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Date: Thu, 28 Jul 2016 15:18:26 +0200 From: LABBE Corentin <clabbe.montjoie@...il.com> To: Arnd Bergmann <arnd@...db.de> Cc: linux-arm-kernel@...ts.infradead.org, robh+dt@...nel.org, mark.rutland@....com, maxime.ripard@...e-electrons.com, wens@...e.org, linux@...linux.org.uk, davem@...emloft.net, devicetree@...r.kernel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com Subject: Re: [PATCH v2 1/5] ethernet: add sun8i-emac driver On Wed, Jul 20, 2016 at 11:56:12AM +0200, Arnd Bergmann wrote: > On Wednesday, July 20, 2016 10:03:16 AM CEST LABBE Corentin wrote: > > + > > + /* Benched on OPIPC with 100M, setting more than 256 does not give any > > + * perf boost > > + */ > > + priv->nbdesc_rx = 128; > > + priv->nbdesc_tx = 256; > > + > > > > 256 tx descriptors can introduce a significant latency. Can you add > support for BQL (netdev_sent_queue/netdev_completed_queue) to limit > the queue size to the minimum? Done, since setting below 256 give lower performance with iperf. > > I also noticed that your tx_lock() prevents you from concurrently > running sun8i_emac_complete_xmit() and sun8i_emac_xmit(). Is that > necessary? I'd think that you can find a way to make them work > concurrently. > > Arnd I will reworked locking and it seems that no locking is necessary. I have added the following comment about the locking strategy: /* Locking strategy: * RX queue does not need any lock since only sun8i_emac_poll() access it. * (All other RX modifiers (ringparam/ndo_stop) disable NAPI and so sun8i_emac_poll()) * TX queue is handled by sun8i_emac_xmit(), sun8i_emac_complete_xmit() and sun8i_emac_tx_timeout() * (All other RX modifiers (ringparam/ndo_stop) disable NAPI and stop queue) * * sun8i_emac_xmit() could fire only once (netif_tx_lock) * sun8i_emac_complete_xmit() could fire only once (called from NAPI) * sun8i_emac_tx_timeout() could fire only once (netif_tx_lock) and couldnt * race with sun8i_emac_xmit (due to netif_tx_lock) and with sun8i_emac_complete_xmit which disable NAPI. * * So only sun8i_emac_xmit and sun8i_emac_complete_xmit could fire at the same time. * But they never could modify the same descriptors: * - sun8i_emac_complete_xmit() will modify only descriptors with empty status * - sun8i_emac_xmit() will modify only descriptors set to DCLEAN * Proper memory barriers ensure that descriptor set to DCLEAN could not be * modified latter by sun8i_emac_complete_xmit(). * */ Does I am right ? Thanks for your review. Regards LABBE Corentin
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