netdev - RE: [PATCH net-next 5/5] net: mvpp2: jumbo frames support

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Date:   Sun, 4 Mar 2018 09:42:39 +0000
From:   Stefan Chulski <stefanc@...vell.com>
To:     Thomas Petazzoni <thomas.petazzoni@...tlin.com>
CC:     Antoine Tenart <antoine.tenart@...tlin.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "maxime.chevallier@...tlin.com" <maxime.chevallier@...tlin.com>,
        "gregory.clement@...tlin.com" <gregory.clement@...tlin.com>,
        "miquel.raynal@...tlin.com" <miquel.raynal@...tlin.com>,
        Nadav Haklai <nadavh@...vell.com>,
        Yan Markman <ymarkman@...vell.com>,
        "mw@...ihalf.com" <mw@...ihalf.com>
Subject: RE: [PATCH net-next 5/5] net: mvpp2: jumbo frames support

> > To perform checksum in HW, HW obviously should work in store and
> forward mode. Store all frame in TX FIFO and then check checksum.
> > If mtu 1500B, everything fine and all port can do this.
> >
> > If mtu is 9KB and 9KB frame transmitted, Port 0 still can do HW checksum.
> But ports 1 and 2 doesn't has enough FIFO for this.
> > So we cannot offload this feature and SW should perform checksum.
> 
> So perhaps the real check should not be "port 0", but whether the MTU is
> higher or lower than the TX FIFO size assigned to the current port.
> This would express in much better way the reason why HW checksum can be
> used or not.

I really don't want involve MTU size here, for each packet we should add to MTU overhead added by HW(offset, CRC, DSA tags and etc).
I prefer just to check: port TX FIFO size is 10KB -> port can support HW checksum offload.
Do you suggest to keep some shadow table with ports TX FIFO sizes for this?  

Thanks,
Stefan.