Re: [PATCH v8 06/12] PCI: liveupdate: Auto-preserve upstream bridges across Live Update

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Jul 28, 2026 at 10:10:00PM +0000, David Matlack wrote:
> When a PCI device is preserved across a Live Update, all of its upstream
> bridges up to the root port must also be preserved. This enables the PCI
> core and any drivers bound to the bridges to manage bridges correctly
> across a Live Update.
> 
> Notably, this will be used in subsequent commits to ensure that
> preserved devices can continue performing memory transactions without a
> disruption or change in routing.
> 
> To preserve bridges, the PCI core tracks the number of downstream
> devices preserved under each bridge using a reference count in struct
> pci_dev_ser. This allows a bridge to remain preserved until all its
> downstream preserved devices are unpreserved or finish their
> participation in the Live Update.
> ...

> +static int pci_liveupdate_unpreserve_device(struct pci_flb_outgoing *outgoing,
> +					    struct pci_dev *dev)
>  {
>  	struct pci_dev_ser *dev_ser = dev->liveupdate.outgoing;
>  
>  	if (!dev_ser) {
>  		pci_warn(dev, "Cannot unpreserve device that is not preserved\n");
> -		return;
> +		return -EINVAL;
> +	}
> +
> +	if (!dev_ser->refcount) {
> +		pci_WARN(dev, 1, "Preserved device has a 0 refcount!\n");
> +		return -EINVAL;
>  	}
>  
> +	if (--dev_ser->refcount)
> +		return 0;

I guess this is a nit: I was looking for a dev_ser->refcount++ in this
patch to correspond with this decrement.  I *guess* the
"dev_ser->refcount = 1" in pci_liveupdate_preserve_device() starts
that, and the "dev->liveupdate.outgoing->refcount++" there is an
increment of the same refcount?  The connections there don't seem
quite obvious to me.

But I guess "dev_ser->refcount = 1" was added by a different patch, so
probably *not* the same?

>  	pci_info(dev, "Device will no longer be preserved across next Live Update\n");
>  	outgoing->ser->nr_devices--;
>  	memset(dev_ser, 0, sizeof(*dev_ser));
>  	dev->liveupdate.outgoing = NULL;
> +	return 0;
> +}

> @@ -385,13 +418,23 @@ static int pci_liveupdate_preserve_device(struct pci_flb_outgoing *outgoing,
>  	}
>  
>  	if (dev->liveupdate.outgoing) {
> -		pci_warn(dev, "Device is already preserved\n");
> -		return -EBUSY;
> -	}
> +		if (!dev->liveupdate.outgoing->refcount) {
> +			pci_WARN(dev, 1, "Preserved device with 0 refcount!\n");
> +			return -EINVAL;
> +		}
>  
> -	if (!pci_is_root_bus(dev->bus)) {
> -		pci_warn(dev, "Cannot preserve devices behind bridges\n");
> -		return -EINVAL;
> +		/*
> +		 * Endpoint devices should not be preserved more than once.
> +		 * Bridges are preserved once for every downstream device that
> +		 * is preserved.
> +		 */
> +		if (!dev->subordinate) {
> +			pci_warn(dev, "Device is already preserved\n");
> +			return -EBUSY;
> +		}
> +
> +		dev->liveupdate.outgoing->refcount++;
> +		return 0;
>  	}
>  
>  	dev_ser = pci_flb_alloc_dev_ser(outgoing);