Re: [PATCH v4] PCI: Fix BAR resize for devices on a root bus

[Ilpo Järvinen]

On Tue, 8 Sep 2026, Liz Fong-Jones wrote:

> pci_do_resource_release_and_resize() releases the device BARs that
> share a bridge window with the BAR being resized, but when the device
> sits directly on a root bus (pdev->bus->self == NULL) it then skips
> resource assignment entirely and returns success, leaving the BARs it
> just released unassigned (IORESOURCE_UNSET).
> 
> Skipping pbus_reassign_bridge_resources() is correct in that case --
> there is no bridge window to adjust -- but the device BARs still have
> to be reassigned. Before the BAR release was consolidated into the PCI
> core, this case worked for amdgpu because the driver released the BARs
> itself and then called pci_assign_unassigned_bus_resources()
> unconditionally after the resize, which assigns unassigned device BARs
> also on a root bus. Commit db92e3fef53e ("drm/amdgpu: Remove driver
> side BAR release before resize") removed that call, so nothing assigns
> the released BARs anymore.
> 
> This breaks amdgpu completely on the SolidRun HoneyComb LX2K (NXP
> LX2160A, arm64, ACPI), where the GPU endpoint is enumerated directly
> on the root bus of its segment (there is no root port device, so
> pdev->bus->self is NULL):
> 
>   amdgpu 0004:01:00.0: BAR 0 [mem 0xa400000000-0xa40fffffff 64bit pref]: releasing
>   amdgpu 0004:01:00.0: BAR 2 [mem 0xa410000000-0xa4101fffff 64bit pref]: releasing
>   amdgpu 0004:01:00.0: sw_init of IP block <gmc_v8_0> failed -19
>   amdgpu 0004:01:00.0: amdgpu_device_ip_init failed
>   amdgpu 0004:01:00.0: Fatal error during GPU init
> 
> No error is logged because the resize path reports success; amdgpu
> then finds BAR0 IORESOURCE_UNSET and bails out with -ENODEV.
> 
> Assign the released BARs directly from the root bus windows when there
> is no upstream bridge. On failure, roll back through the existing
> restore path exactly as in the bridged case.
> 
> The root bus path also had a locking bug that any fix here necessarily
> touches: the old "goto out" jumped to up_read(&pci_bus_sem) without a
> matching down_read() (as does the "goto restore" taken when
> pci_dev_res_add_to_list() fails in the release loop). Take pci_bus_sem
> before the BAR release loop so every path through the function holds
> it exactly once.
> 
> Use pci_upstream_bridge() rather than testing pdev->bus->self
> directly to decide whether the device has an upstream bridge. The two
> are usually equivalent, but pci_is_root_bus() -- which
> pci_upstream_bridge() uses internally -- documents that "virtual"
> buses added for SR-IOV via virtfn_add_bus() also have bus->self ==
> NULL without being root buses. pci_do_resource_release_and_resize()
> is reachable for VF BAR resizes (see the pci_resource_is_iov() checks
> elsewhere in this file and in rebar.c), so testing bus->self directly
> could misroute a VF resize into the root-bus assignment path added
> above. Nothing about the bug being fixed here involves a VF; this is
> purely for correctness on that path.
> 
> Fixes: 337b1b566db0 ("PCI: Fix restoring BARs on BAR resize rollback path")
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://bugs.launchpad.net/ubuntu/+source/linux-hwe-7.0/+bug/2159596
> Assisted-by: Claude:claude-fable-5 checkpatch
> Assisted-by: Claude:claude-sonnet-5
> Reviewed-by: Krzysztof Wilczyński <kwilczynski@xxxxxxxxxx>
> Signed-off-by: Liz Fong-Jones <lizf@xxxxxxxxxxxx>
> ---
> #regzbot introduced: 337b1b566db0
> 
> Observed at runtime on Ubuntu's linux-hwe-7.0 (7.0.0-14, broken) vs
> linux-hwe-6.17 (working), but nothing here is distro-specific: Ubuntu
> carries this code unmodified, and the affected function is identical
> to current mainline. By source inspection the regression window is
> v6.18 (old code paths) to v6.19 (consolidation). Workaround for
> affected users: amdgpu.rebar=0.
> 
> On the topology question raised in review: this is physical hardware,
> not a virtualized guest, running the UEFI/ACPI firmware variant of
> this board (SolidRun also ships a U-Boot/devicetree variant of the
> same hardware). Deferring to Jon Nettleton (CC'd, SolidRun) on the
> exact root complex wiring; see the hardware manual's block diagram:
