Re: [PATCH net-next 2/2] net: phy: Add ability to debug RGMII connections

[Vladimir Oltean]

On Fri, 18 Oct 2019 at 16:23, Russell King - ARM Linux admin
<linux@xxxxxxxxxxxxxxx> wrote:
>
> On Fri, Oct 18, 2019 at 04:09:30PM +0300, Vladimir Oltean wrote:
> > Hi Andrew,
> >
> > On Fri, 18 Oct 2019 at 16:01, Andrew Lunn <andrew@xxxxxxx> wrote:
> > >
> > > > Well, that's the tricky part. You're sending a frame out, with no
> > > > guarantee you'll get the same frame back in. So I'm not sure that any
> > > > identifiers put inside the frame will survive.
> > > > How do the tests pan out for you? Do you actually get to trigger this
> > > > check? As I mentioned, my NIC drops the frames with bad FCS.
> > >
> > > My experience is, the NIC drops the frame and increments some the
> > > counter about bad FCS. I do very occasionally see a frame delivered,
> > > but i guess that is 1/65536 where the FCS just happens to be good by
> > > accident. So i think some other algorithm should be used which is
> > > unlikely to be good when the FCS is accidentally good, or just check
> > > the contents of the packet, you know what is should contain.
> > >
> > > Are there any NICs which don't do hardware FCS? Is that something we
> > > realistically need to consider?
> > >
> > > > Yes, but remember, nobody guarantees that a frame with DMAC
> > > > ff:ff:ff:ff:ff:ff on egress will still have it on its way back. Again,
> > > > this all depends on how you plan to manage the rx-all ethtool feature.
> > >
> > > Humm. Never heard that before. Are you saying some NICs rewrite the
> > > DMAN?
> > >
> >
> > I'm just trying to understand the circumstances under which this
> > kernel thread makes sense.
> > Checking for FCS validity means that the intention was to enable the
> > reception of frames with bad FCS.
> > Bad FCS after bad RGMII setup/hold times doesn't mean there's a small
> > guy in there who rewrites the checksum. It means that frame octets get
> > garbled. All octets are just as likely to get garbled, including the
> > SFD, preamble, DMAC, etc.
> > All I'm saying is that, if the intention of the patch is to actually
> > process the FCS of frames before and after, then it should actually
> > put the interface in promiscuous mode, so that frames with a
> > non-garbled SFD and preamble can still be received, even though their
> > DMAC was the one that got garbled.
>
> Isn't the point of this to see which RGMII setting results in a working
> setup?
>
> So, is it not true that what we're after is receiving a _correct_ frame
> that corresponds to the frame that was sent out?
>

Only true if the MAC does not drop bad frames by itself. Then the FCS
check in the kernel thread is superfluous.

> Hence, if the DMAC got changed, it's irrelevent whether we received the
> packet or not - since "no packet" || "changed packet" = fail.
>
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