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net: docs: update the sample code in driver.rst

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The sample code talks about single-queue devices and uses locks.
Update it to something resembling more modern code.
Make sure we mention use of READ_ONCE() / WRITE_ONCE().

Change the comment which talked about consumer on the xmit side.
AFAIU xmit is the producer and completions are a consumer.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Jakub Kicinski 2023-04-11 18:50:36 -07:00 committed by Paolo Abeni
parent 21cdc87fa9
commit 50762d9af3
1 changed files with 27 additions and 34 deletions
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61
Documentation/networking/driver.rst
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@ -47,30 +47,43 @@ for a driver implementing scatter-gather this means:
.. code-block:: c
static u32 drv_tx_avail(struct drv_ring *dr)
{
u32 used = READ_ONCE(dr->prod) - READ_ONCE(dr->cons);
return dr->tx_ring_size - (used & bp->tx_ring_mask);
}
static netdev_tx_t drv_hard_start_xmit(struct sk_buff *skb,
struct net_device *dev)
{
struct drv *dp = netdev_priv(dev);
struct netdev_queue *txq;
struct drv_ring *dr;
int idx;
idx = skb_get_queue_mapping(skb);
dr = dp->tx_rings[idx];
txq = netdev_get_tx_queue(dev, idx);
lock_tx(dp);
//...
/* This is a hard error log it. */
if (TX_BUFFS_AVAIL(dp) <= (skb_shinfo(skb)->nr_frags + 1)) {
/* This should be a very rare race - log it. */
if (drv_tx_avail(dr) <= skb_shinfo(skb)->nr_frags + 1) {
netif_stop_queue(dev);
unlock_tx(dp);
printk(KERN_ERR PFX "%s: BUG! Tx Ring full when queue awake!\n",
dev->name);
netdev_warn(dev, "Tx Ring full when queue awake!\n");
return NETDEV_TX_BUSY;
}
//... queue packet to card ...
//... update tx consumer index ...
if (TX_BUFFS_AVAIL(dp) <= (MAX_SKB_FRAGS + 1))
netif_stop_queue(dev);
netdev_tx_sent_queue(txq, skb->len);
//... update tx producer index using WRITE_ONCE() ...
if (!netif_txq_maybe_stop(txq, drv_tx_avail(dr),
MAX_SKB_FRAGS + 1, 2 * MAX_SKB_FRAGS))
dr->stats.stopped++;
//...
unlock_tx(dp);
//...
return NETDEV_TX_OK;
}
@ -79,30 +92,10 @@ And then at the end of your TX reclamation event handling:
.. code-block:: c
if (netif_queue_stopped(dp->dev) &&
TX_BUFFS_AVAIL(dp) > (MAX_SKB_FRAGS + 1))
netif_wake_queue(dp->dev);
//... update tx consumer index using WRITE_ONCE() ...
For a non-scatter-gather supporting card, the three tests simply become:
.. code-block:: c
/* This is a hard error log it. */
if (TX_BUFFS_AVAIL(dp) <= 0)
and:
.. code-block:: c
if (TX_BUFFS_AVAIL(dp) == 0)
and:
.. code-block:: c
if (netif_queue_stopped(dp->dev) &&
TX_BUFFS_AVAIL(dp) > 0)
netif_wake_queue(dp->dev);
netif_txq_completed_wake(txq, cmpl_pkts, cmpl_bytes,
drv_tx_avail(dr), 2 * MAX_SKB_FRAGS);
Lockless queue stop / wake helper macros
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~