half-/full-duplex ethernet and ifconfig

[David Young <dyoung%pobox.com@localhost>]

Today I noticed a funny thing about the ethernet media settings that I
can see and choose through 'ifconfig wm0':

        supported Ethernet media:
                media none
                media 10baseT
                media 10baseT mediaopt full-duplex
                media 100baseTX
                media 100baseTX mediaopt full-duplex
                media 1000baseT
                media 1000baseT mediaopt full-duplex
                media autoselect

What's the difference between 'media 10baseT' and 'media 10baseT
mediaopt full-duplex' ?

I thought at first that if I did 'ifconfig wm0 media 10baseT' then it
was up to my NIC and its link partner to negotiate the duplex setting.
I.e., that if I don't specify a duplex setting, then the system assumes
that I don't care.  I also expected that whichever duplex setting was
negotiated, I could read it out again using 'ifconfig wm0'.  So if I set
'ifconfig wm0 media 10baseT', I should see either

        media: Ethernet 10baseT (10baseT full-duplex)
        status: active

or

        media: Ethernet 10baseT (10baseT half-duplex)
        status: active

since the kernel defines non-zero flags for both (IFM_HDX, IFM_FDX).
Instead, I see this:

        media: Ethernet 10baseT
        status: active

Grovelling a bit in the ifconfig(8) code, I see that that
parentheses-less output means the "active" (negotiated) media and the
"current" (selected) media setting are the same: they're both Ethernet
10baseT.

Digging deeper, I find these lines in mii_phy_add_media():

        if (sc->mii_capabilities & BMSR_10THDX) {
                ADD(IFM_MAKEWORD(IFM_ETHER, IFM_10_T, 0, sc->mii_inst),
                    MII_MEDIA_10_T);
                PRINT("10baseT");
        }
        if (sc->mii_capabilities & BMSR_10TFDX) {
                ADD(IFM_MAKEWORD(IFM_ETHER, IFM_10_T, IFM_FDX, sc->mii_inst),
                    MII_MEDIA_10_T_FDX);
                PRINT("10baseT-FDX");
                fdx = 1;
        }

This tells me that we're using 0, not IFM_HDX, to represent
"half-duplex"---or do I have the wrong interpretation---and there is no
way to select a rate setting independent of a duplex setting.

But hold on a minute, I see some PHY drivers are using both IFM_HDX
IFM_FDX at least to tell what the negotiated media is:

% grep IFM_HDX ../../dev/mii/*
../../dev/mii/atphy.c:          mii->mii_media_active |= IFM_HDX;
../../dev/mii/bmtphy.c:                 mii->mii_media_active |= IFM_HDX;
../../dev/mii/brgphy.c:                         mii->mii_media_active |= 
IFM_HDX;
../../dev/mii/etphy.c:          mii->mii_media_active |= IFM_HDX;
../../dev/mii/rdcphy.c:         mii->mii_media_active |= IFM_HDX;
../../dev/mii/rgephy.c:                 mii->mii_media_active |= IFM_HDX;
../../dev/mii/rgephy.c:                 mii->mii_media_active |= IFM_HDX;

I am having a hard time seeing the design intention here, can someone
help me out?

Dave

-- 
David Young
dyoung%pobox.com@localhost    Urbana, IL    (217) 721-9981