[PATCH v2 1/3] drm/amdgpu: reset SI ASIC on MacBookPros with VRAM at 0

[Maciej Kucharek <qchcio@gmail.com>]

On MacBookPros with switchable graphics, the firmware puts VRAM at MC
address 0 when the iGPU is enabled.  The EFI stub enables the iGPU on
these machines through apple_set_os().  On a MacBookPro11,5 (Radeon R9
M370X, 1002:6821, subsystem 106b:0149), the driver adopts this layout
at init:

  MC_VM_FB_LOCATION=0x007f0000, HDP_NONSURFACE_BASE=0
  VRAM at 0x0-0x7fffffff, GART table at 0x1400000

The driver posts the ASIC again on S3 resume and after a GPU reset.
The VBIOS asic_init table then moves VRAM back to the VBIOS default:

  MC_VM_FB_LOCATION=0xf47ff400, HDP_NONSURFACE_BASE=0xf4000000

gmc_v6_0 does not reprogram these registers.  The GART table is then
no longer in VRAM, and the gfx ring test times out:

  amdgpu: ring gfx test failed (-110)
  amdgpu: resume of IP block <gfx_v6_0> failed -110

Reset and post the ASIC at driver load on Apple boards with VRAM at 0.
The driver then uses the VBIOS layout (VRAM at 0xf400000000) from the
start, and later posts keep it.  After the reset, FB_BASE is not 0, so
a module reload does not reset the ASIC again.

The test is the same one that commit c7f44ec5c4ec ("drm/amdgpu: make
Mac FB workaround generic") uses: an Apple subsystem vendor and VRAM at
0.  The VI quirk for the MacBookPro14,3 resets at load for the same
reason:

  commit a22e5e4f2ebb ("drm/amdgpu: reset VI ASIC on MacBookPro14,3")

Other Apple SI boards get the same reset if FB_BASE reads 0 at load.

Tested on a MacBookPro11,5 with 7.2.5 and this patch: the panel comes
up at boot, and 4 of 4 deep S3 cycles resume.  Without the patch, S3
resume fails every time.  The GPU reset path is not tested.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/5978
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Maciej Kucharek <qchcio@gmail.com>
---
 drivers/gpu/drm/amd/amdgpu/si.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/si.c b/drivers/gpu/drm/amd/amdgpu/si.c
index 0adfd4b10..e5e590425 100644
--- a/drivers/gpu/drm/amd/amdgpu/si.c
+++ b/drivers/gpu/drm/amd/amdgpu/si.c
@@ -1512,6 +1512,16 @@ static void si_invalidate_hdp(struct amdgpu_device *adev,
 
 static bool si_need_reset_on_init(struct amdgpu_device *adev)
 {
+	/*
+	 * MacBookPros with switchable graphics put VRAM at 0 when the iGPU
+	 * is enabled.  Any later post (S3 resume, GPU reset) moves it back to
+	 * the vbios default, so reset and post on load to start with the
+	 * vbios layout.
+	 */
+	if (adev->pdev->subsystem_vendor == PCI_VENDOR_ID_APPLE &&
+	    !(RREG32(mmMC_VM_FB_LOCATION) & MC_VM_FB_LOCATION__FB_BASE_MASK))
+		return true;
+
 	return false;
 }
 
-- 
2.55.0