Re: [PATCH] PCI: vmd: Fix secondary bus reset for Intel bridges

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Sep 23, 2022 at 01:37:57PM -0700, francisco.munoz.ruiz@linux.intel.com wrote:
> From: Francisco Munoz <francisco.munoz.ruiz@linux.intel.com>
> 
> The reset was never applied in the current implementation because Intel
> Bridges owned by VMD are parentless. Internally, the reset API applies
> a reset to the parent of the pci device supplied as argument, but in this
> case it failed because there wasn't a parent. This change feeds a child
> device of an Intel Bridge to the reset API and internally the reset is
> applied to its parent.

What kind of problem does this fix?  I guess some devices below a VMD
need to be reset before we can use them?

As a rule, Linux doesn't reset PCI devices at boot, so I'm just
wondering what's different about these.

If this fixes a problem, it's also nice if we can include a symptom in
the commit log so if people are seeing the problem, they can find the
solution, or distros can tell whether they need to include it.

> Signed-off-by: Francisco Munoz <francisco.munoz.ruiz@linux.intel.com>
> Reviewed-by: Nirmal Patel <nirmal.patel@linux.intel.com>
> ---
>  drivers/pci/controller/vmd.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
> index e06e9f4fc50f..34d6ba675440 100644
> --- a/drivers/pci/controller/vmd.c
> +++ b/drivers/pci/controller/vmd.c
> @@ -859,8 +859,16 @@ static int vmd_enable_domain(struct vmd_dev *vmd, unsigned long features)
>  
>  	pci_scan_child_bus(vmd->bus);
>  	vmd_domain_reset(vmd);
> -	list_for_each_entry(child, &vmd->bus->children, node)
> -		pci_reset_bus(child->self);
> +
> +	list_for_each_entry(child, &vmd->bus->children, node) {
> +		if (!list_empty(&child->devices)) {
> +			pci_reset_bus(list_first_entry(&child->devices,
> +						       struct pci_dev,
> +						       bus_list));
> +			break;
> +		}
> +	}
> +
>  	pci_assign_unassigned_bus_resources(vmd->bus);
>  
>  	/*
> -- 
> 2.25.1
>