> 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 <lore...@kernel.org> 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 <lore...@kernel.org>
> >
> > Signed-off-by: Jesper Dangaard Brouer <bro...@redhat.com>
> >
> > > ---
> > > 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
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