PCI: qcom: Add D3cold support

[Steev Klimaszewski]

Hi Krishna,

> This series adds support for putting Qualcomm PCIe host bridges into D3cold
> when downstream conditions allow it, and introduces a small common helper
> to determine D3cold eligibility based on endpoint state.

> On Qualcomm platforms, PCIe host controllers are currently kept powered
> even when there are no active endpoints (i.e. all endpoints are already in
> PCI_D3hot). This prevents the SoC from entering deeper low‑power states
> such as CXPC.

> While PCIe D3cold support exists in the PCI core, host controller drivers
> lack a common mechanism to determine whether it is safe to power off the
> host bridge without breaking active devices or wakeup functionality.
> As a result, controllers either avoid entering D3cold or depend on rough,
> driver‑specific workarounds.

> This series addresses that gap.

> 1. Introduces pci_host_common_can_enter_d3cold(), a helper that determines
>    whether a host bridge may enter D3cold based on downstream PCIe endpoint
>    state. The helper permits D3cold only when all *active* endpoints are
>    already in PCI_D3hot, and any wakeup‑enabled endpoint supports PME
>    from D3cold.

> 2. Updates the Designware PCIe host driver to use this helper in the
>    suspend_noirq() path, replacing the existing heuristic that blocked
>    D3cold whenever L1 ASPM was enabled.

> 3. Enables D3cold support for Qualcomm PCIe controllers by wiring them into
>    the DesignWare common suspend/resume flow and explicitly powering down
>    controller resources when all endpoints are in D3hot.

> The immediate outcome of this series is that Qualcomm PCIe host bridges can
> enter D3cold when all endpoints are in D3hot.

> This is a necessary but not sufficient step toward unblocking CXPC. With
> this series applied, CXPC can be achieved on systems with no attached NVMe
> devices. Support for NVMe‑attached systems requires additional changes
> in NVMe driver, which are being worked on separately.

> Tested on:
>   - Qualcomm Lemans EVK, Monaco & sc7280 platforms.

> Validation steps:
>   - Boot without NVMe attach:
>       * PCIe host enters D3cold during suspend
>       * SoC is able to reach CXPC provided other drivers also remove
> 	their votes as part of suspend.

I have been testing this patchset with Mani's patchset that is supposed to be
related to NVMe on the Thinkpad X13s found at:

https://lore.kernel.org/all/20260414-l1ss-fix-v1-0-adbb4555b5ab@oss.qualcomm.com/

v4 of this patchset *boots* along with Mani's patchset, however, v5 does not,
and unfortunately, the machine does not seem to get to a point where I can even
get logs from it.  Do you know what I might be missing?  I have *not* attempted
to remove the nvme drive and boot off USB to test it.

-- steev