sfc: Correct and expand some comments

These comments have been revised in response to questions raised by Andrew
Morton in <20080501120858.207b6dd6.akpm@linux-foundation.org>.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

1 files changed, 14 insertions, 6 deletions

diff --git a/drivers/net/sfc/efx.c b/drivers/net/sfc/efx.c
index f6131e578b44..11ee0d4407c4 100644
--- a/drivers/net/sfc/efx.c
+++ b/drivers/net/sfc/efx.c
@@ -199,11 +199,12 @@ static inline int efx_process_channel(struct efx_channel *channel, int rx_quota)
  */
 static inline void efx_channel_processed(struct efx_channel *channel)
 {
-        /* Write to EVQ_RPTR_REG.  If a new event arrived in a race
-         * with finishing processing, a new interrupt will be raised.
-         */
+        /* The interrupt handler for this channel may set work_pending
+         * as soon as we acknowledge the events we've seen.  Make sure
+         * it's cleared before then. */
         channel->work_pending = 0;
-        smp_wmb(); /* Ensure channel updated before any new interrupt. */
+        smp_wmb();
+
         falcon_eventq_read_ack(channel);
 }
 
@@ -427,9 +428,12 @@ static void efx_start_channel(struct efx_channel *channel)
                 netif_napi_add(channel->napi_dev, &channel->napi_str,
                                efx_poll, napi_weight);
 
+        /* The interrupt handler for this channel may set work_pending
+         * as soon as we enable it.  Make sure it's cleared before
+         * then.  Similarly, make sure it sees the enabled flag set. */
         channel->work_pending = 0;
         channel->enabled = 1;
-        smp_wmb(); /* ensure channel updated before first interrupt */
+        smp_wmb();
 
         napi_enable(&channel->napi_str);
 
@@ -1332,13 +1336,17 @@ static int efx_net_stop(struct net_device *net_dev)
         return 0;
 }
 
-/* Context: process, dev_base_lock held, non-blocking. */
+/* Context: process, dev_base_lock or RTNL held, non-blocking. */
 static struct net_device_stats *efx_net_stats(struct net_device *net_dev)
 {
         struct efx_nic *efx = net_dev->priv;
         struct efx_mac_stats *mac_stats = &efx->mac_stats;
         struct net_device_stats *stats = &net_dev->stats;
 
+        /* Update stats if possible, but do not wait if another thread
+         * is updating them (or resetting the NIC); slightly stale
+         * stats are acceptable.
+         */
         if (!spin_trylock(&efx->stats_lock))
                 return stats;
         if (efx->state == STATE_RUNNING) {