[PATCH 4.4 20/20] net: systemport: Rewrite __bcm_sysport_tx_reclaim()

Greg Kroah-Hartman
2018-03-29T18:08:30Z

From: Florian Fainelli <f.fainelli@gmail.com>

[ Upstream commit 484d802d0f2f29c335563fcac2a8facf174a1bbc ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/net/ethernet/broadcom/bcmsysport.c | 33 +++++++++++++----------------
drivers/net/ethernet/broadcom/bcmsysport.h | 2 -
2 files changed, 16 insertions(+), 19 deletions(-)

--- a/drivers/net/ethernet/broadcom/bcmsysport.c
+++ b/drivers/net/ethernet/broadcom/bcmsysport.c
@@ -729,37 +729,33 @@ static unsigned int __bcm_sysport_tx_rec
struct bcm_sysport_tx_ring *ring)
{
struct net_device *ndev = priv->netdev;
- unsigned int c_index, last_c_index, last_tx_cn, num_tx_cbs;
unsigned int pkts_compl = 0, bytes_compl = 0;
+ unsigned int txbds_processed = 0;
struct bcm_sysport_cb *cb;
+ unsigned int txbds_ready;
+ unsigned int c_index;
u32 hw_ind;

/* 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(priv, 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;
}

ring->c_index = c_index;
@@ -1229,6 +1225,7 @@ static int bcm_sysport_init_tx_ring(stru
netif_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;
--- a/drivers/net/ethernet/broadcom/bcmsysport.h
+++ b/drivers/net/ethernet/broadcom/bcmsysport.h
@@ -638,7 +638,7 @@ 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 */