Re: [RFC PATCH] PCI: Support Immediate Readiness on devices without PM capabilities

[Sean Christopherson <seanjc@google.com>]

On Wed, Jun 25, 2025, Bjorn Helgaas wrote:
> On Tue, Jun 24, 2025 at 10:16:37AM -0700, Sean Christopherson wrote:
> > +void pci_pm_init(struct pci_dev *dev)
> > +{
> > +	u16 status;
> > +
> > +	device_enable_async_suspend(&dev->dev);
> > +	dev->wakeup_prepared = false;
> > +
> > +	dev->pm_cap = 0;
> > +	dev->pme_support = 0;
> > +
> > +	/*
> > +	 * Note, support for the PCI PM spec is optional for legacy PCI devices
> > +	 * and for VFs.  Continue on even if no PM capabilities are supported.
> > +	 */
> > +	__pci_pm_init(dev);
> >  
> >  	pci_read_config_word(dev, PCI_STATUS, &status);
> >  	if (status & PCI_STATUS_IMM_READY)
> >  		dev->imm_ready = 1;
> 
> I would rather just move this PCI_STATUS read to somewhere else.  I
> don't think there's a great place to put it.  We could put it in
> set_pcie_port_type(), which is sort of a grab bag of PCIe-related
> things.
> 
> I don't know if it's necessarily even a PCIe-specific thing, but it
> would be unexpected if somebody made a conventional PCI device that
> set it, since the bit was reserved (and should be zero) in PCI r3.0
> and defined in PCIe r4.0.
> 
> Maybe we should put it in pci_setup_device() close to where we call
> pci_intx_mask_broken()?

Any reason not to throw it in pci_init_capabilities()?  That has the advantage
of minimizing the travel distance, e.g. to avoid introducing a goof similar to
what happened with 4d4c10f763d7 ("PCI: Explicitly put devices into D0 when initializing").

E.g. something silly like this?  Or maybe pci_misc_init() or so?

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 9e42090fb108..4a1ba5c017cd 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -3205,7 +3205,6 @@ void pci_pm_power_up_and_verify_state(struct pci_dev *pci_dev)
 void pci_pm_init(struct pci_dev *dev)
 {
        int pm;
-       u16 status;
        u16 pmc;
 
        device_enable_async_suspend(&dev->dev);
@@ -3266,9 +3265,6 @@ void pci_pm_init(struct pci_dev *dev)
                pci_pme_active(dev, false);
        }
 
-       pci_read_config_word(dev, PCI_STATUS, &status);
-       if (status & PCI_STATUS_IMM_READY)
-               dev->imm_ready = 1;
 poweron:
        pci_pm_power_up_and_verify_state(dev);
        pm_runtime_forbid(&dev->dev);
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 4b8693ec9e4c..d33b8af37247 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2595,6 +2595,15 @@ void pcie_report_downtraining(struct pci_dev *dev)
        __pcie_print_link_status(dev, false);
 }
 
+static void pci_imm_ready_init(struct pci_dev *dev)
+{
+       u16 status;
+
+       pci_read_config_word(dev, PCI_STATUS, &status);
+       if (status & PCI_STATUS_IMM_READY)
+               dev->imm_ready = 1;
+}
+
 static void pci_init_capabilities(struct pci_dev *dev)
 {
        pci_ea_init(dev);               /* Enhanced Allocation */
@@ -2604,6 +2613,7 @@ static void pci_init_capabilities(struct pci_dev *dev)
        /* Buffers for saving PCIe and PCI-X capabilities */
        pci_allocate_cap_save_buffers(dev);
 
+       pci_imm_ready_init(dev);        /* Immediate Ready */
        pci_pm_init(dev);               /* Power Management */
        pci_vpd_init(dev);              /* Vital Product Data */
        pci_configure_ari(dev);         /* Alternative Routing-ID Forwarding */