netdev - Re: [PATCH net-next 0/3] macb: support the 2-deep Tx queue on at91

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Message-ID: <20201013170358.1a4d282a@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 13 Oct 2020 17:03:58 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Willy Tarreau <w@....eu>
Cc:     Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next 0/3] macb: support the 2-deep Tx queue on at91

On Sun, 11 Oct 2020 11:09:41 +0200 Willy Tarreau wrote:
> while running some tests on my Breadbee board, I noticed poor network
> Tx performance. I had a look at the driver (macb, at91ether variant)
> and noticed that at91ether_start_xmit() immediately stops the queue
> after sending a frame and waits for the interrupt to restart the queue,
> causing a dead time after each packet is sent.
> 
> The AT91RM9200 datasheet states that the controller supports two frames,
> one being sent and the other one being queued, so I performed minimal
> changes to support this. The transmit performance on my board has
> increased by 50% on medium-sized packets (HTTP traffic), and with large
> packets I can now reach line rate.
> 
> Since this driver is shared by various platforms, I tried my best to
> isolate and limit the changes as much as possible and I think it's pretty
> reasonable as-is. I've run extensive tests and couldn't meet any
> unexpected situation (no stall, overflow nor lockup).
> 
> There are 3 patches in this series. The first one adds the missing
> interrupt flag for RM9200 (TBRE, indicating the tx buffer is willing
> to take a new packet). The second one replaces the single skb with a
> 2-array and uses only index 0. It does no other change, this is just
> to prepare the code for the third one. The third one implements the
> queue. Packets are added at the tail of the queue, the queue is
> stopped at 2 packets and the interrupt releases 0, 1 or 2 depending
> on what the transmit status register reports.

LGTM. There's always a chance that this will make other 
designs explode, but short of someone from Cadence giving 
us a timely review we have only one way to find that out.. :)

Applied, thanks!