net: systemport: Rewrite __bcm_sysport_tx_reclaim()

There is no need for complex checking between the last consumed index
and current consumed index, a simple subtraction will do.

This also eliminates the possibility of a permanent transmit queue stall
under the following conditions:

- one CPU bursts ring->size worth of traffic (up to 256 buffers), to the
  point where we run out of free descriptors, so we stop the transmit
  queue at the end of bcm_sysport_xmit()

- because of our locking, we have the transmit process disable
  interrupts which means we can be blocking the TX reclamation process

- when TX reclamation finally runs, we will be computing the difference
  between ring->c_index (last consumed index by SW) and what the HW
  reports through its register

- this register is masked with (ring->size - 1) = 0xff, which will lead
  to stripping the upper bits of the index (register is 16-bits wide)

- we will be computing last_tx_cn as 0, which means there is no work to
  be done, and we never wake-up the transmit queue, leaving it
  permanently disabled

A practical example is e.g: ring->c_index aka last_c_index = 12, we
pushed 256 entries, HW consumer index = 268, we mask it with 0xff = 12,
so last_tx_cn == 0, nothing happens.

Fixes: 80105befdb4b ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/ethernet/broadcom/bcmsysport.c b/drivers/net/ethernet/broadcom/bcmsysport.c
index f15a8fc6dfc97419f8e1492dd1717b9d2e562b84..3fc549b88c43b082bd22023f99ec13fbf1d78525 100644 (file)
--- a/drivers/net/ethernet/broadcom/bcmsysport.c
+++ b/drivers/net/ethernet/broadcom/bcmsysport.c
@@ -855,10 +855,12 @@ static void bcm_sysport_tx_reclaim_one(struct bcm_sysport_tx_ring *ring,
 static unsigned int __bcm_sysport_tx_reclaim(struct bcm_sysport_priv *priv,
                                             struct bcm_sysport_tx_ring *ring)
 {
-       unsigned int c_index, last_c_index, last_tx_cn, num_tx_cbs;
        unsigned int pkts_compl = 0, bytes_compl = 0;
        struct net_device *ndev = priv->netdev;
+       unsigned int txbds_processed = 0;
        struct bcm_sysport_cb *cb;
+       unsigned int txbds_ready;
+       unsigned int c_index;
        u32 hw_ind;
 
        /* Clear status before servicing to reduce spurious interrupts */
@@ -871,29 +873,23 @@ static unsigned int __bcm_sysport_tx_reclaim(struct bcm_sysport_priv *priv,
        /* Compute how many descriptors have been processed since last call */
        hw_ind = tdma_readl(priv, TDMA_DESC_RING_PROD_CONS_INDEX(ring->index));
        c_index = (hw_ind >> RING_CONS_INDEX_SHIFT) & RING_CONS_INDEX_MASK;
-       ring->p_index = (hw_ind & RING_PROD_INDEX_MASK);
-
-       last_c_index = ring->c_index;
-       num_tx_cbs = ring->size;
-
-       c_index &= (num_tx_cbs - 1);
-
-       if (c_index >= last_c_index)
-               last_tx_cn = c_index - last_c_index;
-       else
-               last_tx_cn = num_tx_cbs - last_c_index + c_index;
+       txbds_ready = (c_index - ring->c_index) & RING_CONS_INDEX_MASK;
 
        netif_dbg(priv, tx_done, ndev,
-                 "ring=%d c_index=%d last_tx_cn=%d last_c_index=%d\n",
-                 ring->index, c_index, last_tx_cn, last_c_index);
+                 "ring=%d old_c_index=%u c_index=%u txbds_ready=%u\n",
+                 ring->index, ring->c_index, c_index, txbds_ready);
 
-       while (last_tx_cn-- > 0) {
-               cb = ring->cbs + last_c_index;
+       while (txbds_processed < txbds_ready) {
+               cb = &ring->cbs[ring->clean_index];
                bcm_sysport_tx_reclaim_one(ring, cb, &bytes_compl, &pkts_compl);
 
                ring->desc_count++;
-               last_c_index++;
-               last_c_index &= (num_tx_cbs - 1);
+               txbds_processed++;
+
+               if (likely(ring->clean_index < ring->size - 1))
+                       ring->clean_index++;
+               else
+                       ring->clean_index = 0;
        }
 
        u64_stats_update_begin(&priv->syncp);
@@ -1394,6 +1390,7 @@ static int bcm_sysport_init_tx_ring(struct bcm_sysport_priv *priv,
        netif_tx_napi_add(priv->netdev, &ring->napi, bcm_sysport_tx_poll, 64);
        ring->index = index;
        ring->size = size;
+       ring->clean_index = 0;
        ring->alloc_size = ring->size;
        ring->desc_cpu = p;
        ring->desc_count = ring->size;

diff --git a/drivers/net/ethernet/broadcom/bcmsysport.h b/drivers/net/ethernet/broadcom/bcmsysport.h
index f5a984c1c986535f3421bafd9c851ec995ccf3b0..19c91c76e32763f399ebc8d67c7a0b647da44572 100644 (file)
--- a/drivers/net/ethernet/broadcom/bcmsysport.h
+++ b/drivers/net/ethernet/broadcom/bcmsysport.h
@@ -706,7 +706,7 @@ struct bcm_sysport_tx_ring {
        unsigned int    desc_count;     /* Number of descriptors */
        unsigned int    curr_desc;      /* Current descriptor */
        unsigned int    c_index;        /* Last consumer index */
-       unsigned int    p_index;        /* Current producer index */
+       unsigned int    clean_index;    /* Current clean index */
        struct bcm_sysport_cb *cbs;     /* Transmit control blocks */
        struct dma_desc *desc_cpu;      /* CPU view of the descriptor */
        struct bcm_sysport_priv *priv;  /* private context backpointer */