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

[Jim Quinlan <jim2101024@gmail.com>]

On Thu, May 19, 2022 at 2:04 PM Jim Quinlan <jim2101024@gmail.com> wrote:
>
> On Thu, May 19, 2022 at 12:10 PM Bjorn Helgaas <helgaas@kernel.org> 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 havedev->dev.of_nodei 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?
Hi Bjorn,

I was remiss in not mentioning our biggest actual use of specifying
this sub-subnode: to pass info to the endproint driver.  For example:

pcie@1000110000 {
        compatible = "brcm,bcm7211-pcie";
        /* ... */

        pci@0,0 {
                compatible = "pciclass,0604";
                /* ... */

                pci-ep@0,0 {
                        local-mac-address = [ 00 10 18 d0 3c 51 ];
                        reg = <0x10000 0x0 0x0 0x0 0x0>;
                };
        };
};

The PCIe endpoint driver can just invoke

        of_get_mac_address(dev->dev.of_node, &addr)


Regards,
Jim Quinlan
Broadcom STB


> 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@gmail.com>
> > > ---
> > >  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
> > >