netdev - Re: [PATCH v4 net-next] net: ftmac100: support mtu

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Date:   Mon, 24 Oct 2022 18:50:31 +0300
From:   Sergei Antonov <saproj@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, andrew@...n.ch
Subject: Re: [PATCH v4 net-next] net: ftmac100: support mtu > 1500

On Wed, 19 Oct 2022 at 21:42, Vladimir Oltean <olteanv@...il.com> wrote:
>
> On Wed, Oct 19, 2022 at 09:36:21PM +0300, Sergei Antonov wrote:
> > On Wed, 19 Oct 2022 at 19:55, Vladimir Oltean <olteanv@...il.com> wrote:
> > >
> > > On Wed, Oct 19, 2022 at 07:20:58PM +0300, Sergei Antonov wrote:
> > > > The ftmac100 controller considers some packets FTL (frame
> > > > too long) and drops them. An example of a dropped packet:
> > > > 6 bytes - dst MAC
> > > > 6 bytes - src MAC
> > > > 2 bytes - EtherType IPv4 (0800)
> > > > 1504 bytes - IPv4 packet
> > >
> > > Why do you insist writing these confusing messages?

Working on a better version of the patch. And here is another question.
Unless the flag is set, the controller drops packets bigger than 1518
(that is 1500 payload + 14 Ethernet header + 4 FCS). So if mtu is 1500
the driver can enable the controller's functionality (clear the
FTMAC100_MACCR_RX_FTL flag) and save CPU time. When mtu is less or
greater than 1500, should the driver do the following:
if (ftmac100_rxdes_frame_length(rxdes) > netdev->mtu + ETH_HLEN) {
    drop the packet
}
I mean, is it driver's duty to drop oversized packets?

In the current version of the driver, if, for example, mtu is 1400 and
the incoming packet is 1450 bytes it is not dropped. Or is mtu < 1500
incorrect for Ethernet cards so there is no need to write code
handling it?