[PATCH] drm/ttm/pool: Revert to clear-on-alloc to honor TTM_TT_FLAG_ZERO_ALLOC

[Christian König]

Am 20.06.24 um 18:01 schrieb Nirmoy Das:
> Currently ttm pool is not honoring TTM_TT_FLAG_ZERO_ALLOC flag and
> clearing pages on free. It does help with allocation latency but clearing
> happens even if drm driver doesn't passes the flag. If clear on free
> is needed then a new flag can be added for that purpose.

Mhm, thinking more about it that will most likely get push back from 
others as well.

How about the attached patch? We just skip clearing pages when the 
driver set the ZERO_ALLOC flag again before freeing them.

Maybe rename the flag or add a new one for that, but in general that 
looks like the option with the least impact.

Regards,
Christian.

>
> Cc: Christian Koenig <christian.koenig at amd.com>
> Cc: "Thomas Hellström" <thomas.hellstrom at linux.intel.com>
> Cc: Matthew Auld <matthew.auld at intel.com>
> Signed-off-by: Nirmoy Das <nirmoy.das at intel.com>
> ---
>   drivers/gpu/drm/ttm/ttm_pool.c | 31 +++++++++++++++++--------------
>   1 file changed, 17 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_pool.c b/drivers/gpu/drm/ttm/ttm_pool.c
> index 6e1fd6985ffc..cbbd722185ee 100644
> --- a/drivers/gpu/drm/ttm/ttm_pool.c
> +++ b/drivers/gpu/drm/ttm/ttm_pool.c
> @@ -224,15 +224,6 @@ 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)
>   {
> -	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));
> -	}
> -
>   	spin_lock(&pt->lock);
>   	list_add(&p->lru, &pt->pages);
>   	spin_unlock(&pt->lock);
> @@ -240,15 +231,26 @@ static void ttm_pool_type_give(struct ttm_pool_type *pt, struct page *p)
>   }
>   
>   /* Take pages from a specific pool_type, return NULL when nothing available */
> -static struct page *ttm_pool_type_take(struct ttm_pool_type *pt)
> +static struct page *ttm_pool_type_take(struct ttm_pool_type *pt, bool clear)
>   {
>   	struct page *p;
>   
>   	spin_lock(&pt->lock);
>   	p = list_first_entry_or_null(&pt->pages, typeof(*p), lru);
>   	if (p) {
> +		unsigned int i, num_pages = 1 << pt->order;
> +
>   		atomic_long_sub(1 << pt->order, &allocated_pages);
>   		list_del(&p->lru);
> +		if (clear) {
> +			for (i = 0; i < num_pages; ++i) {
> +				if (PageHighMem(p))
> +					clear_highpage(p + i);
> +				else
> +					clear_page(page_address(p + i));
> +			}
> +		}
> +
>   	}
>   	spin_unlock(&pt->lock);
>   
> @@ -279,7 +281,7 @@ static void ttm_pool_type_fini(struct ttm_pool_type *pt)
>   	list_del(&pt->shrinker_list);
>   	spin_unlock(&shrinker_lock);
>   
> -	while ((p = ttm_pool_type_take(pt)))
> +	while ((p = ttm_pool_type_take(pt, false)))
>   		ttm_pool_free_page(pt->pool, pt->caching, pt->order, p);
>   }
>   
> @@ -330,7 +332,7 @@ static unsigned int ttm_pool_shrink(void)
>   	list_move_tail(&pt->shrinker_list, &shrinker_list);
>   	spin_unlock(&shrinker_lock);
>   
> -	p = ttm_pool_type_take(pt);
> +	p = ttm_pool_type_take(pt, false);
>   	if (p) {
>   		ttm_pool_free_page(pt->pool, pt->caching, pt->order, p);
>   		num_pages = 1 << pt->order;
> @@ -457,10 +459,11 @@ int ttm_pool_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
>   	     num_pages;
>   	     order = min_t(unsigned int, order, __fls(num_pages))) {
>   		struct ttm_pool_type *pt;
> +		bool clear = tt->page_flags & TTM_TT_FLAG_ZERO_ALLOC;
>   
>   		page_caching = tt->caching;
>   		pt = ttm_pool_select_type(pool, tt->caching, order);
> -		p = pt ? ttm_pool_type_take(pt) : NULL;
> +		p = pt ? ttm_pool_type_take(pt, clear) : NULL;
>   		if (p) {
>   			r = ttm_pool_apply_caching(caching, pages,
>   						   tt->caching);
> @@ -480,7 +483,7 @@ int ttm_pool_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
>   				if (num_pages < (1 << order))
>   					break;
>   
> -				p = ttm_pool_type_take(pt);
> +				p = ttm_pool_type_take(pt, clear);
>   			} while (p);
>   		}
>   
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