linux-kernel - Re: Dropping NETIF_F_SG since no checksum feature.

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Message-ID: <adak637bjs3.fsf@cisco.com>
Date:	Tue, 10 Oct 2006 20:42:20 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	David Miller <davem@...emloft.net>
Cc:	mst@...lanox.co.il, shemminger@...l.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	openib-general@...nib.org, rolandd@...co.com
Subject: Re: Dropping NETIF_F_SG since no checksum feature.

    David> Also, if you don't do checksumming on the card we MUST copy
    David> the data (be it from a user buffer, or from a filesystem
    David> page cache page) into a private buffer since if the data
    David> changes the checksum would become invalid, as I mentioned
    David> in another email earlier.

Yes, I get that now -- I replied to Michael's email before I read yours.

    David> Therefore, since we have to copy anyways, it always is
    David> better to checksum in parallel with the copy.

Yes.

    David> So the whole idea of SG without hw-checksum support is
    David> without much merit at all.

Well, on IB it is possible to implement a netdevice (IPoIB connected
mode, I assume that's what Michael is working on) with a large MTU
(64KB is a number thrown around, but really there's not any limit) but
no HW checksum capability.  Doing that in a practical way means we
need to allow non-linear skbs to be passed in.

On the other hand I'm not sure how useful such a netdevice would be --
will non-sendfile() paths generate big packets even if the MTU is 64KB?

Maybe GSO gives us all the real advantages of this anyway?

 - R.
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