netdev - Re: [PATCH net-next 2/2] page_pool: optimize the cpu sync operation when DMA mapping

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Date:   Fri, 20 Aug 2021 08:10:14 +0200
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
        kuba@...nel.org
Cc:     hawk@...nel.org, ilias.apalodimas@...aro.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 2/2] page_pool: optimize the cpu sync operation
 when DMA mapping

On 20.08.2021 04:06, Yunsheng Lin wrote:
> If the DMA_ATTR_SKIP_CPU_SYNC is not set, cpu syncing is
> also done in dma_map_page_attrs(), so set the attrs according
> to pool->p.flags to avoid calling dma sync function again.
> 
> Also mark the dma error as the unlikely case While we are at
> it.
> 
This shouldn't be needed. dma_mapping_error() will be (most likely)
inlined by the compiler, and it includes the unlikely() hint.

> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> ---
>  net/core/page_pool.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 1a69784..8172045 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -191,8 +191,12 @@ static void page_pool_dma_sync_for_device(struct page_pool *pool,
>  
>  static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
>  {
> +	unsigned long attrs = DMA_ATTR_SKIP_CPU_SYNC;
>  	dma_addr_t dma;
>  
> +	if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> +		attrs = 0;
> +
>  	/* Setup DMA mapping: use 'struct page' area for storing DMA-addr
>  	 * since dma_addr_t can be either 32 or 64 bits and does not always fit
>  	 * into page private data (i.e 32bit cpu with 64bit DMA caps)
> @@ -200,15 +204,12 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
>  	 */
>  	dma = dma_map_page_attrs(pool->p.dev, page, 0,
>  				 (PAGE_SIZE << pool->p.order),
> -				 pool->p.dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
> -	if (dma_mapping_error(pool->p.dev, dma))
> +				 pool->p.dma_dir, attrs);
> +	if (unlikely(dma_mapping_error(pool->p.dev, dma)))
>  		return false;
>  
>  	page_pool_set_dma_addr(page, dma);
>  
> -	if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> -		page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
> -
>  	return true;
>  }
>  
>