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Date:      Thu, 5 Dec 2013 10:05:22 -0500
From:      John Baldwin <jhb@freebsd.org>
To:        "=?iso-8859-15?q?Jean-S=E9bastien?= =?iso-8859-15?q?_P=E9dron?=" <dumbbell@freebsd.org>
Cc:        svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org
Subject:   Re: svn commit: r258930 - head/sys/dev/drm2
Message-ID:  <201312051005.23197.jhb@freebsd.org>
In-Reply-To: <52A04D2E.2040705@FreeBSD.org>
References:  <201312041904.rB4J4vbM043709@svn.freebsd.org> <201312041518.33731.jhb@freebsd.org> <52A04D2E.2040705@FreeBSD.org>
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On Thursday, December 05, 2013 4:53:50 am Jean-S=E9bastien P=E9dron wrote:
> On 04.12.2013 21:18, John Baldwin wrote:
> > On Wednesday, December 04, 2013 2:04:57 pm Jean-Sebastien Pedron wrote:
> >> Author: dumbbell
> >> Date: Wed Dec  4 19:04:56 2013
> >> New Revision: 258930
> >> URL: http://svnweb.freebsd.org/changeset/base/258930
> >>
> >> Log:
> >>   drm: Read PCIER_LINK_CAP/PCIER_LINK_CAP2 from the PCI bridge
> >>  =20
> >>   Before this fix, capabilities were read from vgapci and were incorre=
ct.
> >=20
> > Eh, vgapci is the right place to read this.  The LINK_CAP here is telli=
ng
> > you the width of the slot you are plugged into, not the width of the ca=
rd
> > that is plugged into the slot.
>=20
> I'm sorry, my knowledge of PCI is very limited (still learning) and I
> don't understand your comment. Could you please expand on it?
>=20
> I believe this code has the correct result (eg, same result as Linux 3.8
> on the same computer), but maybe I'm doing it wrong and I was just lucky.

(First off, I thought you were reading link width, but you're not, you are
reading the speed of an individual lane.)

Each end of a PCI express link has these config registers.  The link
capability register contains the maximum supported link speed of
that "side" of the link.  So the 'pcib' device is the "top" of the link
(i.e. the physical slot you plug a card into).  When you ask it for its
capable link speed, you are getting the maximum speed the slot supports.
When you ask vgapci0 for its maximum capable link width, you are getting the
maximum speed the card itself supports.  So, for example, if you plug a
2.5 GT/s PCI-e card into a 5.0 GT/s slot, the card ('vgapci0') would report=
 a
maximum speed of 2.5, while the slot ('pcibX') would report a maximum speed
of 5.0.  In addition, each PCI-e device includes a link status register that
contains the current active link speed.  You can see these register values
using 'pciconf -lc'.  For example, on my desktop at home, the parent
device of vgapci0 (an nvidia card), is pcib1:

    pcib0
      pci0
        hostb0
        pcib1
          pci1
            vgapci0
              vgapm0
              nvidia0

In pciconf I see this output for these devices:

pcib1@pci0:0:1:0:       class=3D0x060400 card=3D0x77511462 chip=3D0x0101808=
6 rev=3D0x09 hdr=3D0x01
    cap 0d[88] =3D PCI Bridge card=3D0x77511462
    cap 01[80] =3D powerspec 3  supports D0 D3  current D0
    cap 05[90] =3D MSI supports 1 message=20
    cap 10[a0] =3D PCI-Express 2 root port slot max data 128(128) link x16(=
x16)
                 speed 2.5(5.0) ASPM disabled(L0s/L1)
ecap 0002[100] =3D VC 1 max VC0
ecap 0005[140] =3D Root Complex Link Declaration 1
=2E..
vgapci0@pci0:1:0:0:     class=3D0x030000 card=3D0x15263842 chip=3D0x104010d=
e rev=3D0xa1 hdr=3D0x00
    cap 01[60] =3D powerspec 3  supports D0 D3  current D0
    cap 05[68] =3D MSI supports 1 message, 64 bit=20
    cap 10[78] =3D PCI-Express 2 endpoint max data 128(128) link x16(x16)
                 speed 2.5(2.5) ASPM disabled(L0s/L1)
    cap 09[b4] =3D vendor (length 20)
ecap 0002[100] =3D VC 1 max VC0
ecap 0004[128] =3D Power Budgeting 1
ecap 000b[600] =3D Vendor 1 ID 1

So in this case the "slot" supports a maximum link speed of 5.0 GT/s (it is=
 a
gen3 slot on a SandyBridge motherboard), but the nvidia card is only a gen2
device that supports 2.5GT/s, so the entire link is running at 2.5 GT/s (in
the 'speed A(B)' text above, 'B' is the maximum link speed from LINK_CAP and
'A' is the actual link speed from LINK_STA).

Can you provide pciconf -lc output from your machine and tell me what you t=
hink
the function should be returning (i.e. are you trying to determine the speed
of the slot, or the speed of the card?)

=2D-=20
John Baldwin

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