linux-kernel - Re: [PATCH net-next v6] page_pool: disable dma mapping support for 32-bit arch with 64-bit DMA

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Date:   Wed, 13 Oct 2021 12:21:21 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
        kuba@...nel.org
Cc:     brouer@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxarm@...neuler.org,
        hawk@...nel.org, ilias.apalodimas@...aro.org,
        akpm@...ux-foundation.org, peterz@...radead.org, will@...nel.org,
        jhubbard@...dia.com, yuzhao@...gle.com, mcroce@...rosoft.com,
        fenghua.yu@...el.com, feng.tang@...el.com, jgg@...pe.ca,
        aarcange@...hat.com, guro@...com,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH net-next v6] page_pool: disable dma mapping support for
 32-bit arch with 64-bit DMA



On 13/10/2021 11.19, Yunsheng Lin wrote:
> As the 32-bit arch with 64-bit DMA seems to rare those days,
> and page pool might carry a lot of code and complexity for
> systems that possibly.
> 
> So disable dma mapping support for such systems, if drivers
> really want to work on such systems, they have to implement
> their own DMA-mapping fallback tracking outside page_pool.
> 
> Reviewed-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>
> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> ---
> V6: Drop pp page tracking support
> ---
>   include/linux/mm_types.h | 13 +------------
>   include/net/page_pool.h  | 12 +-----------
>   net/core/page_pool.c     | 10 ++++++----
>   3 files changed, 8 insertions(+), 27 deletions(-)


Acked-by: Jesper Dangaard Brouer <brouer@...hat.com>

This is a nice simplification of struct page and page_pool code, when we 
don't need to handle this 32-bit ARCH with 64-bit DMA case.
It also gets rid of the confusingly named define. Thanks.


> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 7f8ee09c711f..436e0946d691 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -104,18 +104,7 @@ struct page {
>   			struct page_pool *pp;
>   			unsigned long _pp_mapping_pad;
>   			unsigned long dma_addr;
> -			union {
> -				/**
> -				 * dma_addr_upper: might require a 64-bit
> -				 * value on 32-bit architectures.
> -				 */
> -				unsigned long dma_addr_upper;
> -				/**
> -				 * For frag page support, not supported in
> -				 * 32-bit architectures with 64-bit DMA.
> -				 */
> -				atomic_long_t pp_frag_count;
> -			};
> +			atomic_long_t pp_frag_count;
>   		};
>   		struct {	/* slab, slob and slub */
>   			union {
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index a4082406a003..3855f069627f 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -216,24 +216,14 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>   	page_pool_put_full_page(pool, page, true);
>   }
>   
> -#define PAGE_POOL_DMA_USE_PP_FRAG_COUNT	\
> -		(sizeof(dma_addr_t) > sizeof(unsigned long))
> -
>   static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>   {
> -	dma_addr_t ret = page->dma_addr;
> -
> -	if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> -		ret |= (dma_addr_t)page->dma_addr_upper << 16 << 16;
> -
> -	return ret;
> +	return page->dma_addr;
>   }
>   
>   static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>   {
>   	page->dma_addr = addr;
> -	if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> -		page->dma_addr_upper = upper_32_bits(addr);
>   }
>   
>   static inline void page_pool_set_frag_count(struct page *page, long nr)
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 1a6978427d6c..9b60e4301a44 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -49,6 +49,12 @@ static int page_pool_init(struct page_pool *pool,
>   	 * which is the XDP_TX use-case.
>   	 */
>   	if (pool->p.flags & PP_FLAG_DMA_MAP) {
> +		/* DMA-mapping is not supported on 32-bit systems with
> +		 * 64-bit DMA mapping.
> +		 */
> +		if (sizeof(dma_addr_t) > sizeof(unsigned long))
> +			return -EOPNOTSUPP;
> +
>   		if ((pool->p.dma_dir != DMA_FROM_DEVICE) &&
>   		    (pool->p.dma_dir != DMA_BIDIRECTIONAL))
>   			return -EINVAL;
> @@ -69,10 +75,6 @@ static int page_pool_init(struct page_pool *pool,
>   		 */
>   	}
>   
> -	if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT &&
> -	    pool->p.flags & PP_FLAG_PAGE_FRAG)
> -		return -EINVAL;
> -
>   	if (ptr_ring_init(&pool->ring, ring_qsize, GFP_KERNEL) < 0)
>   		return -ENOMEM;
>   
>