Subject: Re: [PATCH v3 net-next 8/8] net: mvneta: add XDP_TX support

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 16 Oct 2019 13:16:38 +0200

On Wed, 16 Oct 2019 13:55:06 +0300
Ilias Apalodimas <[email-masked]> wrote:

> On Wed, Oct 16, 2019 at 12:09:00PM +0200, Lorenzo Bianconi wrote:
> > > On Mon, 14 Oct 2019 12:49:55 +0200, Lorenzo Bianconi wrote:  
> > > > Implement XDP_TX verdict and ndo_xdp_xmit net_device_ops function
> > > > pointer
> > > > 
> > > > Signed-off-by: Lorenzo Bianconi <[email-masked]>  
> > >   
> > > > @@ -1972,6 +1975,109 @@ int mvneta_rx_refill_queue(struct mvneta_port *pp, struct mvneta_rx_queue *rxq)
> > > >  	return i;
> > > >  }
> > > >  
> > > > +static int
> > > > +mvneta_xdp_submit_frame(struct mvneta_port *pp, struct mvneta_tx_queue *txq,
> > > > +			struct xdp_frame *xdpf, bool dma_map)
> > > > +{
> > > > +	struct mvneta_tx_desc *tx_desc;
> > > > +	struct mvneta_tx_buf *buf;
> > > > +	dma_addr_t dma_addr;
> > > > +
> > > > +	if (txq->count >= txq->tx_stop_threshold)
> > > > +		return MVNETA_XDP_CONSUMED;
> > > > +
> > > > +	tx_desc = mvneta_txq_next_desc_get(txq);
> > > > +
> > > > +	buf = &txq->buf[txq->txq_put_index];
> > > > +	if (dma_map) {
> > > > +		/* ndo_xdp_xmit */
> > > > +		dma_addr = dma_map_single(pp->dev->dev.parent, xdpf->data,
> > > > +					  xdpf->len, DMA_TO_DEVICE);
> > > > +		if (dma_mapping_error(pp->dev->dev.parent, dma_addr)) {
> > > > +			mvneta_txq_desc_put(txq);
> > > > +			return MVNETA_XDP_CONSUMED;
> > > > +		}
> > > > +		buf->type = MVNETA_TYPE_XDP_NDO;
> > > > +	} else {
> > > > +		struct page *page = virt_to_page(xdpf->data);
> > > > +
> > > > +		dma_addr = page_pool_get_dma_addr(page) +
> > > > +			   pp->rx_offset_correction + MVNETA_MH_SIZE;
> > > > +		dma_sync_single_for_device(pp->dev->dev.parent, dma_addr,
> > > > +					   xdpf->len, DMA_BIDIRECTIONAL);  
> > > 
> > > This looks a little suspicious, XDP could have moved the start of frame
> > > with adjust_head, right? You should also use xdpf->data to find where
> > > the frame starts, no?  
> > 
> > uhm, right..we need to update the dma_addr doing something like:
> > 
> > dma_addr = page_pool_get_dma_addr(page) + xdpf->data - xdpf;  
> 
> Can we do  page_pool_get_dma_addr(page) + xdpf->headroom as well right?

We actually have to do:
 page_pool_get_dma_addr(page) + xdpf->headroom + sizeof(struct xdp_frame)

As part of part of headroom was reserved to xdpf.  I've considered
several times to change xdpf->headroom to include sizeof(struct xdp_frame)
as use-cases (e.g. in veth and cpumap) ended needing the "full" headroom.

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