Re: [PATCH v6 1/2] drm/ttm: Add a flag to allow drivers to skip clear-on-free

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

Hi, Nirmoy, Christian

On Fri, 2024-08-16 at 15:51 +0200, Nirmoy Das wrote:
> Add TTM_TT_FLAG_CLEARED_ON_FREE, which DRM drivers can set before
> releasing backing stores if they want to skip clear-on-free.
> 
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Suggested-by: Christian König <christian.koenig@amd.com>
> Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
> Reviewed-by: Christian König <christian.koenig@amd.com>

What happens if two devices share the same global TTM pool
 type and one that does its own clearing. Wouldn't there be a pretty
high chance that the the device that doesn't clear its own pages
allocate non-cleared memory from the pool?

/Thomas

> ---
>  drivers/gpu/drm/ttm/ttm_pool.c | 18 +++++++++++-------
>  include/drm/ttm/ttm_tt.h       |  6 +++++-
>  2 files changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_pool.c
> b/drivers/gpu/drm/ttm/ttm_pool.c
> index 8504dbe19c1a..935ab3cfd046 100644
> --- a/drivers/gpu/drm/ttm/ttm_pool.c
> +++ b/drivers/gpu/drm/ttm/ttm_pool.c
> @@ -222,15 +222,18 @@ static void ttm_pool_unmap(struct ttm_pool
> *pool, dma_addr_t dma_addr,
>  }
>  
>  /* Give pages into a specific pool_type */
> -static void ttm_pool_type_give(struct ttm_pool_type *pt, struct page
> *p)
> +static void ttm_pool_type_give(struct ttm_pool_type *pt, struct page
> *p,
> +			       bool cleared)
>  {
>  	unsigned int i, num_pages = 1 << pt->order;
>  
> -	for (i = 0; i < num_pages; ++i) {
> -		if (PageHighMem(p))
> -			clear_highpage(p + i);
> -		else
> -			clear_page(page_address(p + i));
> +	if (!cleared) {
> +		for (i = 0; i < num_pages; ++i) {
> +			if (PageHighMem(p))
> +				clear_highpage(p + i);
> +			else
> +				clear_page(page_address(p + i));
> +		}
>  	}
>  
>  	spin_lock(&pt->lock);
> @@ -394,6 +397,7 @@ static void ttm_pool_free_range(struct ttm_pool
> *pool, struct ttm_tt *tt,
>  				pgoff_t start_page, pgoff_t
> end_page)
>  {
>  	struct page **pages = &tt->pages[start_page];
> +	bool cleared = tt->page_flags & TTM_TT_FLAG_CLEARED_ON_FREE;
>  	unsigned int order;
>  	pgoff_t i, nr;
>  
> @@ -407,7 +411,7 @@ static void ttm_pool_free_range(struct ttm_pool
> *pool, struct ttm_tt *tt,
>  
>  		pt = ttm_pool_select_type(pool, caching, order);
>  		if (pt)
> -			ttm_pool_type_give(pt, *pages);
> +			ttm_pool_type_give(pt, *pages, cleared);
>  		else
>  			ttm_pool_free_page(pool, caching, order,
> *pages);
>  	}
> diff --git a/include/drm/ttm/ttm_tt.h b/include/drm/ttm/ttm_tt.h
> index 2b9d856ff388..cfaf49de2419 100644
> --- a/include/drm/ttm/ttm_tt.h
> +++ b/include/drm/ttm/ttm_tt.h
> @@ -85,6 +85,9 @@ struct ttm_tt {
>  	 * fault handling abuses the DMA api a bit and dma_map_attrs
> can't be
>  	 * used to assure pgprot always matches.
>  	 *
> +	 * TTM_TT_FLAG_CLEARED_ON_FREE: Set this if a drm driver
> handles
> +	 * clearing backing store
> +	 *
>  	 * TTM_TT_FLAG_PRIV_POPULATED: TTM internal only. DO NOT
> USE. This is
>  	 * set by TTM after ttm_tt_populate() has successfully
> returned, and is
>  	 * then unset when TTM calls ttm_tt_unpopulate().
> @@ -94,8 +97,9 @@ struct ttm_tt {
>  #define TTM_TT_FLAG_EXTERNAL		BIT(2)
>  #define TTM_TT_FLAG_EXTERNAL_MAPPABLE	BIT(3)
>  #define TTM_TT_FLAG_DECRYPTED		BIT(4)
> +#define TTM_TT_FLAG_CLEARED_ON_FREE	BIT(5)
>  
> -#define TTM_TT_FLAG_PRIV_POPULATED	BIT(5)
> +#define TTM_TT_FLAG_PRIV_POPULATED	BIT(6)
>  	uint32_t page_flags;
>  	/** @num_pages: Number of pages in the page array. */
>  	uint32_t num_pages;