Re: [PATCH v5] 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.
> 
> When there is no upstream bridge, call pci_bus_assign_resources() on
> the root bus to place the BARs released above, using the same
> alignment-sorted algorithm as normal enumeration instead of a manual
> per-BAR loop. This also walks the rest of the hierarchy under the
> root bus, as pci_assign_unassigned_bus_resources() used to for amdgpu
> before commit db92e3fef53e ("drm/amdgpu: Remove driver side BAR
> release before resize") removed that call -- the core-side fix that
> commit asked for ("such a problem should be fixed inside
> pci_resize_resource() instead").
> 
> pci_bus_assign_resources() returns void, so failure is detected by
> checking whether the released BARs are still assigned afterward; if
> not, roll back as in the bridged case. This is stricter than the
> bridged path -- it fails on any unplaced resource, not just required
> ones -- since a root bus typically has one shared window, and failing
> loudly seemed better than leaving something silently unassigned.
> 
> 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. The two are usually equivalent, but pci_upstream_bridge()
> is the canonical test -- pci_is_root_bus(), which it's built on,
> warns that bus->self == NULL doesn't necessarily mean a root bus
> (SR-IOV virtual buses from virtfn_add_bus() are the same).
> 
> 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
> Suggested-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> Suggested-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> Assisted-by: Claude:claude-fable-5 checkpatch
> Assisted-by: Claude:claude-sonnet-5
> 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 v5:
> - Per Ilpo Järvinen's review of v4: use pci_bus_assign_resources()
>   instead of a manual per-BAR pci_assign_resource() loop in the
>   root-bus branch. Same outcome for the bug being fixed, but reuses
>   the core's alignment-sorted placement algorithm

Great, it worked (I was somewhat afraid there might have been some 
landmine for the case where there's no bridge).

And yes, it's very important in general case to assign the resource in 
correct order because the assignment algorithm is greedy.

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>

-- 
 i.

>   and matches the
>   design intent stated in db92e3fef53e's commit message, rather than
>   reimplementing assignment ad hoc.
> - Drop Reviewed-by from Krzysztof Wilczyński: this revision changes
>   what the root-bus branch actually does (not just a predicate, as
>   in v3's pci_upstream_bridge() change), so his v2 review doesn't
>   carry over. Add Suggested-by for Ilpo instead.
> - Add Suggested-by for Bjorn Helgaas too: overdue credit for the
>   pci_upstream_bridge() suggestion from v3 review, which only ever
>   made it into the cover letter's changelog, not the actual commit
>   trailers.
> - Link to v4: https://patch.msgid.link/20260908-pci-rebar-root-bus-v4-1-2780e9c22e08@xxxxxxxxxxxx
> 
> 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 | 23 +++++++++++++++++------
>  1 file changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> index e8c94aa1d3c12..ed16ef7c26fa7 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 +2418,21 @@ 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 {
> +		/* No bridge window to adjust; let the core reassign the bus. */
> +		pci_bus_assign_resources(bus);
>  
> -	down_read(&pci_bus_sem);
> -	ret = pbus_reassign_bridge_resources(bus, res, &saved);
> -	if (ret)
> -		goto restore;
> +		list_for_each_entry(dev_res, &saved, list) {
> +			if (!resource_assigned(dev_res->res)) {
> +				ret = -ENOSPC;
> +				goto restore;
> +			}
> +		}
> +	}
>  
>  out:
>  	up_read(&pci_bus_sem);
> 
> ---
> base-commit: 893e11787f78e43b534e252249ac3fff4d1333f8
> change-id: 20260704-pci-rebar-root-bus-f3123fe10fc7
> 
> Best regards,
> --  
> Liz Fong-Jones <lizf@xxxxxxxxxxxx>
>