Re: [PATCH v2] drm/amd/amdgpu: add firmware file fallback for APU VBIOS discovery

[Oz Tiram <oz@shift-computing.de>]

Hi Mario,

Tested on a Morefine MNAS X1 AI Workstation (AMD Ryzen 7 Pro 8845HS / 
Radeon 780M iGPU) with pci=realloc,assign-busses.

The VFCT entry for the iGPU has PCIBus=106 (0x6A, recorded at POST) 
while the
runtime bus is 11 (0x0B). Your patch fires exactly as expected:

   amdgpu 0000:0b:00.0: amdgpu: VFCT bus number mismatch: table 106 != 
runtime 11,
       matching by device identity (vendor 0x1002 device 0x1900)
   amdgpu 0000:0b:00.0: amdgpu: Fetched VBIOS from VFCT

The iGPU initialises fully and drives the framebuffer.

One minor nit: the dev_notice format string ends with \\n (two 
characters) rather
than \n. The resulting kernel message has a literal "\n" at the end. 
Same issue
exists in the nearby "too short #2" dev_info -- not introduced by your 
patch, but
might be worth cleaning up.

Tested-by: Oz Tiram <oz@shift-computing.de>

On 7/6/26 02:56, Mario Limonciello wrote:
>
>
> 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 <oz@shift-computing.de>
>>>> ---
>>>> 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;
>>>