Re: [PATCH v2 4/4] pci: save the boot pcie link speed and restore it on fini

From: Bjorn Helgaas <helgaas@kernel.org>
To: Karol Herbst <kherbst@redhat.com>
Cc: nouveau <nouveau@lists.freedesktop.org>,
	Lyude Paul <lyude@redhat.com>,
	linux-pci@vger.kernel.org
Subject: Re: [PATCH v2 4/4] pci: save the boot pcie link speed and restore it on fini
Date: Tue, 21 May 2019 08:10:34 -0500	[thread overview]
Message-ID: <20190521131033.GC57618@google.com> (raw)
In-Reply-To: <CACO55tsHi+4gRMC=XqOypJttztKNe5oKxxk0eqEVxGZoMzS+4Q@mail.gmail.com>

On Tue, May 21, 2019 at 12:30:38AM +0200, Karol Herbst wrote:
> On Mon, May 20, 2019 at 11:20 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Tue, May 07, 2019 at 10:12:45PM +0200, Karol Herbst wrote:
> > > Apperantly things go south if we suspend the device with a different PCIE
> > > link speed set than it got booted with. Fixes runtime suspend on my gp107.
> > >
> > > This all looks like some bug inside the pci subsystem and I would prefer a
> > > fix there instead of nouveau, but maybe there is no real nice way of doing
> > > that outside of drivers?
> >
> > I agree it would be nice to fix this in the PCI core if that's
> > feasible.
> >
> > It looks like this driver changes the PCIe link speed using some
> > device-specific mechanism.  When we suspend, we put the device in
> > D3cold, so it loses all its state.  When we resume, the link probably
> > comes up at the boot speed because nothing did that device-specific
> > magic to change it, so you probably end up with the link being slow
> > but the driver thinking it's configured to be fast, and maybe that
> > combination doesn't work.
> >
> > If it requires something device-specific to change that link speed, I
> > don't know how to put that in the PCI core.  But maybe I'm missing
> > something?
> >
> > Per the PCIe spec (r4.0, sec 1.2):
> >
> >   Initialization – During hardware initialization, each PCI Express
> >   Link is set up following a negotiation of Lane widths and frequency
> >   of operation by the two agents at each end of the Link. No firmware
> >   or operating system software is involved.
> >
> > I have been assuming that this means device-specific link speed
> > management is out of spec, but it seems pretty common that devices
> > don't come up by themselves at the fastest possible link speed.  So
> > maybe the spec just intends that devices can operate at *some* valid
> > speed.
> 
> I would expect that devices kind of have to figure out what they can
> operate on and the operating system kind of just checks what the
> current state is and doesn't try to "restore" the old state or
> something?

The devices at each end of the link negotiate the width and speed of
the link.  This is done directly by the hardware without any help from
the OS.

The OS can read the current link state (Current Link Speed and
Negotiated Link Width, both in the Link Status register).  The OS has
very little control over that state.  It can't directly restore the
state because the hardware has to negotiate a width & speed that
result in reliable operation.

> We don't do anything in the driver after the device was suspended. And
> the 0x88000 is a mirror of the PCI config space, but we also got some
> PCIe stuff at 0x8c000 which is used by newer GPUs for gen3 stuff
> essentially. I have no idea how much of this is part of the actual pci
> standard and how much is driver specific. But the driver also wants to
> have some control over the link speed as it's tight to performance
> states on GPU.

As far as I'm aware, there is no generic PCIe way for the OS to
influence the link width or speed.  If the GPU driver needs to do
that, it would be via some device-specific mechanism.

> The big issue here is just, that the GPU boots with 8.0, some on-gpu
> init mechanism decreases it to 2.5. If we suspend, the GPU or at least
> the communication with the controller is broken. But if we set it to
> the boot speed, resuming the GPU just works. So my assumption was,
> that _something_ (might it be the controller or the pci subsystem)
> tries to force to operate on an invalid link speed and because the
> bridge controller is actually powered down as well (as all children
> are in D3cold) I could imagine that something in the pci subsystem
> actually restores the state which lets the controller fail to
> establish communication again?

