On 7/5/26 14:10, Oz Tiram wrote:
Hi Mario,
To make sure I understand correctly: are you suggesting that the bus
number in the VFCT was legitimate at BIOS POST time, and that
pci=realloc,assign-busses is what changes it at runtime, causing the
mismatch?
That's what it sounds like right now. You can easily drop all the
superfluous kernel command line optiosn and see.
I'm not familiar enough with the PCI subsystem to know the right way to
implement that — could you point me in the right direction?
Well there's a variety of ways to do it. But how about we start here -
if we make that specific busnr match optional and instead make a VID/DID
match.
See if the attached patch helps.
Oz
On 7/5/26 20:37, Mario Limonciello wrote:
On 7/5/26 05:04, Oz Tiram wrote:
APUs (e.g. AMD Radeon 780M / HawkPoint, PCI 1002:1900) have no
dedicated VBIOS ROM chip. amdgpu_get_bios_apu() attempts four paths
before giving up:
1. ACPI VFCT table
2. VRAM BAR read
3. ROM BAR read
4. platform BIOS
On some systems all four fail. The specific case motivating this patch
is a hybrid graphics machine (dGPU + APU) where:
- The VFCT table contains the iGPU entry but with a stale PCIBus
value
from BIOS POST time (0x6A). When the kernel boots with
pci=realloc,assign-busses, PCI bus numbers are reassigned
dynamically
and the iGPU lands on bus 0x0B at runtime. amdgpu_acpi_vfct_bios()
matches entries by bus number, so the entry is never found.
- The VRAM BAR is unmapped at probe time.
- The ROM BAR is zero (PCI firmware did not assign it).
- No platform BIOS mapping exists.
The UEFI GOP driver initialises the iGPU successfully for early display,
confirming the hardware is functional. The VBIOS image data embedded in
the VFCT is also valid; only the PCIBus metadata is wrong.
So the BIOS on this machine is actually totally fine; it's just when
the kernel is booted to reassign busses there is a problem?
In that case; why not detect the kernel was booted this way and keep
track of the original bus number when reassigned to avoid the issue?
The firmware
file can be extracted directly from the VFCT using dd:
dd if=/sys/firmware/acpi/tables/VFCT bs=1 skip=$((0x68))
count=16896 \
of=/lib/firmware/amdgpu/1002_1900.bin
(0x68 is the byte offset of the VBIOS image after the ACPI table header
and VFCT_IMAGE_HEADER; the image length 16896 comes from the ImageLength
field in VFCT_IMAGE_HEADER.)
The driver then prints "Unable to locate a BIOS ROM" and refuses to
bind, leaving the APU completely unusable under Linux.
Add a fifth fallback: request a firmware file named
"amdgpu/<vendor>_<device>.bin" (e.g. "amdgpu/1002_1900.bin") via
request_firmware(). This allows a VBIOS image extracted as above to be
placed in /lib/firmware/ and makes the binding succeed without patching
ACPI tables or BIOS.
The fallback is only reached if all existing paths have already failed,
so there is no regression risk for boards where VFCT or ROM BAR work.
What happens if the VBIOS changes in another way one boot to another?
You might have some other stateful information that isn't updated.
The whole thing to me feels like a hack for a behavior we can control
in the kernel when doing reassignments.
Signed-off-by: Oz Tiram <[email protected]>
---
v2: Fix commit message: clarify that VFCT contains the iGPU entry but
with a stale PCIBus from BIOS POST that mismatches the runtime bus
number assigned by pci=realloc,assign-busses. Explain that the
VBIOS
image data is valid and document the dd extraction command and byte
offsets. Note that the UEFI GOP driver initialises the iGPU
successfully, confirming the hardware is functional.
drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c b/drivers/gpu/
drm/amd/amdgpu/amdgpu_bios.c
index aa039e148a5e..86064c753b09 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
@@ -26,6 +26,7 @@
* Jerome Glisse
*/
+#include <linux/firmware.h>
#include "amdgpu.h"
#include "atom.h"
@@ -457,6 +458,28 @@ static bool amdgpu_get_bios_apu(struct
amdgpu_device *adev)
goto success;
}
+ {
+ const struct firmware *fw;
+ char fw_name[32];
+ size_t fw_size;
+
+ snprintf(fw_name, sizeof(fw_name), "amdgpu/%04x_%04x.bin",
+ adev->pdev->vendor, adev->pdev->device);
+ if (request_firmware(&fw, fw_name, adev->dev) == 0) {
+ adev->bios = kmemdup(fw->data, fw->size, GFP_KERNEL);
+ fw_size = fw->size;
+ release_firmware(fw);
+ if (!adev->bios || !check_atom_bios(adev, fw_size)) {
+ amdgpu_bios_release(adev);
+ } else {
+ adev->bios_size = fw_size;
+ dev_info(adev->dev, "Fetched VBIOS from firmware
file %s\n",
+ fw_name);
+ goto success;
+ }
+ }
+ }
+
dev_err(adev->dev, "Unable to locate a BIOS ROM\n");
return false;
From 42ac2fe3085cc3593523ac3e514bc36616078b4f Mon Sep 17 00:00:00 2001
From: Mario Limonciello <[email protected]>
Date: Sun, 5 Jul 2026 15:48:05 -0500
Subject: [PATCH] drm/amdgpu/radeon: Fix VFCT bus number matching with soft
filter
On systems where PCI bus renumbering occurs (e.g. pci=realloc,
resource conflicts), the runtime bus number may differ from the
BIOS POST bus number recorded in the VFCT table. This causes
amdgpu_acpi_vfct_bios() to fail finding the VBIOS even though
the correct device entry exists.
Introduce amdgpu_acpi_vfct_match() which treats the bus number
as a soft filter: vendor/device/function identity is the hard
requirement, while exact bus match is the preferred path. When
bus numbers disagree but device identity matches, accept the
VFCT entry and log a dev_notice for diagnostics.
Apply the same fix to radeon_acpi_vfct_bios().
While updating radeon, also modernize the error handling:
- Replace DRM_ERROR with dev_info/dev_notice
- Remove legacy goto out + acpi_put_table pattern (acpi_put_table
was removed from the kernel)
- Use early return instead of goto for cleaner control flow
Signed-off-by: Mario Limonciello <[email protected]>
---
drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c | 48 +++++++++++++--
drivers/gpu/drm/radeon/radeon_bios.c | 78 +++++++++++++++++-------
2 files changed, 99 insertions(+), 27 deletions(-)
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
index 35d04e69aec0..fa730af5e979 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
@@ -370,6 +370,46 @@ static bool amdgpu_read_disabled_bios(struct amdgpu_device *adev)
false : amdgpu_asic_read_disabled_bios(adev);
}
+/**
+ * amdgpu_acpi_vfct_match() - Check if a VFCT entry matches the device
+ * @adev: AMDGPU device
+ * @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.
+ *
+ * Returns: 0 on match, -ENODEV on no match
+ */
+static int amdgpu_acpi_vfct_match(struct amdgpu_device *adev,
+ VFCT_IMAGE_HEADER *vhdr)
+{
+ /* Vendor and device IDs must always match */
+ if (vhdr->VendorID != adev->pdev->vendor ||
+ vhdr->DeviceID != adev->pdev->device)
+ return -ENODEV;
+
+ if (vhdr->PCIDevice != PCI_SLOT(adev->pdev->devfn) ||
+ vhdr->PCIFunction != PCI_FUNC(adev->pdev->devfn))
+ return -ENODEV;
+
+ /* Exact bus number match - preferred */
+ if (vhdr->PCIBus == adev->pdev->bus->number)
+ return 0;
+
+ /* Bus mismatch but device identity matches (PCI renumbering case) */
+ 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,
+ adev->pdev->vendor, adev->pdev->device);
+ return 0;
+}
+
#ifdef CONFIG_ACPI
static bool amdgpu_acpi_vfct_bios(struct amdgpu_device *adev)
{
@@ -406,11 +446,7 @@ static bool amdgpu_acpi_vfct_bios(struct amdgpu_device *adev)
}
if (vhdr->ImageLength &&
- vhdr->PCIBus == adev->pdev->bus->number &&
- vhdr->PCIDevice == PCI_SLOT(adev->pdev->devfn) &&
- vhdr->PCIFunction == PCI_FUNC(adev->pdev->devfn) &&
- vhdr->VendorID == adev->pdev->vendor &&
- vhdr->DeviceID == adev->pdev->device) {
+ !amdgpu_acpi_vfct_match(adev, vhdr)) {
adev->bios = kmemdup(&vbios->VbiosContent,
vhdr->ImageLength,
GFP_KERNEL);
@@ -424,7 +460,7 @@ static bool amdgpu_acpi_vfct_bios(struct amdgpu_device *adev)
}
}
- dev_info(adev->dev, "ACPI VFCT table present but broken (too short #2),skipping\n");
+ dev_info(adev->dev, "ACPI VFCT table present but broken (too short #2),skipping\\n");
return false;
}
#else
diff --git a/drivers/gpu/drm/radeon/radeon_bios.c b/drivers/gpu/drm/radeon/radeon_bios.c
index 3a8c5199a0fe..5938a8ed6e73 100644
--- a/drivers/gpu/drm/radeon/radeon_bios.c
+++ b/drivers/gpu/drm/radeon/radeon_bios.c
@@ -596,21 +596,60 @@ static bool radeon_read_disabled_bios(struct radeon_device *rdev)
return legacy_read_disabled_bios(rdev);
}
+/**
+ * radeon_acpi_vfct_match() - Check if a VFCT entry matches the device
+ * @rdev: Radeon device
+ * @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.
