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From:   Sven Van Asbroeck <thesven73@gmail.com>
Date:   Tue, 8 Dec 2020 18:02:30 -0500
Message-ID: <CAGngYiVp2u-A07rrkbeJCbqPW9efjkJUNC+NBxrtCM2JtXGpVA@mail.gmail.com>
Subject: Re: [PATCH net v1 2/2] lan743x: boost performance: limit PCIe
 bandwidth requirement
To:     Andrew Lunn <andrew@lunn.ch>
Cc:     Jakub Kicinski <kuba@kernel.org>,
        Bryan Whitehead <bryan.whitehead@microchip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@microchip.com>,
        David S Miller <davem@davemloft.net>,
        netdev <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi Andrew,

On Tue, Dec 8, 2020 at 5:51 PM Andrew Lunn <andrew@lunn.ch> wrote:
>
> >
> > So I assumed that it's a PCIe dma bandwidth issue, but I could be wrong -
> > I didn't do any PCIe bandwidth measurements.
>
> Sometimes it is actually cache operations which take all the
> time. This needs to invalidate the cache, so that when the memory is
> then accessed, it get fetched from RAM. On SMP machines, cache
> invalidation can be expensive, due to all the cross CPU operations.
> I've actually got better performance by building a UP kernel on some
> low core count ARM CPUs.
>
> There are some tricks which can be played. Do you actually need all
> 9K? Does the descriptor tell you actually how much is used? You can
> get a nice speed up if you just unmap 64 bytes for a TCP ACK, rather
> than the full 9K.
>

Thank you for the suggestion! The original driver developer chose 9K because
presumably that's the largest frame size supported by the chip.

Yes, I believe the chip will tell us via the descriptor how much it has
written, I would have to double-check. I was already looking for a
"trick" to transfer only the required number of bytes, but I was led to
believe that dma_map_single() and dma_unmap_single() always needed to match.

So:
dma_map_single(9K) followed by dma_unmap_single(9K) is correct, and
dma_map_single(9K) followed by dma_unmap_single(1500 bytes) means trouble.

How can we get around that?