[PATCH] drm/amdgpu: Add comments to gmc structure

[Zeng, Oak]

Thanks Alex. 

> AGP is also used for page tables in system memory.
I am not aware of this usage. I thought page table are all in frame buffer today. Was this a use case in old asics?

Oak

-----Original Message-----
From: Alex Deucher <alexdeucher at gmail.com> 
Sent: Wednesday, November 6, 2019 12:37 PM
To: Zeng, Oak <Oak.Zeng at amd.com>
Cc: amd-gfx at lists.freedesktop.org; Kuehling, Felix <Felix.Kuehling at amd.com>; Koenig, Christian <Christian.Koenig at amd.com>
Subject: Re: [PATCH] drm/amdgpu: Add comments to gmc structure

On Wed, Nov 6, 2019 at 12:21 PM Zeng, Oak <Oak.Zeng at amd.com> wrote:
>
> 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.
>
> Change-Id: I02c2a27cd0dbc205498eb86aafa722f2e0c25fe6
> Signed-off-by: Oak Zeng <Oak.Zeng at amd.com>

A few comments below, otherwise looks good to me.

> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.h | 25 
> +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.h 
> b/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.h
> index 555d8e5..8003201 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.h
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.h
> @@ -127,18 +127,43 @@ struct amdgpu_xgmi {  };
>
>  struct amdgpu_gmc {
> +       /* FB's physical address in MMIO space (for CPU to
> +        * map FB). This is different compared to the apg/

apg -> 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;
> +       /* APG aperture start and end in MC address space

APG -> AGP

> +        * 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 is used to simulate FB in ZFB case
> +        */

You may want to add a comment that the AGP aperture just maps to physical bus or IOVA addresses on the platform.  It's also used for page tables in system memory.

>         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.
> +        */
>         u64                     vram_start;
>         u64                     vram_end;
>         /* FB region , it's same as local vram region in single GPU, 
> in XGMI
> --
> 2.7.4
>
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