[PATCH v2 3/6] PCI: brcmstb: Add "refusal mode" to preclude PCIe-induced CPU aborts

[Jim Quinlan]

https://lore.kernel.org/linux-pci/20171215201434.GY30595@bhelgaas-glaptop.roam.corp.google.com/
On Wed, Jul 20, 2022 at 6:06 PM Bjorn Helgaas <helgaas at kernel.org> wrote:
>
> On Sat, Jul 16, 2022 at 06:24:50PM -0400, Jim Quinlan wrote:
> > Our PCIe RC HW has an atypical behavior: if it does not have PCIe link
> > established between itself and downstream, any subsequent config space
> > access causes a CPU abort.  This commit sets a "refusal mode" if the PCIe
> > link-up fails, and this has our pci_ops map_bus function returning a NULL
> > address, which in turn precludes the access from happening.
> >
> > Right now, "refusal mode" is window dressing.  It will become relevant
> > in a future commit when brcm_pcie_start_link() is invoked during
> > enumeration instead of before it.
> >
> > Signed-off-by: Jim Quinlan <jim2101024 at gmail.com>
> > ---
> >  drivers/pci/controller/pcie-brcmstb.c | 24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> >
> > diff --git a/drivers/pci/controller/pcie-brcmstb.c b/drivers/pci/controller/pcie-brcmstb.c
> > index c026446d5830..72219a4f3964 100644
> > --- a/drivers/pci/controller/pcie-brcmstb.c
> > +++ b/drivers/pci/controller/pcie-brcmstb.c
> > @@ -255,6 +255,7 @@ struct brcm_pcie {
> >       u32                     hw_rev;
> >       void                    (*perst_set)(struct brcm_pcie *pcie, u32 val);
> >       void                    (*bridge_sw_init_set)(struct brcm_pcie *pcie, u32 val);
> > +     bool                    refusal_mode;
> >  };
> >
> >  static inline bool is_bmips(const struct brcm_pcie *pcie)
> > @@ -687,6 +688,19 @@ static void __iomem *brcm_pcie_map_conf(struct pci_bus *bus, unsigned int devfn,
> >       if (pci_is_root_bus(bus))
> >               return PCI_SLOT(devfn) ? NULL : base + where;
> >
> > +     if (pcie->refusal_mode) {
> > +             /*
> > +              * At this point we do not have PCIe link-up.  If there is
> > +              * a config read or write access besides those targeting
> > +              * the host bridge, our PCIe HW throws a CPU abort.  To
> > +              * prevent this we return the NULL address.  The calling
> > +              * functions -- pci_generic_config_*() -- will notice this
> > +              * and not perform the access, and if it is a read access,
> > +              * 0xffffffff is returned.
> > +              */
> > +             return NULL;
> > +     }
>
> Is this any different from all the other .map_bus() implementations
> that return NULL when the link is down?

Not really,,but long ago I submitted code that gated the config spec
access based on link status and was advised not to do it  [1].
I'll be happy to make it look like the others.

Regards,
Jim Quinlan
Broadcom STB

[1] https://lore.kernel.org/linux-pci/20171215201434.GY30595@bhelgaas-glaptop.roam.corp.google.com/

>
>   cdns_pci_map_bus()
>   dw_pcie_other_conf_map_bus()
>   nwl_pcie_map_bus() (see nwl_pcie_valid_device())
>   xilinx_pcie_map_bus() (see xilinx_pcie_valid_device())
>
> If you can implement this the same way, i.e., using
> brcm_pcie_link_up(), it would be nice.
>
> >       /* For devices, write to the config space index register */
> >       idx = PCIE_ECAM_OFFSET(bus->number, devfn, 0);
> >       writel(idx, pcie->base + PCIE_EXT_CFG_INDEX);
> > @@ -704,6 +718,11 @@ static void __iomem *brcm_pcie_map_conf32(struct pci_bus *bus, unsigned int devf
> >       if (pci_is_root_bus(bus))
> >               return PCI_SLOT(devfn) ? NULL : base + (where & ~0x3);
> >
> > +     if (pcie->refusal_mode) {
> > +             /* See note above in brcm_pcie_map_conf() */
> > +             return NULL;
> > +     }
> > +
> >       /* For devices, write to the config space index register */
> >       idx = PCIE_ECAM_OFFSET(bus->number, devfn, (where & ~3));
> >       writel(idx, base + IDX_ADDR(pcie));
> > @@ -989,6 +1008,7 @@ static int brcm_pcie_start_link(struct brcm_pcie *pcie)
> >               dev_err(dev, "link down\n");
> >               return -ENODEV;
> >       }
> > +     pcie->refusal_mode = false;
> >
> >       if (!brcm_pcie_rc_mode(pcie)) {
> >               dev_err(dev, "PCIe misconfigured; is in EP mode\n");
> > @@ -1134,6 +1154,8 @@ static void brcm_pcie_turn_off(struct brcm_pcie *pcie)
> >       void __iomem *base = pcie->base;
> >       int tmp;
> >
> > +     pcie->refusal_mode = true;
> > +
> >       if (brcm_pcie_link_up(pcie))
> >               brcm_pcie_enter_l23(pcie);
> >       /* Assert fundamental reset */
> > @@ -1185,6 +1207,7 @@ static int brcm_pcie_resume(struct device *dev)
> >       u32 tmp;
> >       int ret;
> >
> > +     pcie->refusal_mode = true;
> >       base = pcie->base;
> >       ret = clk_prepare_enable(pcie->clk);
> >       if (ret)
> > @@ -1361,6 +1384,7 @@ static int brcm_pcie_probe(struct platform_device *pdev)
> >       pcie->type = data->type;
> >       pcie->perst_set = data->perst_set;
> >       pcie->bridge_sw_init_set = data->bridge_sw_init_set;
> > +     pcie->refusal_mode = true;
> >
> >       pcie->base = devm_platform_ioremap_resource(pdev, 0);
> >       if (IS_ERR(pcie->base))
> > --
> > 2.17.1
> >
> >
> > _______________________________________________
> > linux-arm-kernel mailing list
> > linux-arm-kernel at lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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