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

From: Heiner Kallweit
Date: Fri Aug 20 2021 - 02:10:32 EST


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@xxxxxxxxxx>
> ---
> 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;
> }
>
>