sfc: farch: fix TX queue lookup in TX flush done handling

commit 5b1faa9 upstream.

We're starting from a TXQ instance number ('qid'), not a TXQ type, so
 efx_get_tx_queue() is inappropriate (and could return NULL, leading
 to panics).

Fixes: 1280479 ("sfc: decouple TXQ type from label")
Reported-by: Trevor Hemsley <themsley@voiceflex.com>
Cc: stable@vger.kernel.org
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/ethernet/sfc/farch.c

@@ -1081,16 +1081,16 @@ static void
 efx_farch_handle_tx_flush_done(struct efx_nic *efx, efx_qword_t *event)
 {
 	struct efx_tx_queue *tx_queue;
+	struct efx_channel *channel;
 	int qid;
 
 	qid = EFX_QWORD_FIELD(*event, FSF_AZ_DRIVER_EV_SUBDATA);
 	if (qid < EFX_MAX_TXQ_PER_CHANNEL * (efx->n_tx_channels + efx->n_extra_tx_channels)) {
-		tx_queue = efx_get_tx_queue(efx, qid / EFX_MAX_TXQ_PER_CHANNEL,
-					    qid % EFX_MAX_TXQ_PER_CHANNEL);
-		if (atomic_cmpxchg(&tx_queue->flush_outstanding, 1, 0)) {
+		channel = efx_get_tx_channel(efx, qid / EFX_MAX_TXQ_PER_CHANNEL);
+		tx_queue = channel->tx_queue + (qid % EFX_MAX_TXQ_PER_CHANNEL);
+		if (atomic_cmpxchg(&tx_queue->flush_outstanding, 1, 0))
 			efx_farch_magic_event(tx_queue->channel,
 					      EFX_CHANNEL_MAGIC_TX_DRAIN(tx_queue));
-		}
 	}
 }