tech-net archive

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Old Index]

Re: half-/full-duplex ethernet and ifconfig

Hi, David.

(2012/11/14 8:56), David Young wrote:
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 think if mediaopt full-duplex isn't set, it act as half-duplex
and it really act as I expected.

 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


         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

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),
         if (sc->mii_capabilities & BMSR_10TFDX) {
                 ADD(IFM_MAKEWORD(IFM_ETHER, IFM_10_T, IFM_FDX, sc->mii_inst),
                 fdx = 1;

The BMSR register give us the "capability" of the PHY. The ANAR register
is set from mii_capabilities and argument of SIOCSIFMEDIA.

This tells me that we're using 0, not IFM_HDX, to represent
"half-duplex"---or do I have the wrong interpretation

 In my opinion, IFM_HDX should be removed. If the duplex has a state other
than full-duplex and half-duplex, it's ok to have bot IFM_FDX and IFM_HDX.
If the duplex setting has only two state (i.e. full and half), the best way
is to use only one flag to identify it.

---and there is no
way to select a rate setting independent of a duplex setting.

 Currently, we have no API to specify an advertise setting for auto
negotiation particularly. I think it's useful on some cases.

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 |= 
../../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?


                SAITOH Masanobu (

Home | Main Index | Thread Index | Old Index