netdev - RE: [PATCH net v2 3/4] bnxt: make sure xmit_more + errors does not miss doorbells

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Date:   Fri, 13 Aug 2021 08:35:39 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jakub Kicinski' <kuba@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "michael.chan@...adcom.com" <michael.chan@...adcom.com>,
        "huangjw@...adcom.com" <huangjw@...adcom.com>,
        "eddie.wai@...adcom.com" <eddie.wai@...adcom.com>,
        "prashant@...adcom.com" <prashant@...adcom.com>,
        "gospo@...adcom.com" <gospo@...adcom.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "edwin.peer@...adcom.com" <edwin.peer@...adcom.com>
Subject: RE: [PATCH net v2 3/4] bnxt: make sure xmit_more + errors does not
 miss doorbells

From: Jakub Kicinski
> Sent: 11 August 2021 22:38
> 
> skbs are freed on error and not put on the ring. We may, however,
> be in a situation where we're freeing the last skb of a batch,
> and there is a doorbell ring pending because of xmit_more() being
> true earlier. Make sure we ring the door bell in such situations.
> 
> Since errors are rare don't pay attention to xmit_more() and just
> always flush the pending frames.
> 
...
> +tx_free:
>  	dev_kfree_skb_any(skb);
> +tx_kick_pending:
> +	tx_buf->skb = NULL;
> +	if (txr->kick_pending)
> +		bnxt_txr_db_kick(bp, txr, txr->tx_prod);
>  	return NETDEV_TX_OK;

Is this case actually so unlikely that the 'kick' can be
done unconditionally?
Then all the conditionals can be removed from the hot path.

	David

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