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

From: Lorenzo Bianconi <[email-masked]>
Date: Thu, 10 Oct 2019 11:20:04 +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

Thx a lot Jesper for detailed report, I will include this info in the commit log.

Regards,
Lorenzo

> 
> 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