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Date:   Tue, 22 Dec 2020 00:25:34 +0200
From:   Vladimir Oltean <>
To:     Florian Fainelli <>
        "David S. Miller" <>,
        Jakub Kicinski <>,
        Murali Krishna Policharla <>,
        Vladimir Oltean <>,
        open list <>
Subject: Re: [PATCH net] net: systemport: set dev->max_mtu to

On Mon, Dec 21, 2020 at 01:49:03PM -0800, Florian Fainelli wrote:
> On 12/18/2020 1:17 PM, Florian Fainelli wrote:
> >>>>>>> SYSTEMPORT Lite does not actually validate the frame length, so setting
> >>>>>>> a maximum number to the buffer size we allocate could work, but I don't
> >>>>>>> see a reason to differentiate the two types of MACs here.
> >>>>>>
> >>>>>> And if the Lite doesn't validate the frame length, then shouldn't it
> >>>>>> report a max_mtu equal to the max_mtu of the attached DSA switch, plus
> >>>>>> the Broadcom tag length? Doesn't the b53 driver support jumbo frames?
> >>>>>
> >>>>> And how would I do that without create a horrible layering violation in
> >>>>> either the systemport driver or DSA? Yes the b53 driver supports jumbo
> >>>>> frames.
> >>>>
> >>>> Sorry, I don't understand where is the layering violation (maybe it doesn't
> >>>> help me either that I'm not familiar with Broadcom architectures).
> >>>>
> >>>> Is the SYSTEMPORT Lite always used as a DSA master, or could it also be
> >>>> used standalone? What would be the issue with hardcoding a max_mtu value
> >>>> which is large enough for b53 to use jumbo frames?
> >>>
> >>> SYSTEMPORT Lite is always used as a DSA master AFAICT given its GMII
> >>> Integration Block (GIB) was specifically designed with another MAC and
> >>> particularly that of a switch on the other side.
> >>>
> >>> The layering violation I am concerned with is that we do not know ahead
> >>> of time which b53 switch we are going to be interfaced with, and they
> >>> have various limitations on the sizes they support. Right now b53 only
> >>> concerns itself with returning JMS_MAX_SIZE, but I am fairly positive
> >>> this needs fixing given the existing switches supported by the driver.
> >>
> >> Maybe we don't need to over-engineer this. As long as you report a large
> >> enough max_mtu in the SYSTEMPORT Lite driver to accomodate for all
> >> possible revisions of embedded switches, and the max_mtu of the switch
> >> itself is still accurate and representative of the switch revision limits,
> >> I think that's good enough.
> >
> > I suppose that is fair, v2 coming, thanks!
> I was going to issue a v2 for this patch, but given that we don't
> allocate buffers larger than 2KiB and there is really no need to
> implement ndo_change_mtu(), is there really a point not to use
> UMAC_MAX_MTU_SIZE for both variants of the SYSTEMPORT MAC?

After your first reply that "the Lite doesn't validate the frame length", I was
under the impression that it is sufficient to declare a larger max_mtu such as
JMS_MAX_SIZE and 9K jumbo frames would just work. But with the current buffer
allocation in bcm_sysport_rx_refill it clearly wouldn't. A stupid confusion
really. So yeah, sorry for having you resend a v2 with no change.
If it helps you could add to the patch:

Reviewed-by: Vladimir Oltean <>

Thanks again for explaining.

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