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SubjectRe: [PATCH v2 2/6] PCI: brcmstb: Split brcm_pcie_setup() into two funcs
On Sat, Jul 16, 2022 at 06:24:49PM -0400, Jim Quinlan wrote:
> Currently, the function does the setup for establishing PCIe link-up
> with the downstream device, and it does the actual link-up as well.
> The calling sequence is (roughly) the following in the probe:
>
> -> brcm_pcie_probe()
> -> brcm_pcie_setup(); /* Set-up and link-up */
> -> pci_host_probe(bridge);
>
> This commit splits the setup function in two: brcm_pcie_setup(), which only
> does the set-up, and brcm_pcie_start_link(), which only does the link-up.
> The reason why we are doing this is to lay a foundation for subsequent
> commits so that we can turn on any power regulators, as described in the
> root port's DT node, prior to doing link-up.

All drivers that care about power regulators turn them on before
link-up, but typically those regulators are described directly under
the host bridge itself.

IIUC the difference here is that you have regulators described under
Root Ports (not the host bridge/Root Complex itself), so you don't
know about them until you've enumerated the Root Ports.
brcm_pcie_probe() can't turn them on directly because it doesn't know
what Root Ports are present and doesn't know about regulators below
them.

So I think brcm_pcie_setup() does initialization that doesn't depend
on the link or any downstream devices, and brcm_pcie_start_link() does
things that depend on the link being up. Right?

If so, "start_link" might be a slight misnomer since AFAICT
brcm_pcie_start_link() doesn't do anything to initiate link-up except
maybe deasserting fundamental reset. Some drivers start the LTSSM or
explicitly enable link training, but brcm_pcie_start_link() doesn't
seem to do anything like that.

brcm_pcie_start_link() still does brcm_pcie_set_outbound_win(). Does
that really depend on the link being up? If that only affects the
Root Port, maybe it could be done before link-up?

> We do this by defining an
> add_bus() callback which is invoked during enumeraion. At the end of this
> patchset the probe function trace will look something like this:
>
> -> brcm_pcie_probe()
> -> brcm_pcie_setup(); /* Set-up only */
> -> pci_host_probe(bridge);
> -> [enumeration]
> -> pci_alloc_child_bus()
> -> bus->ops->add_bus(bus); /* We've set this op */
> -> brcm_pcie_add_bus() /* Our callback */
> -> [turn on regulators] /* Main objective! */
> -> brcm_pcie_start_link() /* Link-up */

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