Re: [PATCH v1] PCI: brcmstb: Fix regression regarding missing PCIe linkup

[Jim Quinlan]

On Thu, May 19, 2022 at 12:10 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> [+to Rob for my naive DT questions]
>
> On Wed, May 18, 2022 at 03:42:11PM -0400, Jim Quinlan wrote:
> > commit 93e41f3fca3d ("PCI: brcmstb: Add control of subdevice voltage regulators")
> >
> > introduced a regression on the PCIe RPi4 Compute Module.  If the PCIe
> > endpoint node described in [2] was missing, no linkup would be attempted,
> > and subsequent accesses would cause a panic because this particular PCIe HW
> > causes a CPU abort on illegal accesses (instead of returning 0xffffffff).
> >
> > We fix this by allowing the DT endpoint subnode to be missing.  This is
> > important for platforms like the CM4 which have a standard PCIe socket and
> > the endpoint device is unknown.
>
> I assume you're referring specifically to making this optional in the
> DT:
>
>     /* PCIe endpoint */
>     pci-ep@0,0 {
>             assigned-addresses =
>                 <0x82010000 0x0 0xf8000000 0x6 0x00000000 0x0 0x2000>;
>             reg = <0x0 0x0 0x0 0x0 0x0>;
>             compatible = "pci14e4,1688";
>     };
>
Actually, both that and the node that contains it, i.e. pci@0,0.

> I don't really understand what's going on here, but I assume this
> describes a [14e4:1688] device, which the PCI database says is a
> NetXtreme BCM5761 10/100/1000BASE-T Ethernet
> (https://pci-ids.ucw.cz/read/PC/14e4/1688)

Yes.  I use an assortment of PCIe endpoint devices for testing.
>
> Why do you *ever* need this stanza?  "git grep pci-ep
> Documentation/devicetree/bindings/pci/" says no other DT has one.

You'll find one in
"Documentation/devicetree/bindings/pci/nvidia,tegra-pcie.txt", line
~240, although this
is a board DTS example.  They use "pci@0,0" for endpoint 02:00.0,
whereas I find "pci-ep" to
be more descriptive.

Note that  the "pci-ep@0,0" node is in the "example" section of
brcm,stb-pcie.yaml; but nothing
says it is required.  I believe it was added it because a reviewer
asked me to, but if I remember
incorrectly,  it does illustrate that "pcie@0,0" is not the endpoint
device node as many would think.

Note that the regression occurred because "pci@0,0" was missing, not
"pci-ep@0,0" as I first thought.

>
> If the link does come up, I assume normal PCI enumeration would
> discover the [14e4:1688] or whatever device is plugged into a CM4
> socket, and it would read and assign BARs as needed.  Why do we need
> to describe any of this in the DT?
The only  reason one needs to describe this node is  when a regulator is
under the root port, in my case pci@0,0.  In the example this is

                            vpcie3v3-supply = <&vreg7>;

This was the entire reason behind the original patchset.
>
> If the link doesn't come up, it looks like you set the "refusal_mode"
> so subsequent config accesses fail gracefully instead of with a CPU
> abort.
Yes.
>
> [Tangent: since you never clear "refusal_mode", I assume there's no
> possibility of hot-adding a device.  A device must be put in the slot
> before power-up, right?]
Yes, we do not have the HW functionality to support hotplug.

>
> > [1] https://bugzilla.kernel.org/show_bug.cgi?id=215925
> > [2] Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
> >
> > Fixes: 93e41f3fca3d ("PCI: brcmstb: Add control of subdevice voltage regulators")
> > Fixes: 830aa6f29f07 ("PCI: brcmstb: Split brcm_pcie_setup() into two funcs")
> > Link: https://bugzilla.kernel.org/show_bug.cgi?id=215925
> > Signed-off-by: Jim Quinlan <jim2101024@xxxxxxxxx>
> > ---
> >  drivers/pci/controller/pcie-brcmstb.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/pci/controller/pcie-brcmstb.c b/drivers/pci/controller/pcie-brcmstb.c
> > index ba5c120816b2..adca74e235cb 100644
> > --- a/drivers/pci/controller/pcie-brcmstb.c
> > +++ b/drivers/pci/controller/pcie-brcmstb.c
> > @@ -540,16 +540,18 @@ static int pci_subdev_regulators_add_bus(struct pci_bus *bus)
> >
> >  static int brcm_pcie_add_bus(struct pci_bus *bus)
> >  {
> > -     struct device *dev = &bus->dev;
> >       struct brcm_pcie *pcie = (struct brcm_pcie *) bus->sysdata;
> >       int ret;
> >
> > -     if (!dev->of_node || !bus->parent || !pci_is_root_bus(bus->parent))
> > +     /* Only busno==1 requires us to linkup */
> > +     if ((int)bus->number != 1)
>
> It's a big leap from "DT endpoint is optional" to "bus->number == 1 if
> DT endpoint is missing" (if that's even what it means).  Help me
> connect the dots here.
The brcm_pcie_add_bus() function returned immediately and skipped linkup
when (!dev->of_node). That clause was removed from that function, which
is the true fix for the regression,  but you can see thiscondition
is still tested in pci_subdev_regulators_add_bus().

I added the "busno != 1" as an added precaution,
as the brcmstb RC driver only cares about pcie linkup and turning on
regulators when busno==1.

If this regulator mechanism becomes a feature any RC driver may use --
as it was in
v8 of the original patch but was moved to pcie-brcamstb only to avoid conflicts
with Pali's upcoming RC functionality improvements -- I would probably consider
removing the busno==1 clause.

Regards and thanks,
Jim Quinlan
Broadcom S


>
> I *guess* this is really saying "we only want to bring the link up for
> RPs"?
>
> And "bus->number == 1" assumes the RP is on bus 0, there's only one
> RP, and that RP's secondary bus is 1?  So it's only in that case
> (we're adding the secondary bus of the RP), that we need to manually
> bring up the link?
>
> >               return 0;
> >
> >       ret = pci_subdev_regulators_add_bus(bus);
> > -     if (ret)
> > +     if (ret) {
> > +             pcie->refusal_mode = true;
>
> Is this related?  It doesn't *look* related to making the DT endpoint
> optional.
>
> >               return ret;
> > +     }
> >
> >       /* Grab the regulators for suspend/resume */
> >       pcie->sr = bus->dev.driver_data;
> >
> > base-commit: ef1302160bfb19f804451d0e919266703501c875
> > prerequisite-patch-id: 23a425390a4226bd70bbff459148c80f5e28379c
> > prerequisite-patch-id: e3f2875124b46b2b1cf9ea28883bf0c864b79479
> > prerequisite-patch-id: 9cdd706ee2038c7b393c4d65ff76a1873df1ca03
> > prerequisite-patch-id: 332ac90be6e4e4110e27bdd1caaff212c129f547
> > prerequisite-patch-id: 32a74f87cbfe9e8d52c34a4edeee6d271925665a
> > prerequisite-patch-id: f57cdf7ec7080bb8c95782bc7c3ec672db8ec1ce
> > prerequisite-patch-id: 18dc9236aed47f708f5c854afd832f3c80be5ea7
> > prerequisite-patch-id: dd147c6854c4ca12a9a8bd4f5714968a59d60e4e
> > --
> > 2.17.1
> >