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    Subject[PATCH 4.15 44/47] net: systemport: Rewrite __bcm_sysport_tx_reclaim()
    Date
    4.15-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    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
    @@ -855,10 +855,12 @@ static void bcm_sysport_tx_reclaim_one(s
    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_rec
    /* 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);
    @@ -1406,6 +1402,7 @@ static int bcm_sysport_init_tx_ring(stru
    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;
    --- 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 */

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