netdev - Re: [PATCH net-next] page_pool: add DMA_ATTR_WEAK

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Message-ID: <6bd4adc4-8d0f-85c2-fa6a-8ce277e52f4e@redhat.com>
Date:   Tue, 18 Apr 2023 13:23:45 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Christoph Hellwig <hch@...radead.org>
Cc:     brouer@...hat.com, netdev@...r.kernel.org, edumazet@...gle.com,
        pabeni@...hat.com, michael.chan@...adcom.com, hawk@...nel.org,
        ilias.apalodimas@...aro.org, davem@...emloft.net
Subject: Re: [PATCH net-next] page_pool: add DMA_ATTR_WEAK_ORDERING on all
 mappings

To Hellwig,

On 17/04/2023 17.28, Jakub Kicinski wrote:
> Commit c519fe9a4f0d ("bnxt: add dma mapping attributes") added
> DMA_ATTR_WEAK_ORDERING to DMA attrs on bnxt. It has since spread
> to a few more drivers (possibly as a copy'n'paste).
> 
> DMA_ATTR_WEAK_ORDERING only seems to matter on Sparc and PowerPC/cell,
> the rarity of these platforms is likely why we never bothered adding
> the attribute in the page pool, even though it should be safe to add.
> 
> To make the page pool migration in drivers which set this flag less
> of a risk (of regressing the precious sparc database workloads or
> whatever needed this) let's add DMA_ATTR_WEAK_ORDERING on all
> page pool DMA mappings.
> 

This sounds reasonable to me, but I don't know the DMA APIs well enough.
Thus, I would like to hear if Hellwig thinks this is okay?

> We could make this a driver opt-in but frankly I don't think it's
> worth complicating the API. I can't think of a reason why device
> accesses to packet memory would have to be ordered.
> 
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
> CC: hawk@...nel.org
> CC: ilias.apalodimas@...aro.org
> ---
>   net/core/page_pool.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 2f6bf422ed30..97f20f7ff4fc 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -316,7 +316,8 @@ 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);
> +				 pool->p.dma_dir, DMA_ATTR_SKIP_CPU_SYNC |
> +						  DMA_ATTR_WEAK_ORDERING);
>   	if (dma_mapping_error(pool->p.dev, dma))
>   		return false;
>   
> @@ -484,7 +485,7 @@ void page_pool_release_page(struct page_pool *pool, struct page *page)
>   	/* When page is unmapped, it cannot be returned to our pool */
>   	dma_unmap_page_attrs(pool->p.dev, dma,
>   			     PAGE_SIZE << pool->p.order, pool->p.dma_dir,
> -			     DMA_ATTR_SKIP_CPU_SYNC);
> +			     DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_WEAK_ORDERING);
>   	page_pool_set_dma_addr(page, 0);
>   skip_dma_unmap:
>   	page_pool_clear_pp_info(page);