Re: [PATCH] drm/amdgpu: Gate VFCT bus fallback on pci=realloc

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Alex, Ilpo, Lukas, linux-acpi]

On Thu, Oct 15, 2026 at 07:48:42AM -0500, Mario Limonciello wrote:
> The soft filter system introduced by commit 11c141672045f ("drm/amdgpu:
> Fix VFCT bus number matching with soft filter") has a deficiency that
> it won't work when there are multiple GPUs of the exact same model.
> 
> As it was introduced specifically to help the case of pci=realloc, gate
> it to only run when that's in use.
> 
> Fixes: 11c141672045f ("drm/amdgpu: Fix VFCT bus number matching with soft filter")
> Reported-by: Reza Nikzad Chaleshori <rezanikzadch@gmail.com>
> Closes: https://lore.kernel.org/amd-gfx/20261014080523.186533-1-rezanikzadch@gmail.com/
> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c | 15 ++++++++-------
>  drivers/pci/setup-bus.c                  |  5 +++++
>  include/linux/pci.h                      |  1 +
>  3 files changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
> index 08ed1eb695586..75af9a59671ae 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
> @@ -384,12 +384,10 @@ static bool amdgpu_read_disabled_bios(struct amdgpu_device *adev)
>   * @vhdr: VFCT image header to check
>   *
>   * VFCT entries contain the PCI bus number as recorded during BIOS POST.
> - * On systems where the kernel renumbers PCI buses (e.g. pci=realloc or
> - * resource conflicts), the runtime bus number may differ from the POST
> - * value.  Match by device identity (vendor + device + function) and use
> - * the bus number as a preference: exact bus match is preferred, but when
> - * the bus numbers disagree we accept the entry if the device identity
> - * matches.
> + * On systems where the user requested PCI resource reallocation, the
> + * runtime bus number may differ from the POST value. Match by device
> + * identity (vendor + device + function), but only treat the bus number as
> + * a preference when pci=realloc was explicitly requested.

This is a generic problem, isn't it?  

I think any table from firmware that has PCI bus numbers in it could
trip over issues like this.  Seems like we need a more generic
solution.

>   *
>   * Returns: 0 on match, -ENODEV on no match
>   */
> @@ -409,7 +407,10 @@ static int amdgpu_acpi_vfct_match(struct amdgpu_device *adev,
>  	if (vhdr->PCIBus == adev->pdev->bus->number)
>  		return 0;
>  
> -	/* Bus mismatch but device identity matches (PCI renumbering case) */
> +	/* Allow bus renumbering only when the user requested pci=realloc. */
> +	if (!pci_realloc_enabled_by_user())
> +		return -ENODEV;
> +
>  	dev_notice(adev->dev,
>  		   "VFCT bus number mismatch: table %u != runtime %u, matching by device identity (vendor 0x%04x device 0x%04x)\n",
>  		   vhdr->PCIBus, adev->pdev->bus->number,
> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> index ed16ef7c26fa7..d9450585082c0 100644
> --- a/drivers/pci/setup-bus.c
> +++ b/drivers/pci/setup-bus.c
> @@ -1799,6 +1799,11 @@ static bool pci_realloc_enabled(enum enable_type enable)
>  	return enable >= user_enabled;
>  }
>  
> +bool pci_realloc_enabled_by_user(void)
> +{
> +	return pci_realloc_enable == user_enabled;
> +}

I don't like this.  I think the same bus renumbering can happen in the
"auto_enabled" case, and I don't think we set pci_realloc_enable in
that case.

But more generally, I think ultimately the PCI core should be able to
reassign bus numbers arbitrarily to accommodate hotplugs (given the
appropriate driver support to tolerate that, of course).

>  #if defined(CONFIG_PCI_IOV) && defined(CONFIG_PCI_REALLOC_ENABLE_AUTO)
>  static int iov_resources_unassigned(struct pci_dev *dev, void *data)
>  {
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index d31a8d107b1ef..844734da9250f 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -610,6 +610,7 @@ static inline struct pci_dev *pci_physfn(struct pci_dev *dev)
>  }
>  
>  struct pci_dev *pci_alloc_dev(struct pci_bus *bus);
> +bool pci_realloc_enabled_by_user(void);
>  
>  #define	to_pci_dev(n) container_of(n, struct pci_dev, dev)
>  #define for_each_pci_dev(d) while ((d = pci_get_device(PCI_ANY_ID, PCI_ANY_ID, d)) != NULL)
> -- 
> 2.43.0
>