Re: [PATCH V10 2/5] PCI: Create device tree node for bridge

Bjorn Helgaas
2023-06-29T22:56:38Z

On Thu, Jun 29, 2023 at 10:19:47AM -0700, Lizhi Hou wrote:
> The PCI endpoint device such as Xilinx Alveo PCI card maps the register
> spaces from multiple hardware peripherals to its PCI BAR. Normally,
> the PCI core discovers devices and BARs using the PCI enumeration process.
> There is no infrastructure to discover the hardware peripherals that are
> present in a PCI device, and which can be accessed through the PCI BARs.

IIUC this is basically a multi-function device except that instead of
each device being a separate PCI Function, they all appear in a single
Function. That would mean all the devices share the same config space
so a single PCI Command register controls all of them, they all share
the same IRQs (either INTx or MSI/MSI-X), any MMIO registers are likely
in a shared BAR, etc., right?

Obviously PCI enumeration only sees the single Function and binds a
single driver to it. But IIUC, you want to use existing drivers for
each of these sub-devices, so this series adds a DT node for the
single Function (using the quirks that call of_pci_make_dev_node()).
And I assume that when the PCI driver claims the single Function, it
will use that DT node to add platform devices, and those existing
drivers can claim those?

I don't see the PCI driver for the single Function in this series. Is
that coming? Is this series useful without it?

> Apparently, the device tree framework requires a device tree node for the
> PCI device. Thus, it can generate the device tree nodes for hardware
> peripherals underneath. Because PCI is self discoverable bus, there might
> not be a device tree node created for PCI devices. Furthermore, if the PCI
> device is hot pluggable, when it is plugged in, the device tree nodes for
> its parent bridges are required. Add support to generate device tree node
> for PCI bridges.

Can you remind me why hot-adding a PCI device requires DT nodes for
parent bridges? I don't think we have those today, so maybe the DT
node for the PCI device requires a DT parent? How far up does that
go? From this patch, I guess a Root Port would be the top DT node on
a PCIe system, since that's the top PCI-to-PCI bridge?

This patch adds a DT node for *every* PCI bridge in the system. We
only actually need that node for these unusual devices. Is there some
way the driver for the single PCI Function could add that node when it
is needed? Sorry if you've answered this in the past; maybe the
answer could be in the commit log or a code comment in case somebody
else wonders.

> @@ -340,6 +340,8 @@ void pci_bus_add_device(struct pci_dev *dev)
> */
> pcibios_bus_add_device(dev);
> pci_fixup_device(pci_fixup_final, dev);
> + if (pci_is_bridge(dev))
> + of_pci_make_dev_node(dev);

It'd be nice to have a clue here about why we need this, since this is
executed for *every* system, even ACPI platforms that typically don't
use OF things.

> pci_create_sysfs_dev_files(dev);
> pci_proc_attach_device(dev);
> pci_bridge_d3_update(dev);
> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> index 2c25f4fa0225..9786ae407948 100644
> --- a/drivers/pci/of.c
> +++ b/drivers/pci/of.c
> @@ -487,6 +487,15 @@ static int of_irq_parse_pci(const struct pci_dev *pdev, struct of_phandle_args *
> } else {
> /* We found a P2P bridge, check if it has a node */
> ppnode = pci_device_to_OF_node(ppdev);
> +#if IS_ENABLED(CONFIG_PCI_DYNAMIC_OF_NODES)

I would use plain #ifdef here instead of IS_ENABLED(), as you did in
pci.h below. IS_ENABLED() is true if the Kconfig symbol is set to
either "y" or "m".

Using IS_ENABLED() suggests that the config option *could* be a
module, which is not the case here because CONFIG_PCI_DYNAMIC_OF_NODES
is a bool.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/kconfig.h?id=v6.4#n69

> @@ -617,6 +626,85 @@ int devm_of_pci_bridge_init(struct device *dev, struct pci_host_bridge *bridge)
> return pci_parse_request_of_pci_ranges(dev, bridge);
> }
>
> +#if IS_ENABLED(CONFIG_PCI_DYNAMIC_OF_NODES)

Same here, of course.

> +void of_pci_remove_node(struct pci_dev *pdev)
> +{
> + struct device_node *np;
> +
> + np = pci_device_to_OF_node(pdev);
> + if (!np || !of_node_check_flag(np, OF_DYNAMIC))

> + * Each entry in the ranges table is a tuple containing the child address,
> + * the parent address, and the size of the region in the child address space.
> + * Thus, for PCI, in each entry parent address is an address on the primary
> + * side and the child address is the corresponding address on the secondary
> + * side.
> + */
> +struct of_pci_range {
> + u32 child_addr[OF_PCI_ADDRESS_CELLS];
> + u32 parent_addr[OF_PCI_ADDRESS_CELLS];
> + u32 size[OF_PCI_SIZE_CELLS];

> + if (pci_is_bridge(pdev)) {
> + memcpy(rp[i].child_addr, rp[i].parent_addr,
> + sizeof(rp[i].child_addr));
> + } else {
> + /*
> + * For endpoint device, the lower 64-bits of child
> + * address is always zero.

I think this connects with the secondary side comment above, right? I
think I would comment this as:

/*
* PCI-PCI bridges don't translate addresses, so the child
* (secondary side) address is identical to the parent (primary
* side) address.
*/

and

/*
* Non-bridges have no child (secondary side) address, so clear it
* out.
*/

> + */
> + rp[i].child_addr[0] = j;

> + ret = of_changeset_add_empty_prop(ocs, np, "dynamic");

It seems slightly confusing to use a "dynamic" property here when we
also have the OF_DYNAMIC dynamic flag above. I think they have
different meanings, don't they?

Bjorn