Re: [RFC PATCH v2 7/8] drm/amdgpu/ttm: Allocate/Free 4K MMIO_REMAP Singleton BO

[Alex Deucher <alexdeucher@gmail.com>]

On Sat, Aug 23, 2025 at 3:20 AM Srinivasan Shanmugam
<srinivasan.shanmugam@amd.com> wrote:
>
> Add amdgpu_ttm_mmio_remap_bo_init()/fini() to manage the kernel-owned
> one-page(4K) MMIO_REMAP BO. The allocator runs during TTM init when the
> hardware exposes a remap base (adev->rmmio_base) and the host
> PAGE_SIZE is <= AMDGPU_GPU_PAGE_SIZE (4K).
>
> The helper is idempotent (returns 0 if already allocated) and only
> returns an error when the actual allocation fails.
>
> This keeps MMIO_REMAP lifetime handling localized and prepares for the
> subsequent patch that exposes a userspace handle.
>
> Cc: Christian König <christian.koenig@amd.com>
> Suggested-by: Alex Deucher <alexander.deucher@amd.com>
> Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 69 +++++++++++++++++++++++++
>  1 file changed, 69 insertions(+)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> index 58b6ab1be4c1..c76c41a312b0 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> @@ -1853,6 +1853,68 @@ static void amdgpu_ttm_pools_fini(struct amdgpu_device *adev)
>         adev->mman.ttm_pools = NULL;
>  }
>
> +/**
> + * amdgpu_ttm_mmio_remap_bo_init - allocate the singleton 4K MMIO_REMAP BO
> + * @adev: amdgpu device
> + *
> + * Allocates the kernel-owned one-page buffer in AMDGPU_GEM_DOMAIN_MMIO_REMAP
> + * when the hardware exposes a remap base (adev->rmmio_remap.base) and the host
> + * PAGE_SIZE is <= AMDGPU_GPU_PAGE_SIZE (4K). If either condition is not met, the
> + * function returns 0 and leaves adev->rmmio_remap.bo as NULL.
> + *
> + * If the BO is already allocated, the function does nothing and returns 0.
> + * Only errors during actual allocation (e.g., amdgpu_bo_create_kernel()) are
> + * propagated as negative returns.
> + *
> + * Return:
> + *  * 0 on success or intentional skip (feature not present/unsupported)
> + *  * negative errno on allocation failure
> + */
> +static int amdgpu_ttm_mmio_remap_bo_init(struct amdgpu_device *adev)
> +{
> +       int r;
> +
> +       if (!adev->rmmio_base)

Check for adev->rmmio_remap.bus_addr here.

> +               return 0;
> +
> +       /* Hardware remap page is fixed 4K; skip on larger PAGE_SIZE. */
> +       if (PAGE_SIZE > AMDGPU_GPU_PAGE_SIZE) {
> +               dev_warn(adev->dev, "MMIO_REMAP disabled: PAGE_SIZE=%lu > 4K\n", PAGE_SIZE);

No need to warn here.

> +               return 0;
> +       }
> +
> +       if (adev->rmmio_remap.bo)
> +               return 0;

Why is this here?

> +
> +       /* Create exactly ONE kernel-owned BO in the MMIO_REMAP domain */
> +       r = amdgpu_bo_create_kernel(adev,
> +                                   AMDGPU_GPU_PAGE_SIZE, AMDGPU_GPU_PAGE_SIZE,
> +                                   AMDGPU_GEM_DOMAIN_MMIO_REMAP,
> +                                   &adev->rmmio_remap.bo,
> +                                   NULL, NULL);
> +       if (r) {
> +               dev_err(adev->dev, "MMIO_REMAP: BO create failed (%d)\n", r);
> +               return r;
> +       }
> +
> +       return 0;
> +}
> +
> +/**
> + * amdgpu_ttm_mmio_remap_bo_fini - free the singleton MMIO_REMAP BO
> + * @adev: amdgpu device
> + *
> + * Frees the kernel-owned MMIO_REMAP BO if it was allocated by
> + * amdgpu_ttm_mmio_remap_bo_init().
> + */
> +static void amdgpu_ttm_mmio_remap_bo_fini(struct amdgpu_device *adev)
> +{
> +       if (adev->rmmio_remap.bo) {
> +               amdgpu_bo_free_kernel(&adev->rmmio_remap.bo, NULL, NULL);
> +               adev->rmmio_remap.bo = NULL;
> +       }
> +}
> +
>  /*
>   * amdgpu_ttm_init - Init the memory management (ttm) as well as various
>   * gtt/vram related fields.
> @@ -2027,6 +2089,11 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
>                 return r;
>         }
>
> +       /* Allocate the singleton MMIO_REMAP BO (4K) if supported */
> +       r = amdgpu_ttm_mmio_remap_bo_init(adev);
> +       if (r)
> +               return r;
> +
>         /* Initialize preemptible memory pool */
>         r = amdgpu_preempt_mgr_init(adev);
>         if (r) {
> @@ -2090,6 +2157,8 @@ void amdgpu_ttm_fini(struct amdgpu_device *adev)
>         amdgpu_bo_free_kernel(&adev->mman.sdma_access_bo, NULL,
>                                         &adev->mman.sdma_access_ptr);
>
> +       /* Drop the singleton MMIO_REMAP BO (if allocated) */
> +       amdgpu_ttm_mmio_remap_bo_fini(adev);
>         amdgpu_ttm_fw_reserve_vram_fini(adev);
>         amdgpu_ttm_drv_reserve_vram_fini(adev);
>
> --
> 2.34.1
>