Re: [PATCH v5 1/5] PCI: host-common: Add helper to determine host bridge D3cold eligibility

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, May 19, 2026 at 05:39:01PM -0500, Bjorn Helgaas wrote:
> On Wed, Apr 29, 2026 at 12:12:23PM +0530, Krishna Chaitanya Chundru wrote:
> > Add a common helper, pci_host_common_d3cold_possible(), to determine
> > whether PCIe devices under host bridge can safely transition to D3cold.
> ...

> > +static int __pci_host_common_d3cold_possible(struct pci_dev *pdev, void *userdata)
> > +{
> > +	u32 *flags = userdata;
> > +	int type;
> > +
> > +	/* Ignore conventional PCI devices */
> > +	if (!pci_is_pcie(pdev))
> > +		return 0;
> > +
> > +	type = pci_pcie_type(pdev);
> > +	if (type != PCI_EXP_TYPE_ENDPOINT &&
> > +	    type != PCI_EXP_TYPE_LEG_END &&
> > +	    type != PCI_EXP_TYPE_RC_END)
> > +		return 0;
> 
> From https://sashiko.dev/#/patchset/20260429-d3cold-v5-0-89e9735b9df6%40oss.qualcomm.com:
> 
>   If the topology contains an active conventional PCI device or an
>   intermediate PCIe switch in PCI_D0, returning 0 here allows
>   pci_walk_bus() to continue without clearing the
>   PCI_HOST_D3COLD_ALLOWED flag.
> 
>   Does this create a situation where the host bridge might
>   aggressively power off the link, dropping power to these active
>   components?
> 
> I guess this is intentional, since you have comment about ignoring
> conventional PCI devices.  But this does seem like a potential
> problem.  Why should we ignore switches here?  And I think it's still
> fairly common to have a PCIe-to-PCI bridge leading to a conventional
> PCI device, and I don't know why we should ignore them.
> 
> The commit log consistently refers to "PCIe" devices and endpoints, so
> maybe there's some reason that I'm missing.
> 
> There are other sashiko comments on this series that I think should
> also be looked at.

This series is all in pci/next, so you and Mani can decide on whether
any sashiko comments need to be addressed.

Even if there's no code change, I think it'd be nice to have a brief
comment here about why conventional PCI and switches are ignored.