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

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Message-ID: <20250627155133.GA1669946@bhelgaas>
Date: Fri, 27 Jun 2025 10:51:33 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
	Vipin Sharma <vipinsh@...gle.com>,
	Aaron Lewis <aaronlewis@...gle.com>
Subject: Re: [RFC PATCH] PCI: Support Immediate Readiness on devices without
 PM capabilities

On Thu, Jun 26, 2025 at 03:17:48PM -0700, Sean Christopherson wrote:
> 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").

The only reason I suggested doing it earlier was to enable a potential
pci_find_capability() optimization.  But this could easily be moved
if/when that happens, so I think the patch below would be fine.

> 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 */