Re: [Intel-gfx] [PATCH v3 06/12] drm/ttm: add TTM_PAGE_FLAG_EXTERNAL_MAPPABLE

["Christian König" <christian.koenig@amd.com>]

Am 15.09.21 um 20:59 schrieb Matthew Auld:
> In commit:
>
> commit 667a50db0477d47fdff01c666f5ee1ce26b5264c
> Author: Thomas Hellstrom <thellstrom@vmware.com>
> Date:   Fri Jan 3 11:17:18 2014 +0100
>
>      drm/ttm: Refuse to fault (prime-) imported pages
>
> we introduced the restriction that imported pages should not be directly
> mappable through TTM(this also extends to userptr). In the next patch we
> want to introduce a shmem_tt backend, which should follow all the
> existing rules with TTM_PAGE_FLAG_EXTERNAL, since it will need to handle
> swapping itself, but with the above mapping restriction lifted.
>
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Christian König <christian.koenig@amd.com>
> ---
>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 6 ++++--
>   include/drm/ttm/ttm_tt.h        | 7 +++++++
>   2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> index 708390588c7c..fd6e18f12f50 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -163,8 +163,10 @@ vm_fault_t ttm_bo_vm_reserve(struct ttm_buffer_object *bo,
>   	 * (if at all) by redirecting mmap to the exporter.
>   	 */
>   	if (bo->ttm && (bo->ttm->page_flags & TTM_PAGE_FLAG_EXTERNAL)) {
> -		dma_resv_unlock(bo->base.resv);
> -		return VM_FAULT_SIGBUS;
> +		if (!(bo->ttm->page_flags & TTM_PAGE_FLAG_EXTERNAL_MAPPABLE)) {
> +			dma_resv_unlock(bo->base.resv);
> +			return VM_FAULT_SIGBUS;
> +		}
>   	}
>   
>   	return 0;
> diff --git a/include/drm/ttm/ttm_tt.h b/include/drm/ttm/ttm_tt.h
> index 7f54a83c95ef..800c9edb3e10 100644
> --- a/include/drm/ttm/ttm_tt.h
> +++ b/include/drm/ttm/ttm_tt.h
> @@ -66,11 +66,18 @@ struct ttm_tt {
>   	 * Note that enum ttm_bo_type.ttm_bo_type_sg objects will always enable
>   	 * this flag.
>   	 *
> +	 * TTM_PAGE_FLAG_EXTERNAL_MAPPABLE: Same behaviour as
> +	 * TTM_PAGE_FLAG_EXTERNAL, but with the reduced restriction that it is
> +	 * still valid to use TTM to map the pages directly. This is useful when
> +	 * implementing a ttm_tt backend which still allocates driver owned
> +	 * pages underneath(say with shmem).
> +	 *
>   	 * TTM_PAGE_FLAG_PRIV_POPULATED: TTM internal only. DO NOT USE.
>   	 */
>   #define TTM_PAGE_FLAG_SWAPPED		(1 << 0)
>   #define TTM_PAGE_FLAG_ZERO_ALLOC	(1 << 1)
>   #define TTM_PAGE_FLAG_EXTERNAL		(1 << 2)
> +#define TTM_PAGE_FLAG_EXTERNAL_MAPPABLE	(1 << 3 | TTM_PAGE_FLAG_EXTERNAL)

That's really bad practice because an "if (!(flags & 
TTM_PAGE_FLAG_EXTERNAL_MAPPABLE))" has a different semantics as an "if 
(flags & TTM_PAGE_FLAG_EXTERNAL_MAPPABLE)".

Rather add a TTM_PAGE_FLAG_UNMAPPABLE and make sure that it is set as 
appropriated.

Regards,
Christian.

>   
>   #define TTM_PAGE_FLAG_PRIV_POPULATED	(1 << 31)
>   	uint32_t page_flags;