Subject: Re: [PATCH v2 net-next 4/8] net: mvneta: sync dma buffers before refilling hw queues

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 10 Oct 2019 09:08:31 +0200

On Thu, 10 Oct 2019 01:18:34 +0200
Lorenzo Bianconi <[email-masked]> wrote:

> mvneta driver can run on not cache coherent devices so it is
> necessary to sync dma buffers before sending them to the device
> in order to avoid memory corruption. This patch introduce a performance
> penalty and it is necessary to introduce a more sophisticated logic
> in order to avoid dma sync as much as we can

Report with benchmarks here:
 https://github.com/xdp-project/xdp-project/blob/master/areas/arm64/board_espressobin08_bench_xdp.org

We are testing this on an Espressobin board, and do see a huge
performance cost associated with this DMA-sync.   Regardless we still
want to get this patch merged, to move forward with XDP support for
this driver. 

We promised each-other (on IRC freenode #xdp) that we will follow-up
with a solution/mitigation, after this patchset is merged.  There are
several ideas, that likely should get separate upstream review.

> Signed-off-by: Lorenzo Bianconi <[email-masked]>

Signed-off-by: Jesper Dangaard Brouer <[email-masked]>

> ---
>  drivers/net/ethernet/marvell/mvneta.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> index 79a6bac0192b..ba4aa9bbc798 100644
> --- a/drivers/net/ethernet/marvell/mvneta.c
> +++ b/drivers/net/ethernet/marvell/mvneta.c
> @@ -1821,6 +1821,7 @@ static int mvneta_rx_refill(struct mvneta_port *pp,
>  			    struct mvneta_rx_queue *rxq,
>  			    gfp_t gfp_mask)
>  {
> +	enum dma_data_direction dma_dir;
>  	dma_addr_t phys_addr;
>  	struct page *page;
>  
> @@ -1830,6 +1831,9 @@ static int mvneta_rx_refill(struct mvneta_port *pp,
>  		return -ENOMEM;
>  
>  	phys_addr = page_pool_get_dma_addr(page) + pp->rx_offset_correction;
> +	dma_dir = page_pool_get_dma_dir(rxq->page_pool);
> +	dma_sync_single_for_device(pp->dev->dev.parent, phys_addr,
> +				   MVNETA_MAX_RX_BUF_SIZE, dma_dir);
>  	mvneta_rx_desc_fill(rx_desc, phys_addr, page, rxq);
>  
>  	return 0;

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer