drm/amdgpu: Add comments to gmc structure

Explain fields like aper_base, agp_start etc. The definition
of those fields are confusing as they are from different view
(CPU or GPU). Add comments for easier understand.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Alex Deucher <Alex.Deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.h
index 555d8e5..406736a 100644 (file)
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.h
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.h
@@ -127,18 +127,48 @@ struct amdgpu_xgmi {
 };
 
 struct amdgpu_gmc {
+       /* FB's physical address in MMIO space (for CPU to
+        * map FB). This is different compared to the agp/
+        * gart/vram_start/end field as the later is from
+        * GPU's view and aper_base is from CPU's view.
+        */
        resource_size_t         aper_size;
        resource_size_t         aper_base;
        /* for some chips with <= 32MB we need to lie
         * about vram size near mc fb location */
        u64                     mc_vram_size;
        u64                     visible_vram_size;
+       /* AGP aperture start and end in MC address space
+        * Driver find a hole in the MC address space
+        * to place AGP by setting MC_VM_AGP_BOT/TOP registers
+        * Under VMID0, logical address == MC address. AGP
+        * aperture maps to physical bus or IOVA addressed.
+        * AGP aperture is used to simulate FB in ZFB case.
+        * AGP aperture is also used for page table in system
+        * memory (mainly for APU).
+        *
+        */
        u64                     agp_size;
        u64                     agp_start;
        u64                     agp_end;
+       /* GART aperture start and end in MC address space
+        * Driver find a hole in the MC address space
+        * to place GART by setting VM_CONTEXT0_PAGE_TABLE_START/END_ADDR
+        * registers
+        * Under VMID0, logical address inside GART aperture will
+        * be translated through gpuvm gart page table to access
+        * paged system memory
+        */
        u64                     gart_size;
        u64                     gart_start;
        u64                     gart_end;
+       /* Frame buffer aperture of this GPU device. Different from
+        * fb_start (see below), this only covers the local GPU device.
+        * Driver get fb_start from MC_VM_FB_LOCATION_BASE (set by vbios)
+        * and calculate vram_start of this local device by adding an
+        * offset inside the XGMI hive.
+        * Under VMID0, logical address == MC address
+        */
        u64                     vram_start;
        u64                     vram_end;
        /* FB region , it's same as local vram region in single GPU, in XGMI