> https://dev.solid-run.com/nxp/lx2160a/com-som/lx2160a-com-hardware-user-manual#block-diagram
> ---
> Changes in v4:
> - No functional changes. Resending per Thorsten Leemhuis (regressions
>   tracking) -- this looked to have fallen through the cracks after
>   v3. Confirmed still broken today on v7.3-rc2 (mainline) and v7.2.4
>   (stable): pci_do_resource_release_and_resize() still has the
>   original "if (!bus->self) goto out;" with no reassignment on that
>   path.
> - Rebase onto current mainline.
> - Link to v3: https://patch.msgid.link/20260731-pci-rebar-root-bus-v3-1-6732e233fc99@xxxxxxxxxxxx
> 
> Changes in v3:
> - Add Reviewed-by from Krzysztof Wilczyński, received on v2.
> - Use pci_upstream_bridge() instead of testing pdev->bus->self
>   directly, per Bjorn Helgaas's review: bus->self == NULL doesn't
>   necessarily mean a root bus (SR-IOV virtfn_add_bus() buses are the
>   same), and this function is reachable for VF BAR resizes elsewhere
>   in this file. Doesn't affect the bug being fixed here (not a VF)
>   but is more correct in general. Reviewed-by kept from v2 -- this is
>   a narrow, mechanical change to a path this device doesn't exercise,
>   not a substantive rework of what was reviewed.
> - Rebase onto current mainline.
> - Add Jacob Martin to Cc: he independently triaged and accepted the
>   corresponding Ubuntu report (LP: #2159596) on the Canonical kernel
>   team, so he has direct interest in this landing.
> - Move Ilpo Järvinen to To: for final review, per Bjorn.
> - Link to v2: https://patch.msgid.link/20260712-pci-rebar-root-bus-v2-1-a1b9107a82dc@xxxxxxxxxxxx
> 
> Changes in v2:
> - Add Assisted-by tags (missing from v1; required per
>   Documentation/process/coding-assistants.rst)
> - Add Link: to the corresponding Ubuntu bug report
> - Drop the "# v6.19+" annotation on Cc: stable; unnecessary noise
>   given the Fixes: tag already lets the stable team derive applicable
>   versions (per stable-kernel-rules.rst)
> - Link to v1: https://patch.msgid.link/20260705-pci-rebar-root-bus-v1-1-55df70cbdd88@xxxxxxxxxxxx
> 
> To: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> To: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> Cc: linux-pci@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: regressions@xxxxxxxxxxxxxxx
> Cc: amd-gfx@xxxxxxxxxxxxxxxxxxxxx
> Cc: Jon Nettleton <jon@xxxxxxxxxxxxx>
> Cc: Jon Nettleton <jon.nettleton@xxxxxxxxx>
> Cc: Jacob Martin <jacob.martin@xxxxxxxxxxxxx>
> Cc: Thorsten Leemhuis <regressions@xxxxxxxxxxxxx>
> ---
>  drivers/pci/setup-bus.c | 26 ++++++++++++++++++++------
>  1 file changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> index e8c94aa1d3c1..9907dc5521aa 100644
> --- a/drivers/pci/setup-bus.c
> +++ b/drivers/pci/setup-bus.c
> @@ -2380,6 +2380,7 @@ int pci_do_resource_release_and_resize(struct pci_dev *pdev, int resno, int size
>  	struct resource *res = pci_resource_n(pdev, resno);
>  	struct pci_dev_resource *dev_res;
>  	struct pci_bus *bus = pdev->bus;
> +	struct pci_dev *bridge = pci_upstream_bridge(pdev);
>  	struct resource *b_win, *r;
>  	LIST_HEAD(saved);
>  	unsigned int i;
> @@ -2397,6 +2398,8 @@ int pci_do_resource_release_and_resize(struct pci_dev *pdev, int resno, int size
>  	if (ret)
>  		return ret;
>  
> +	down_read(&pci_bus_sem);
> +
>  	pci_dev_for_each_resource(pdev, r, i) {
>  		if (i >= PCI_BRIDGE_RESOURCES)
>  			break;
> @@ -2415,13 +2418,24 @@ int pci_do_resource_release_and_resize(struct pci_dev *pdev, int resno, int size
>  
>  	pci_resize_resource_set_size(pdev, resno, size);
>  
> -	if (!bus->self)
> -		goto out;
> +	if (bridge) {
> +		ret = pbus_reassign_bridge_resources(bus, res, &saved);
> +		if (ret)
> +			goto restore;
> +	} else {
> +		/*
> +		 * A device on a root bus has no bridge windows to adjust.
> +		 * Assign the BARs released above directly from the root bus
> +		 * windows.
> +		 */
> +		list_for_each_entry(dev_res, &saved, list) {
> +			i = pci_resource_num(pdev, dev_res->res);
>  
> -	down_read(&pci_bus_sem);
> -	ret = pbus_reassign_bridge_resources(bus, res, &saved);
> -	if (ret)
> -		goto restore;
> +			ret = pci_assign_resource(pdev, i);
> +			if (ret)
> +				goto restore;

Maybe this is what has to be done here, but I wonder if this could use 
pci_bus_assign_resources()?

-- 
 i.