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 11:18:15 +0200

> Hi Lorenzo, Jesper,
> 
> On Thu, Oct 10, 2019 at 09:08:31AM +0200, Jesper Dangaard Brouer wrote:
> > 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.
> 
> I think mentioning that the patch *introduces* a performance penalty is a bit
> misleading. 
> The dma sync does have a performance penalty but it was always there. 
> The initial driver was mapping the DMA with DMA_FROM_DEVICE, which implies
> syncing as well. In page_pool we do not explicitly sync buffers on allocation
> and leave it up the driver writer (and allow him some tricks to avoid that),
> thus this patch is needed.

Reviewing the commit log I definitely agree, I will rewrite it in v3. Thx

Regards,
Lorenzo

> 
> In any case what Jesper mentions is correct, we do have a plan :)
> 
> > 
> > > 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
> 
> Thanks!
> /Ilias