netdev - Re: [PATCH wpan-next v2 08/11] net: mac802154: Introduce a tx queue flushing mechanism

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Date:   Sun, 15 May 2022 18:23:04 -0400
From:   Alexander Aring <aahringo@...hat.com>
To:     Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     Alexander Aring <alex.aring@...il.com>,
        Stefan Schmidt <stefan@...enfreihafen.org>,
        linux-wpan - ML <linux-wpan@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        David Girault <david.girault@...vo.com>,
        Romuald Despres <romuald.despres@...vo.com>,
        Frederic Blain <frederic.blain@...vo.com>,
        Nicolas Schodet <nico@...fr.eu.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH wpan-next v2 08/11] net: mac802154: Introduce a tx queue
 flushing mechanism

Hi,

On Thu, May 12, 2022 at 10:34 AM Miquel Raynal
<miquel.raynal@...tlin.com> wrote:
>
> Right now we are able to stop a queue but we have no indication if a
> transmission is ongoing or not.
>
> Thanks to recent additions, we can track the number of ongoing
> transmissions so we know if the last transmission is over. Adding on top
> of it an internal wait queue also allows to be woken up asynchronously
> when this happens. If, beforehands, we marked the queue to be held and
> stopped it, we end up flushing and stopping the tx queue.
>
> Thanks to this feature, we will soon be able to introduce a synchronous
> transmit API.
>
> Signed-off-by: Miquel Raynal <miquel.raynal@...tlin.com>
> ---
>  include/net/cfg802154.h      |  1 +
>  net/ieee802154/core.c        |  1 +
>  net/mac802154/cfg.c          |  2 +-
>  net/mac802154/ieee802154_i.h |  1 +
>  net/mac802154/tx.c           | 26 ++++++++++++++++++++++++--
>  net/mac802154/util.c         |  6 ++++--
>  6 files changed, 32 insertions(+), 5 deletions(-)
>
> diff --git a/include/net/cfg802154.h b/include/net/cfg802154.h
> index ad3f438e4583..8b6326aa2d42 100644
> --- a/include/net/cfg802154.h
> +++ b/include/net/cfg802154.h
> @@ -218,6 +218,7 @@ struct wpan_phy {
>         struct mutex queue_lock;
>         atomic_t ongoing_txs;
>         atomic_t hold_txs;
> +       wait_queue_head_t sync_txq;
>
>         char priv[] __aligned(NETDEV_ALIGN);
>  };
> diff --git a/net/ieee802154/core.c b/net/ieee802154/core.c
> index d81b7301e013..f13e3082d988 100644
> --- a/net/ieee802154/core.c
> +++ b/net/ieee802154/core.c
> @@ -129,6 +129,7 @@ wpan_phy_new(const struct cfg802154_ops *ops, size_t priv_size)
>         wpan_phy_net_set(&rdev->wpan_phy, &init_net);
>
>         init_waitqueue_head(&rdev->dev_wait);
> +       init_waitqueue_head(&rdev->wpan_phy.sync_txq);
>
>         mutex_init(&rdev->wpan_phy.queue_lock);
>
> diff --git a/net/mac802154/cfg.c b/net/mac802154/cfg.c
> index b51100fd9e3f..93df24f75572 100644
> --- a/net/mac802154/cfg.c
> +++ b/net/mac802154/cfg.c
> @@ -46,7 +46,7 @@ static int ieee802154_suspend(struct wpan_phy *wpan_phy)
>         if (!local->open_count)
>                 goto suspend;
>
> -       ieee802154_hold_queue(local);
> +       ieee802154_sync_and_hold_queue(local);
>         synchronize_net();
>
>         /* stop hardware - this must stop RX */
> diff --git a/net/mac802154/ieee802154_i.h b/net/mac802154/ieee802154_i.h
> index e34db1d49ef4..a057827fc48a 100644
> --- a/net/mac802154/ieee802154_i.h
> +++ b/net/mac802154/ieee802154_i.h
> @@ -124,6 +124,7 @@ extern struct ieee802154_mlme_ops mac802154_mlme_wpan;
>
>  void ieee802154_rx(struct ieee802154_local *local, struct sk_buff *skb);
>  void ieee802154_xmit_sync_worker(struct work_struct *work);
> +int ieee802154_sync_and_hold_queue(struct ieee802154_local *local);
>  netdev_tx_t
>  ieee802154_monitor_start_xmit(struct sk_buff *skb, struct net_device *dev);
>  netdev_tx_t
> diff --git a/net/mac802154/tx.c b/net/mac802154/tx.c
> index 607019b8f8ab..38f74b8b6740 100644
> --- a/net/mac802154/tx.c
> +++ b/net/mac802154/tx.c
> @@ -44,7 +44,8 @@ void ieee802154_xmit_sync_worker(struct work_struct *work)
>  err_tx:
>         /* Restart the netif queue on each sub_if_data object. */
>         ieee802154_release_queue(local);
> -       atomic_dec(&local->phy->ongoing_txs);
> +       if (!atomic_dec_and_test(&local->phy->ongoing_txs))
> +               wake_up(&local->phy->sync_txq);
>         kfree_skb(skb);
>         netdev_dbg(dev, "transmission failed\n");
>  }
> @@ -100,12 +101,33 @@ ieee802154_tx(struct ieee802154_local *local, struct sk_buff *skb)
>
>  err_wake_netif_queue:
>         ieee802154_release_queue(local);
> -       atomic_dec(&local->phy->ongoing_txs);
> +       if (!atomic_dec_and_test(&local->phy->ongoing_txs))
> +               wake_up(&local->phy->sync_txq);
>  err_free_skb:
>         kfree_skb(skb);
>         return NETDEV_TX_OK;
>  }
>
> +static int ieee802154_sync_queue(struct ieee802154_local *local)
> +{
> +       int ret;
> +
> +       ieee802154_hold_queue(local);
> +       ieee802154_disable_queue(local);
> +       wait_event(local->phy->sync_txq, !atomic_read(&local->phy->ongoing_txs));
> +       ret = local->tx_result;
> +       ieee802154_release_queue(local);

I am curious, why this extra hold, release here?

- Alex