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

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

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 VFCT table is absent or contains only the discrete GPU entry
    (e.g. when a custom ACPI override is present for the dGPU only).
  - The VRAM BAR is unmapped at probe time.
  - The ROM BAR is zero (PCI firmware did not assign it; observed even
    with pci=realloc,assign-busses).
  - No platform BIOS mapping exists.

The driver then prints "Unable to locate a BIOS ROM" and refuses to
bind, leaving the APU completely unusable under Linux even though the
hardware is functional.

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 from the
running hardware to be shipped as a firmware blob in /lib/firmware/ and
makes the binding succeed without any change to the ACPI tables.

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.

Signed-off-by: Oz Tiram <oz@shift-computing.de>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
index aa039e148a5e..491f88f495a6 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 +458,24 @@ static bool amdgpu_get_bios_apu(struct amdgpu_device *adev)
 		goto success;
 	}
 
+	{
+		const struct firmware *fw;
+		char fw_name[32];
+
+		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);
+			adev->bios_size = fw->size;
+			release_firmware(fw);
+			if (adev->bios) {
+				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;
 
-- 
2.53.0