drm/amdgpu: reset SI ASIC on MacBookPros with VRAM at 0

Reset and post the SI GPU at driver load on Apple boards whose firmware
left VRAM at MC address 0. Fixes deep sleep (S3) resume and GPU reset
with amdgpu on the MacBookPro11,5 (Radeon R9 M370X). See hardware.md
("Deep sleep on amdgpu") for the cause.

Submitted upstream to amd-gfx: v1 on 2026-10-14, v2 on 2026-10-15 as
patch 1 of a series that adds the same check for CIK and VI. The code is
the same in both versions. Only the comment changed in v2.
https://gitlab.freedesktop.org/drm/amd/-/issues/5978
Applies to linux 7.2.x and amd-staging-drm-next (checked: 7.2.5).

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;
 }