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

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Message-ID: <20250625230313.GA1593493@bhelgaas>
Date: Wed, 25 Jun 2025 18:03:13 -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 Tue, Jun 24, 2025 at 10:16:37AM -0700, Sean Christopherson wrote:
> Query support for Immediate Readiness irrespective of whether or not the
> device supports PM capabilities, as nothing in the PCIe spec suggests that
> Immediate Readiness is in any way dependent on PM functionality.

Huh, I forgot that we had support for Immediate Readiness at all.

I agree, Immediate Readiness has nothing to do with PM except that we
take advantage of it in a PM path, and I think pci_pm_init() was
probably not the best place to put this.

> Opportunistically add a comment to explain why "errors" during PM setup
> are effectively ignored.
> 
> Fixes: d6112f8def51 ("PCI: Add support for Immediate Readiness")
> Cc: David Matlack <dmatlack@...gle.com>
> Cc: Vipin Sharma <vipinsh@...gle.com>
> Cc: Aaron Lewis <aaronlewis@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
> 
> RFC as I'm not entirely sure this is useful/correct.
> 
> Found by inspection when debugging a VFIO VF passthrough issue that turned out
> to be 907a7a2e5bf4 ("PCI/PM: Set up runtime PM even for devices without PCI PM").
> 
> The folks on the Cc list are looking at parallelizing VF assignment to avoid
> serializing the 100ms wait on FLR.  I'm hoping we'll get lucky and the VFs in
> question do (or can) support PCI_STATUS_IMM_READY.
> 
>  drivers/pci/pci.c | 40 +++++++++++++++++++++++++---------------
>  1 file changed, 25 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 9e42090fb108..cd91adbf0269 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3198,33 +3198,22 @@ void pci_pm_power_up_and_verify_state(struct pci_dev *pci_dev)
>  	pci_update_current_state(pci_dev, PCI_D0);
>  }
>  
> -/**
> - * pci_pm_init - Initialize PM functions of given PCI device
> - * @dev: PCI device to handle.
> - */
> -void pci_pm_init(struct pci_dev *dev)
> +static void __pci_pm_init(struct pci_dev *dev)
>  {
>  	int pm;
> -	u16 status;
>  	u16 pmc;
>  
> -	device_enable_async_suspend(&dev->dev);
> -	dev->wakeup_prepared = false;
> -
> -	dev->pm_cap = 0;
> -	dev->pme_support = 0;
> -
>  	/* find PCI PM capability in list */
>  	pm = pci_find_capability(dev, PCI_CAP_ID_PM);
>  	if (!pm)
> -		goto poweron;
> +		return;
>  	/* Check device's ability to generate PME# */
>  	pci_read_config_word(dev, pm + PCI_PM_PMC, &pmc);
>  
>  	if ((pmc & PCI_PM_CAP_VER_MASK) > 3) {
>  		pci_err(dev, "unsupported PM cap regs version (%u)\n",
>  			pmc & PCI_PM_CAP_VER_MASK);
> -		goto poweron;
> +		return;
>  	}
>  
>  	dev->pm_cap = pm;
> @@ -3265,11 +3254,32 @@ void pci_pm_init(struct pci_dev *dev)
>  		/* Disable the PME# generation functionality */
>  		pci_pme_active(dev, false);
>  	}
> +}
> +
> +/**
> + * pci_pm_init - Initialize PM functions of given PCI device
> + * @dev: PCI device to handle.
> + */
> +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()?

Both PCI_STATUS_IMM_READY and PCI_STATUS_CAP_LIST are read-only, and
we currently read PCI_STATUS for every single pci_find_capability()
call, which is kind of stupid.  Maybe someday we can optimize that and
read PCI_STATUS once for both PCI_STATUS_CAP_LIST and
PCI_STATUS_IMM_READY.

> -poweron:
> +
>  	pci_pm_power_up_and_verify_state(dev);
>  	pm_runtime_forbid(&dev->dev);
>  	pm_runtime_set_active(&dev->dev);
> 
> base-commit: 86731a2a651e58953fc949573895f2fa6d456841
> -- 
> 2.50.0.714.g196bf9f422-goog
>