Re: [PATCH v3] PCI: Only enable IO window if supported

[Yinghai Lu]

On Thu, Jul 30, 2015 at 7:15 PM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> The PCI subsystem always assumes that I/O is supported on PCIe bridges
> and tries to assign an I/O window to each child bus even if that is not
> the case.
>
> This may result in messages such as:
>
>   pcieport 0000:02:00.0: res[7]=[io  0x1000-0x0fff] get_res_add_size add_size 1000
>   pcieport 0000:02:00.0: BAR 7: no space for [io  size 0x1000]
>   pcieport 0000:02:00.0: BAR 7: failed to assign [io  size 0x1000]
>
> for each bridge port, even if a bus or its parent does not support I/O in
> the first place.
>
> To avoid this message, check if a bus supports I/O before trying to enable
> it.  Also check if the root bus has an IO window assigned; if not, it does
> not make sense to try to assign one to any of its child busses.
>
>
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index cefd636681b6..d9e02ba34035 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -332,6 +332,24 @@ static void pci_read_bases(struct pci_dev *dev, unsigned int howmany, int rom)
>         }
>  }
>
> +static int pci_bridge_supports_io(struct pci_dev *bridge)
> +{
> +       u16 io;
> +
> +       pci_read_config_word(bridge, PCI_IO_BASE, &io);
> +       if (io)
> +               return 1;
> +
> +       /* IO_BASE/LIMIT is either hard-wired to zero or programmed to zero */
> +       pci_write_config_word(bridge, PCI_IO_BASE, 0xe0f0);
> +       pci_read_config_word(bridge, PCI_IO_BASE, &io);
> +       pci_write_config_word(bridge, PCI_IO_BASE, 0x0);
> +       if (io)
> +               return 1;
> +
> +       return 0;
> +}
> +
>  static void pci_read_bridge_io(struct pci_bus *child)
>  {
>         struct pci_dev *dev = child->self;
> @@ -340,6 +358,15 @@ static void pci_read_bridge_io(struct pci_bus *child)
>         struct pci_bus_region region;
>         struct resource *res;
>
> +       if (!(child->bus_flags & PCI_BUS_FLAGS_SUPPORTS_IO))
> +               return;
> +
> +       if (!pci_bridge_supports_io(dev)) {
> +               dev_printk(KERN_DEBUG, &dev->dev, "  no I/O window\n");
> +               child->bus_flags &= ~PCI_BUS_FLAGS_SUPPORTS_IO;
> +               return;
> +       }
> +

It only can avoid warning with bridge, and still have warning on
devices under the bridge.

also would have problem on transparent bridges, like

BRIDGE_A ---- BRIDGE_AA----DEVICE_AA
                   |
                   \-- BRIDGE_AB ---DEVICE_AB

if only BRIDGE_A and BRIDGE_AA are transparent, and BRIDGE_AB is not.

and if pci_bridge_supports_io() return false for BRIDGE_A.

We will have all three bridges have PCI_BUS_FLAGS_SUPPORTS_IO cleared.
so all will not been sized and will not get assigned io port resource.

later DEVICE_AA could root bus io port as parent, and get io resource assigned.
but DEVICE_AB will not get assigned.

so we should get rid of pci_bridge_supports_io(), and just check root
resource IO port.
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