Re: [PATCH v2 02/33] drm/ttm/pool: Add ttm_pool_page_order_nodma() helper

[Christian König]

On 7/10/26 23:54, Matthew Brost wrote:
> TTM records the allocation order of a multi-order page in page->private at
> allocation time. For pools that do not use dma_alloc the order is stored
> directly, while dma_alloc pools store a struct ttm_pool_dma pointer there
> instead. Drivers that do their own DMA mapping (e.g. Xe) need the per-page
> order to walk a populated ttm_tt one chunk at a time, but cannot rely on
> folio_order(): TTM allocates high-order pages with alloc_pages_node()
> without __GFP_COMP, so they are not compound and folio_order() always
> returns 0.

Well what is the justification of XE doing the DMA mapping themselves?

Regards,
Christian.

> 
> Expose ttm_pool_page_order_nodma(), a thin wrapper that returns the order
> recorded in page->private. It is only valid for pages from a pool that does
> not use dma_alloc; whether a TTM device uses dma_alloc is fixed at ttm
> device init time, so callers know from their device configuration that this
> helper applies. Use it from the existing internal ttm_pool_page_order() to
> keep a single source of truth.
> 
> Cc: Carlos Santa <carlos.santa@xxxxxxxxx>
> Cc: Ryan Neph <ryanneph@xxxxxxxxxx>
> Cc: Christian Koenig <christian.koenig@xxxxxxx>
> Cc: Huang Rui <ray.huang@xxxxxxx>
> Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> Cc: Maxime Ripard <mripard@xxxxxxxxxx>
> Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
> Cc: David Airlie <airlied@xxxxxxxxx>
> Cc: Simona Vetter <simona@xxxxxxxx>
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> Assisted-by: GitHub_Copilot:claude-opus-4.8
> Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> ---
>  drivers/gpu/drm/ttm/ttm_pool.c | 25 ++++++++++++++++++++++++-
>  include/drm/ttm/ttm_pool.h     |  2 ++
>  2 files changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_pool.c b/drivers/gpu/drm/ttm/ttm_pool.c
> index dbe977412a81..d34592d4dbc7 100644
> --- a/drivers/gpu/drm/ttm/ttm_pool.c
> +++ b/drivers/gpu/drm/ttm/ttm_pool.c
> @@ -461,6 +461,29 @@ static unsigned int ttm_pool_shrink(int nid, unsigned long num_to_free)
>  	return num_pages;
>  }
>  
> +/**
> + * ttm_pool_page_order_nodma() - Allocation order of a non-dma_alloc TTM page
> + * @p: The page to query.
> + *
> + * Return the allocation order that the TTM pool allocator recorded in
> + * @p->private at allocation time. This only works for pages that were
> + * allocated from a pool which does *not* use dma_alloc (i.e. pages backed by
> + * alloc_pages_node()), where TTM stores the order directly in page->private.
> + *
> + * For dma_alloc pools, page->private instead holds a struct ttm_pool_dma
> + * pointer and this helper must not be used. Whether a TTM device uses
> + * dma_alloc is fixed at ttm device init time, so callers are expected to know
> + * from their TTM device configuration that their pages are not dma_alloc
> + * backed before using this helper.
> + *
> + * Return: The allocation order of the page.
> + */
> +unsigned int ttm_pool_page_order_nodma(struct page *p)
> +{
> +	return p->private;
> +}
> +EXPORT_SYMBOL(ttm_pool_page_order_nodma);
> +
>  /* Return the allocation order based for a page */
>  static unsigned int ttm_pool_page_order(struct ttm_pool *pool, struct page *p)
>  {
> @@ -470,7 +493,7 @@ static unsigned int ttm_pool_page_order(struct ttm_pool *pool, struct page *p)
>  		return dma->vaddr & ~PAGE_MASK;
>  	}
>  
> -	return p->private;
> +	return ttm_pool_page_order_nodma(p);
>  }
>  
>  /*
> diff --git a/include/drm/ttm/ttm_pool.h b/include/drm/ttm/ttm_pool.h
> index 26ee592e1994..753203980e2c 100644
> --- a/include/drm/ttm/ttm_pool.h
> +++ b/include/drm/ttm/ttm_pool.h
> @@ -94,6 +94,8 @@ long ttm_pool_backup(struct ttm_pool *pool, struct ttm_tt *ttm,
>  int ttm_pool_restore_and_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
>  			       const struct ttm_operation_ctx *ctx);
>  
> +unsigned int ttm_pool_page_order_nodma(struct page *p);
> +
>  int ttm_pool_mgr_init(unsigned long num_pages);
>  void ttm_pool_mgr_fini(void);
>