Re: [PATCH 3/3] PCI: Set pref for mem64 resource of pcie device

[Bjorn Helgaas]

On Tue, Mar 31, 2015 at 07:57:49PM -0700, Yinghai Lu wrote:
> We still get "no compatible bridge window" warning on sparc T5-8
> after we add support for 64bit resource parsing for root bus.
> 
>  PCI: scan_bus[/pci@300/pci@1/pci@0/pci@6] bus no 8
>  PCI: Claiming 0000:00:01.0: Resource 15: 0000800100000000..00008004afffffff [220c]
>  PCI: Claiming 0000:01:00.0: Resource 15: 0000800100000000..00008004afffffff [220c]
>  PCI: Claiming 0000:02:04.0: Resource 15: 0000800100000000..000080012fffffff [220c]
>  PCI: Claiming 0000:03:00.0: Resource 15: 0000800100000000..000080012fffffff [220c]
>  PCI: Claiming 0000:04:06.0: Resource 14: 0000800100000000..000080010fffffff [220c]
>  PCI: Claiming 0000:05:00.0: Resource 0: 0000800100000000..0000800100001fff [204]
>  pci 0000:05:00.0: can't claim BAR 0 [mem 0x800100000000-0x800100001fff]: no compatible bridge window
> 
> All the bridges 64-bit resource have pref bit, but the device resource does not
> have pref set, then we can not find parent for the device resource,
> as we can not put non-pref mem under pref mem.
> 
> According to pcie spec errta
> https://www.pcisig.com/specifications/pciexpress/base2/PCIe_Base_r2.1_Errata_08Jun10.pdf
> page 13, in some case it is ok to mark some as pref.

That implementation note is included in the PCIe spec r3.0, sec 7.5.2.1.  I
think that's a better reference to cite.

The most straightforward way to read the implementation note is that it is
safe for the *device*, i.e., the hardware, to set the Prefetchable bit in
some BARs, even if those BARs don't actually support prefetching.  But I
don't think the device can really tell when it would be safe, because the
device can't tell whether the entire path leading to it is over PCIe.

I don't think it makes sense for *software* to set the Prefetchable bit in
a BAR, because the spec says bits 0-3 (including Prefetchable) are supposed
to be read-only (conventional PCI spec r3.0, sec 6.2.5.1, p226).

If the intent were to suggest that system software could *treat* a
non-prefetchable BAR as though it were prefetchable, that would make sense,
and it would have been easy to write the implementation note to actually
say that.

I guess I'm OK with doing this as in your patch below even though it really
doesn't match the language in the spec, because I can't see any other
useful way to interpret the spec.

But this is a general change that affects all platforms, and it's late in
the cycle for something as invasive as this.  I'd rather include your patch
in the v4.1 merge window, and revert d63e2e1f3df9 ("sparc/PCI: Clip bridge
windows to fit in upstream windows") for v4.0.

Bjorn

> Only set pref for 64bit mmio when the entire path from the host to the adapter is
> over PCI Express.
> 
> Fixes: commit d63e2e1f3df9 ("sparc/PCI: Clip bridge windows to fit in upstream windows")
> Link: http://lkml.kernel.org/r/CAE9FiQU1gJY1LYrxs+ma5LCTEEe4xmtjRG0aXJ9K_Tsu+m9Wuw@xxxxxxxxxxxxxx
> Reported-by: David Ahern <david.ahern@xxxxxxxxxx>
> Tested-by: David Ahern <david.ahern@xxxxxxxxxx>
> Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> #3.19
> ---
>  drivers/pci/probe.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 50 insertions(+)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index f71cb7c..4e0cd46 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1507,6 +1507,53 @@ static void pci_init_capabilities(struct pci_dev *dev)
>  	pci_enable_acs(dev);
>  }
>  
> +static bool pci_up_path_over_pcie(struct pci_bus *bus)
> +{
> +	if (!bus)
> +		return true;
> +
> +	if (bus->self && !pci_is_pcie(bus->self))
> +		return false;
> +
> +	return pci_up_path_over_pcie(bus->parent);
> +}
> +
> +/*
> + * According to
> + * https://www.pcisig.com/specifications/pciexpress/base2/PCIe_Base_r2.1_Errata_08Jun10.pdf
> + * page 13, system firmware could put some 64bit non-pref under 64bit pref,
> + * on some cases.
> + * Let's set pref bit for 64bit mmio when entire path from the host to
> + * the adapter is over PCI Express.
> + */
> +static void set_pcie_64bit_pref(struct pci_dev *dev)
> +{
> +	int i;
> +
> +	if (!pci_is_pcie(dev))
> +		return;
> +
> +	if (!pci_up_path_over_pcie(dev->bus))
> +		return;
> +
> +	for (i = 0; i < PCI_BRIDGE_RESOURCES; i++) {
> +		struct resource *res = &dev->resource[i];
> +		enum pci_bar_type type;
> +		int reg;
> +
> +		if (!(res->flags & IORESOURCE_MEM_64))
> +			continue;
> +
> +		if (res->flags & IORESOURCE_PREFETCH)
> +			continue;
> +
> +		reg = pci_resource_bar(dev, i, &type);
> +		dev_printk(KERN_DEBUG, &dev->dev, "reg 0x%x %pR + pref\n",
> +			   reg, res);
> +		res->flags |= IORESOURCE_PREFETCH;
> +	}
> +}
> +
>  void pci_device_add(struct pci_dev *dev, struct pci_bus *bus)
>  {
>  	int ret;
> @@ -1536,6 +1583,9 @@ void pci_device_add(struct pci_dev *dev, struct pci_bus *bus)
>  	/* Initialize various capabilities */
>  	pci_init_capabilities(dev);
>  
> +	/* After pcie_cap is assigned and sriov bar is probed */
> +	set_pcie_64bit_pref(dev);
> +
>  	/*
>  	 * Add the device to our list of discovered devices
>  	 * and the bus list for fixup functions, etc.
> -- 
> 1.8.4.5
> 
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