linux-kernel - Re: [PATCH v2 1/2] PCI/VPD: Add runtime power management to sysfs interface

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Date:   Fri, 11 Aug 2023 14:25:43 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, eric.auger@...hat.com
Subject: Re: [PATCH v2 1/2] PCI/VPD: Add runtime power management to sysfs
 interface

On Thu, Aug 03, 2023 at 11:12:32AM -0600, Alex Williamson wrote:
> Unlike default access to config space through sysfs, the vpd read and
> write function don't actively manage the runtime power management state
> of the device during access.  Since commit 7ab5e10eda02 ("vfio/pci: Move
> the unused device into low power state with runtime PM"), the vfio-pci
> driver will use runtime power management and release unused devices to
> make use of low power states.  Attempting to access VPD information in
> this low power state can result in incorrect information or kernel
> crashes depending on the system behavior.

I guess this specifically refers to D3cold, right?  VPD is accessed
via config space, which should work in all D0-D3hot states, but not in
D3cold.  I don't see anything in the spec about needing to be in D0 to
access VPD.

I assume there's no public problem report we could cite here?  I
suppose the behavior in D3cold is however the system handles a UR
error.

> Wrap the vpd read/write bin attribute handlers in runtime PM and take
> into account the potential quirk to select the correct device to wake.
> 
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
>  drivers/pci/vpd.c | 34 ++++++++++++++++++++++++++++++++--
>  1 file changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/vpd.c b/drivers/pci/vpd.c
> index a4fc4d0690fe..81217dd4789f 100644
> --- a/drivers/pci/vpd.c
> +++ b/drivers/pci/vpd.c
> @@ -275,8 +275,23 @@ static ssize_t vpd_read(struct file *filp, struct kobject *kobj,
>  			size_t count)
>  {
>  	struct pci_dev *dev = to_pci_dev(kobj_to_dev(kobj));
> +	struct pci_dev *vpd_dev = dev;
> +	ssize_t ret;
> +
> +	if (dev->dev_flags & PCI_DEV_FLAGS_VPD_REF_F0) {
> +		vpd_dev = pci_get_func0_dev(dev);
> +		if (!vpd_dev)
> +			return -ENODEV;
> +	}
> +
> +	pci_config_pm_runtime_get(vpd_dev);
> +	ret = pci_read_vpd(vpd_dev, off, count, buf);
> +	pci_config_pm_runtime_put(vpd_dev);
> +
> +	if (dev != vpd_dev)
> +		pci_dev_put(vpd_dev);
>  
> -	return pci_read_vpd(dev, off, count, buf);
> +	return ret;
>  }
>  
>  static ssize_t vpd_write(struct file *filp, struct kobject *kobj,
> @@ -284,8 +299,23 @@ static ssize_t vpd_write(struct file *filp, struct kobject *kobj,
>  			 size_t count)
>  {
>  	struct pci_dev *dev = to_pci_dev(kobj_to_dev(kobj));
> +	struct pci_dev *vpd_dev = dev;
> +	ssize_t ret;
> +
> +	if (dev->dev_flags & PCI_DEV_FLAGS_VPD_REF_F0) {
> +		vpd_dev = pci_get_func0_dev(dev);
> +		if (!vpd_dev)
> +			return -ENODEV;
> +	}
> +
> +	pci_config_pm_runtime_get(vpd_dev);
> +	ret = pci_write_vpd(vpd_dev, off, count, buf);
> +	pci_config_pm_runtime_put(vpd_dev);
> +
> +	if (dev != vpd_dev)
> +		pci_dev_put(vpd_dev);
>  
> -	return pci_write_vpd(dev, off, count, buf);
> +	return ret;
>  }
>  static BIN_ATTR(vpd, 0600, vpd_read, vpd_write, 0);
>  
> -- 
> 2.40.1
>