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From:   Rafal Ozieblo <rafalo@cadence.com>
To:     David Miller <davem@davemloft.net>
CC:     "nicolas.ferre@microchip.com" <nicolas.ferre@microchip.com>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Piotr Sroka <piotrs@cadence.com>,
        Lukasz Tyrala <ltyrala@cadence.com>
Subject: RE: [PATCH 0/3] Receive Side Coalescing for macb driver
Thread-Topic: [PATCH 0/3] Receive Side Coalescing for macb driver
Thread-Index: AQHT1DKrh4uO12ninEi+8bwBioxkGqQDgUeAgAEnuEA=
Date:   Tue, 17 Apr 2018 08:59:35 +0000
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From: David Miller [mailto:davem@davemloft.net]=20
Sent: 16 kwietnia 2018 17:09

> From: Rafal Ozieblo <rafalo@cadence.com>
> Date: Sat, 14 Apr 2018 21:53:07 +0100
>
>> This patch series adds support for receive side coalescing for Cadence=20
>> GEM driver. Receive segmentation coalescing is a mechanism to reduce=20
>> CPU overhead. This is done by coalescing received TCP message segments=20
>> together into a single large message. This means that when the message
>> is complete the CPU only has to process the single header and act upon=20
>> the one data payload.
>
> You're going to have to think more deeply about enabling this feature.
>
> If you can't adjust the receive buffer offset, then the various packet he=
ader fields will be unaligned.
>
> On certain architectures this will result in unaligned traps all over the=
 networking stack as the packet is being processed.
>
> So enabling this by default will hurt performance on such systems a lot.
>
> The whole "skb_reserve(skb, NET_IP_ALIGN)" is not just for fun, it is abs=
olutely essential.

I totally agree with you. But the issue is with IP cores which has this fea=
ture implemented in.
Even when user does not want to use that feature but he bought IP with conf=
iguration supported RSC, then he has to switch off IP alignment.
There is no IP alignment with RSC in the GEM:
"When the gem rsc define has been set the receive buffer offset cannot be c=
hanged in the network configuration register."
If IP supports RSC and skb has 2B reserved for alignment we end up with non=
e packets receive correctly (2B missing in the each skb).
We can either leave few customers without support in Linux driver or let th=
em use the driver with decrease performance.