  1) At boot-time, the Port and the GPU hardware negotiate 8.0 GT/s
     without OS/driver intervention.

  2) Some mechanism reduces link speed to 2.5 GT/s.  This probably
     requires driver intervention or at least some ACPI method.

  3) Suspend puts GPU into D3cold (powered off).

  4) Resume restores GPU to D0, and the Port and GPU hardware again
     negotiate 8.0 GT/s without OS/driver intervention, just like at
     initial boot.

  5) Now the driver thinks the GPU is at 2.5 GT/s but it's actually at
     8.0 GT/s.

Without knowing more about the transition to 2.5 GT/s, I can't guess
why the GPU wouldn't work after resume.  From a PCIe point of view,
the link is supposed to work and the device should be reachable
independent of the link speed.  But maybe there's some weird
dependency between the GPU and the driver here.

It sounds like things work if you return to 8.0 GT/s before suspend,
things work.  That would make sense to me because then the driver's
idea of the link state after resume would match the actual state.

But I don't see a way to deal with this in the PCI core.  The PCI core
does save and restore most of the architected config space around
suspend/resume, but since this appears to be a device-specific thing,
the PCI core would have no idea how to save/restore it.

> > > Signed-off-by: Karol Herbst <kherbst@redhat.com>
> > > Reviewed-by: Lyude Paul <lyude@redhat.com>
> > > ---
> > >  drm/nouveau/include/nvkm/subdev/pci.h |  5 +++--
> > >  drm/nouveau/nvkm/subdev/pci/base.c    |  9 +++++++--
> > >  drm/nouveau/nvkm/subdev/pci/pcie.c    | 24 ++++++++++++++++++++----
> > >  drm/nouveau/nvkm/subdev/pci/priv.h    |  2 ++
> > >  4 files changed, 32 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/drm/nouveau/include/nvkm/subdev/pci.h b/drm/nouveau/include/nvkm/subdev/pci.h
> > > index 1fdf3098..b23793a2 100644
> > > --- a/drm/nouveau/include/nvkm/subdev/pci.h
> > > +++ b/drm/nouveau/include/nvkm/subdev/pci.h
> > > @@ -26,8 +26,9 @@ struct nvkm_pci {
> > >       } agp;
> > >
> > >       struct {
> > > -             enum nvkm_pcie_speed speed;
> > > -             u8 width;
> > > +             enum nvkm_pcie_speed cur_speed;
> > > +             enum nvkm_pcie_speed def_speed;
> > > +             u8 cur_width;
> > >       } pcie;
> > >
> > >       bool msi;
> > > diff --git a/drm/nouveau/nvkm/subdev/pci/base.c b/drm/nouveau/nvkm/subdev/pci/base.c
> > > index ee2431a7..d9fb5a83 100644
> > > --- a/drm/nouveau/nvkm/subdev/pci/base.c
> > > +++ b/drm/nouveau/nvkm/subdev/pci/base.c
> > > @@ -90,6 +90,8 @@ nvkm_pci_fini(struct nvkm_subdev *subdev, bool suspend)
> > >
> > >       if (pci->agp.bridge)
> > >               nvkm_agp_fini(pci);
> > > +     else if (pci_is_pcie(pci->pdev))
> > > +             nvkm_pcie_fini(pci);
> > >
> > >       return 0;
> > >  }
> > > @@ -100,6 +102,8 @@ nvkm_pci_preinit(struct nvkm_subdev *subdev)
> > >       struct nvkm_pci *pci = nvkm_pci(subdev);
> > >       if (pci->agp.bridge)
> > >               nvkm_agp_preinit(pci);
