Re: [PATCH 1/1] PCI: X-Gene: Disable Configuration Request Retry Status for X-Gene v1 PCIe

Bjorn Helgaas
2015-06-12T21:59:08Z

Hi Duc,

On Thu, Jun 11, 2015 at 01:08:14PM -0700, Duc Dang wrote:
> X-Gene v1 PCIe controller has a bug in Configuration Request Retry
> Status (CRS) logic:
> When CPU tries to read Vendor ID and Device ID of not-existed
> remote device, the controller returns 0xFFFF0001 instead of
> 0xFFFFFFFF; this will add significant delay in boot time as
> pci_bus_read_dev_vendor_id will wait for 60 seconds before
> giving up.

OK, help me understand how this works. I think this is related to the
problem I reported where if the slot is empty, "lspci" doesn't show
anything, not even the Root Port leading to the slot.

I think this happens because when we try to read the Root Port's config
space,

- the slot below the Root Port is empty
- the Root Port's link is down
- xgene_pcie_map_bus() returns NULL because !port->link_up
- pci_generic_config_read32() returns PCIBIOS_DEVICE_NOT_FOUND

so it looks like the Root Port itself doesn't exist.

I proposed to change xgene_pcie_map_bus() so it didn't check whether the
link was up. That change makes reads of the Root Port's config space work.

After we learn the Root Port exists, the PCI core enumerates devices below
the Root Port, e.g., on bus 01. X-Gene advertises that it supports CRS, so
we enable it. When we try to read the Vendor ID of 01:00.0, there's no
response from the device (because the slot is empty), and the Root Complex
should complete the read by fabricating data of all ones, i.e., 0xFFFFFFFF.
But apparently X-Gene supplies 0xFFFF0001 instead, which means "there's a
device here, but it's not ready yet," so the PCI core retries the read for
60 seconds before timing out.

This patch is basically a quirk that keeps X-Gene from advertising CRS
support, so the PCI core won't enable CRS. In the example above, I guess
that means the Root Complex will supply 0xFFFFFFFF and the core will see
that the slot is empty.

But this patch leaves the "!port->link_up" test in xgene_pcie_map_bus().
Doesn't that mean the core will still not discover the Root Port when the
slot is empty?

It seems to me that you would want both the xgene_pcie_map_bus() change and
this patch. The first would fix the problem that we don't enumerate Root
Ports leading to empty slots, and the second would fix the problem that we
enable CRS and timeout when enumerating below those Root Ports.

One more question below:

> So for X-Gene v1 PCIe controllers, disable CRS capability
> advertisement by clearing CRS Software Visibility bit before
> returning the Root Capability value to the callers. This is done
> by implementing X-Gene PCIe specific xgene_pcie_config_read32 for
> CFG read accesses to replace the generic default pci_generic_config_read32
> function.
>
> Signed-off-by: Duc Dang <dhdang@apm.com>
> ---
> drivers/pci/host/pci-xgene.c | 48 +++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 47 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pci/host/pci-xgene.c b/drivers/pci/host/pci-xgene.c
> index ee082c0..741a253 100644
> --- a/drivers/pci/host/pci-xgene.c
> +++ b/drivers/pci/host/pci-xgene.c
> @@ -59,6 +59,12 @@
> #define SZ_1T (SZ_1G*1024ULL)
> #define PIPE_PHY_RATE_RD(src) ((0xc000 & (u32)(src)) >> 0xe)
>
> +#define ROOT_CAP_AND_CTRL 0x5C
> +
> +/* PCIe IP version */
> +#define XGENE_PCIE_IP_VER_UNKN 0
> +#define XGENE_PCIE_IP_VER_1 1
> +
> struct xgene_pcie_port {
> struct device_node *node;
> struct device *dev;
> @@ -67,6 +73,7 @@ struct xgene_pcie_port {
> void __iomem *cfg_base;
> unsigned long cfg_addr;
> bool link_up;
> + u32 version;
> };
>
> static inline u32 pcie_bar_low_val(u32 addr, u32 flags)
> @@ -140,9 +147,44 @@ static void __iomem *xgene_pcie_map_bus(struct pci_bus *bus, unsigned int devfn,
> return xgene_pcie_get_cfg_base(bus) + offset;
> }
>
> +int xgene_pcie_config_read32(struct pci_bus *bus, unsigned int devfn,
> + int where, int size, u32 *val)
> +{
> + void __iomem *addr;
> + struct xgene_pcie_port *port = bus->sysdata;
> +
> + addr = bus->ops->map_bus(bus, devfn, where & ~0x3);
> + if (!addr) {
> + *val = ~0;
> + return PCIBIOS_DEVICE_NOT_FOUND;
> + }
> +
> + *val = readl(addr);

Can't you just call pci_generic_config_read32() directly instead of
duplicating its code here?

> + /*
> + * X-Gene v1 PCIe controller has a bug in Configuration Request
> + * Retry Status (CRS) logic:
> + * When CPU tries to read Vendor ID and Device ID of not-existed
> + * remote device, the controller returns 0xFFFF0001 instead of
> + * 0xFFFFFFFF; this will add significant delay in boot time as
> + * pci_bus_read_dev_vendor_id will wait for 60 seconds before
> + * giving up.
> + * So for X-Gene v1 PCIe controllers, disable CRS capability
> + * advertisement by clearing CRS Software Visibility bit before
> + * returning the Root Capability value to the callers.
> + */
> + if (pci_is_root_bus(bus) && (port->version == XGENE_PCIE_IP_VER_1) &&
> + ((where & ~0x3) == ROOT_CAP_AND_CTRL))
> + *val &= ~(PCI_EXP_RTCAP_CRSVIS << 16);
> +
> + if (size <= 2)
> + *val = (*val >> (8 * (where & 3))) & ((1 << (size * 8)) - 1);
> +
> + return PCIBIOS_SUCCESSFUL;
> +}
> +
> static struct pci_ops xgene_pcie_ops = {
> .map_bus = xgene_pcie_map_bus,
> - .read = pci_generic_config_read32,
> + .read = xgene_pcie_config_read32,
> .write = pci_generic_config_write32,
> };
>
> @@ -483,6 +525,10 @@ static int xgene_pcie_probe_bridge(struct platform_device *pdev)
> port->node = of_node_get(pdev->dev.of_node);
> port->dev = &pdev->dev;
>
> + port->version = XGENE_PCIE_IP_VER_UNKN;
> + if (of_device_is_compatible(port->node, "apm,xgene-pcie"))
> + port->version = XGENE_PCIE_IP_VER_1;
> +
> ret = xgene_pcie_map_reg(port, pdev);
> if (ret)
> return ret;
> --
> 1.9.1
>