so the fact it's in a bad state will
not be noticed.  Buffer leak is the only problem.  Don't try to move
buffers in the ring if the read/write pointers indicate the ring was
never used or have already been reset.

nfp_net_clear_config_and_disable() is now fully idempotent.

Found by code inspection, FW communication failures are very rare,
and reconfiguring a live device is not common either, so it's unlikely
anyone has ever noticed the leak.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/netronome/nfp/nfp_net_common.c

@@ -1078,7 +1078,7 @@ static bool nfp_net_xdp_complete(struct nfp_net_tx_ring *tx_ring)
  * @dp:		NFP Net data path struct
  * @tx_ring:	TX ring structure
  *
- * Assumes that the device is stopped
+ * Assumes that the device is stopped, must be idempotent.
  */
 static void
 nfp_net_tx_ring_reset(struct nfp_net_dp *dp, struct nfp_net_tx_ring *tx_ring)
@@ -1280,13 +1280,18 @@ static void nfp_net_rx_give_one(const struct nfp_net_dp *dp,
  * nfp_net_rx_ring_reset() - Reflect in SW state of freelist after disable
  * @rx_ring:	RX ring structure
  *
- * Warning: Do *not* call if ring buffers were never put on the FW freelist
- *	    (i.e. device was not enabled)!
+ * Assumes that the device is stopped, must be idempotent.
  */
 static void nfp_net_rx_ring_reset(struct nfp_net_rx_ring *rx_ring)
 {
 	unsigned int wr_idx, last_idx;
 
+	/* wr_p == rd_p means ring was never fed FL bufs.  RX rings are always
+	 * kept at cnt - 1 FL bufs.
+	 */
+	if (rx_ring->wr_p == 0 && rx_ring->rd_p == 0)
+		return;
+
 	/* Move the empty entry to the end of the list */
 	wr_idx = D_IDX(rx_ring, rx_ring->wr_p);
 	last_idx = rx_ring->cnt - 1;
@@ -2508,6 +2513,8 @@ static void nfp_net_vec_clear_ring_data(struct nfp_net *nn, unsigned int idx)
 /**
  * nfp_net_clear_config_and_disable() - Clear control BAR and disable NFP
  * @nn:      NFP Net device to reconfigure
+ *
+ * Warning: must be fully idempotent.
  */
 static void nfp_net_clear_config_and_disable(struct nfp_net *nn)
 {