netdev - Re: [PATCH v4 06/11] net: ethernet: mtk-eth-mac: new driver

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Date:   Fri, 22 May 2020 09:49:49 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Stephane Le Provost <stephane.leprovost@...iatek.com>,
        Pedro Tsai <pedro.tsai@...iatek.com>,
        Andrew Perepech <andrew.perepech@...iatek.com>,
        Rob Herring <robh+dt@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Matthias Brugger <matthias.bgg@...il.com>,
        John Crispin <john@...ozen.org>,
        Sean Wang <sean.wang@...iatek.com>,
        Mark Lee <Mark-MC.Lee@...iatek.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Fabien Parent <fparent@...libre.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Edwin Peer <edwin.peer@...adcom.com>,
        DTML <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/Mediatek SoC..." 
        <linux-mediatek@...ts.infradead.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH v4 06/11] net: ethernet: mtk-eth-mac: new driver

On Fri, May 22, 2020 at 9:44 AM Bartosz Golaszewski <brgl@...ev.pl> wrote:
> śr., 20 maj 2020 o 23:23 Arnd Bergmann <arnd@...db.de> napisał(a):
> > On Wed, May 20, 2020 at 7:35 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:
> > > śr., 20 maj 2020 o 16:37 Arnd Bergmann <arnd@...db.de> napisał(a):

> > > My thinking was this: if I mask the relevant interrupt (TX/RX
> > > complete) and ack it right away, the status bit will be asserted on
> > > the next packet received/sent but the process won't get interrupted
> > > and when I unmask it, it will fire right away and I won't have to
> > > recheck the status register. I noticed that if I ack it at the end of
> > > napi poll callback, I end up missing certain TX complete interrupts
> > > and end up seeing a lot of retransmissions even if I reread the status
> > > register. I'm not yet sure where this race happens.
> >
> > Right, I see. If you just ack at the end of the poll function, you need
> > to check the rings again to ensure you did not miss an interrupt
> > between checking observing both rings to be empty and the irq-ack.
> >
> > I suspect it's still cheaper to check the two rings with an uncached
> > read from memory than to to do the read-modify-write on the mmio,
> > but you'd have to measure that to be sure.
> >
>
> Unfortunately the PHY on the board I have is 100Mbps which is the
> limiting factor in benchmarking this driver. :(
>
> If you're fine with this - I'd like to fix the minor issues you
> pointed out and stick with the current approach for now. We can always
> fix the implementation in the future once a board with a Gigabit PHY
> is out. Most ethernet drivers don't use such fine-grained interrupt
> control anyway. I expect the performance differences to be miniscule
> really.

Ok, fair enough. The BQL limiting is the part that matters the most
for performance on slow lines (preventing long latencies from
buffer bloat), and  you have that now.

       Arnd