> > > +     else if (pci_is_pcie(pci->pdev))
> > > +             nvkm_pcie_preinit(pci);
> > >       return 0;
> > >  }
> > >
> > > @@ -193,8 +197,9 @@ nvkm_pci_new_(const struct nvkm_pci_func *func, struct nvkm_device *device,
> > >       pci->func = func;
> > >       pci->pdev = device->func->pci(device)->pdev;
> > >       pci->irq = -1;
> > > -     pci->pcie.speed = -1;
> > > -     pci->pcie.width = -1;
> > > +     pci->pcie.cur_speed = -1;
> > > +     pci->pcie.def_speed = -1;
> > > +     pci->pcie.cur_width = -1;
> > >
> > >       if (device->type == NVKM_DEVICE_AGP)
> > >               nvkm_agp_ctor(pci);
> > > diff --git a/drm/nouveau/nvkm/subdev/pci/pcie.c b/drm/nouveau/nvkm/subdev/pci/pcie.c
> > > index 70ccbe0d..731dd30e 100644
> > > --- a/drm/nouveau/nvkm/subdev/pci/pcie.c
> > > +++ b/drm/nouveau/nvkm/subdev/pci/pcie.c
> > > @@ -85,6 +85,13 @@ nvkm_pcie_oneinit(struct nvkm_pci *pci)
> > >       return 0;
> > >  }
> > >
> > > +int
> > > +nvkm_pcie_preinit(struct nvkm_pci *pci)
> > > +{
> > > +     pci->pcie.def_speed = nvkm_pcie_get_speed(pci);
> > > +     return 0;
> > > +}
> > > +
> > >  int
> > >  nvkm_pcie_init(struct nvkm_pci *pci)
> > >  {
> > > @@ -105,12 +112,21 @@ nvkm_pcie_init(struct nvkm_pci *pci)
> > >       if (pci->func->pcie.init)
> > >               pci->func->pcie.init(pci);
> > >
> > > -     if (pci->pcie.speed != -1)
> > > -             nvkm_pcie_set_link(pci, pci->pcie.speed, pci->pcie.width);
> > > +     if (pci->pcie.cur_speed != -1)
> > > +             nvkm_pcie_set_link(pci, pci->pcie.cur_speed,
> > > +                                pci->pcie.cur_width);
> > >
> > >       return 0;
> > >  }
> > >
> > > +int
> > > +nvkm_pcie_fini(struct nvkm_pci *pci)
> > > +{
> > > +     if (!IS_ERR_VALUE(pci->pcie.def_speed))
> > > +             return nvkm_pcie_set_link(pci, pci->pcie.def_speed, 16);
> > > +     return 0;
> > > +}
> > > +
> > >  int
> > >  nvkm_pcie_set_link(struct nvkm_pci *pci, enum nvkm_pcie_speed speed, u8 width)
> > >  {
> > > @@ -146,8 +162,8 @@ nvkm_pcie_set_link(struct nvkm_pci *pci, enum nvkm_pcie_speed speed, u8 width)
> > >               speed = max_speed;
> > >       }
> > >
> > > -     pci->pcie.speed = speed;
> > > -     pci->pcie.width = width;
> > > +     pci->pcie.cur_speed = speed;
> > > +     pci->pcie.cur_width = width;
> > >
> > >       if (speed == cur_speed) {
> > >               nvkm_debug(subdev, "requested matches current speed\n");
> > > diff --git a/drm/nouveau/nvkm/subdev/pci/priv.h b/drm/nouveau/nvkm/subdev/pci/priv.h
> > > index a0d4c007..e7744671 100644
> > > --- a/drm/nouveau/nvkm/subdev/pci/priv.h
> > > +++ b/drm/nouveau/nvkm/subdev/pci/priv.h
> > > @@ -60,5 +60,7 @@ enum nvkm_pcie_speed gk104_pcie_max_speed(struct nvkm_pci *);
> > >  int gk104_pcie_version_supported(struct nvkm_pci *);
> > >
> > >  int nvkm_pcie_oneinit(struct nvkm_pci *);
> > > +int nvkm_pcie_preinit(struct nvkm_pci *);
> > >  int nvkm_pcie_init(struct nvkm_pci *);
> > > +int nvkm_pcie_fini(struct nvkm_pci *);
> > >  #endif
> > > --
> > > 2.21.0
> > >