+ *
+ * Returns: 0 on match, -ENODEV on no match
+ */
+static int radeon_acpi_vfct_match(struct radeon_device *rdev,
+ VFCT_IMAGE_HEADER *vhdr)
+{
+ /* Vendor and device IDs must always match */
+ if (vhdr->VendorID != rdev->pdev->vendor ||
+ vhdr->DeviceID != rdev->pdev->device)
+ return -ENODEV;
+
+ if (vhdr->PCIDevice != PCI_SLOT(rdev->pdev->devfn) ||
+ vhdr->PCIFunction != PCI_FUNC(rdev->pdev->devfn))
+ return -ENODEV;
+
+ /* Exact bus number match - preferred */
+ if (vhdr->PCIBus == rdev->pdev->bus->number)
+ return 0;
+
+ /* Bus mismatch but device identity matches (PCI renumbering case) */
+ dev_notice(&rdev->pdev->dev,
+ "VFCT bus number mismatch: table %u != runtime %u, "
+ "matching by device identity (vendor 0x%04x device 0x%04x)\n",
+ vhdr->PCIBus, rdev->pdev->bus->number,
+ rdev->pdev->vendor, rdev->pdev->device);
+ return 0;
+}
+
#ifdef CONFIG_ACPI
static bool radeon_acpi_vfct_bios(struct radeon_device *rdev)
{
struct acpi_table_header *hdr;
acpi_size tbl_size;
UEFI_ACPI_VFCT *vfct;
- unsigned offset;
- bool r = false;
+ unsigned int offset;
if (!ACPI_SUCCESS(acpi_get_table("VFCT", 1, &hdr)))
return false;
tbl_size = hdr->length;
if (tbl_size < sizeof(UEFI_ACPI_VFCT)) {
- DRM_ERROR("ACPI VFCT table present but broken (too short #1)\n");
- goto out;
+ dev_info(&rdev->pdev->dev, "ACPI VFCT table present but broken (too short #1),skipping\n");
+ return false;
}
vfct = (UEFI_ACPI_VFCT *)hdr;
@@ -622,37 +661,34 @@ static bool radeon_acpi_vfct_bios(struct radeon_device *rdev)
offset += sizeof(VFCT_IMAGE_HEADER);
if (offset > tbl_size) {
- DRM_ERROR("ACPI VFCT image header truncated\n");
- goto out;
+ dev_info(&rdev->pdev->dev, "ACPI VFCT image header truncated,skipping\n");
+ return false;
}
offset += vhdr->ImageLength;
if (offset > tbl_size) {
- DRM_ERROR("ACPI VFCT image truncated\n");
- goto out;
+ dev_info(&rdev->pdev->dev, "ACPI VFCT image truncated,skipping\n");
+ return false;
}
if (vhdr->ImageLength &&
- vhdr->PCIBus == rdev->pdev->bus->number &&
- vhdr->PCIDevice == PCI_SLOT(rdev->pdev->devfn) &&
- vhdr->PCIFunction == PCI_FUNC(rdev->pdev->devfn) &&
- vhdr->VendorID == rdev->pdev->vendor &&
- vhdr->DeviceID == rdev->pdev->device) {
+ !radeon_acpi_vfct_match(rdev, vhdr)) {
rdev->bios = kmemdup(&vbios->VbiosContent,
vhdr->ImageLength,
GFP_KERNEL);
- if (rdev->bios)
- r = true;
- goto out;
+ if (!rdev->bios ||
+ rdev->bios[0] != 0x55 || rdev->bios[1] != 0xaa) {
+ kfree(rdev->bios);
+ rdev->bios = NULL;
+ return false;
+ }
+ return true;
}
}
- DRM_ERROR("ACPI VFCT table present but broken (too short #2)\n");
-
-out:
- acpi_put_table(hdr);
- return r;
+ dev_info(&rdev->pdev->dev, "ACPI VFCT table present but broken (too short #2),skipping\n");
+ return false;
}
#else
static inline bool radeon_acpi_vfct_bios(struct radeon_device *rdev)
--
2.53.0
