Subject: Re: [net-next, PATCH, v2] net: netsec: Sync dma for device on buffer allocation

From: Ilias Apalodimas <[email-masked]>
Date: Thu, 4 Jul 2019 20:52:50 +0300

On Thu, Jul 04, 2019 at 07:39:44PM +0200, Jesper Dangaard Brouer wrote:
> On Thu,  4 Jul 2019 17:46:09 +0300
> Ilias Apalodimas <[email-masked]> wrote:
> 
> > Quoting Arnd,
> > 
> > We have to do a sync_single_for_device /somewhere/ before the
> > buffer is given to the device. On a non-cache-coherent machine with
> > a write-back cache, there may be dirty cache lines that get written back
> > after the device DMA's data into it (e.g. from a previous memset
> > from before the buffer got freed), so you absolutely need to flush any
> > dirty cache lines on it first.
> > 
> > Since the coherency is configurable in this device make sure we cover
> > all configurations by explicitly syncing the allocated buffer for the
> > device before refilling it's descriptors
> > 
> > Signed-off-by: Ilias Apalodimas <[email-masked]>
> > ---
> > 
> > Changes since V1: 
> > - Make the code more readable
> >  
> >  drivers/net/ethernet/socionext/netsec.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/socionext/netsec.c b/drivers/net/ethernet/socionext/netsec.c
> > index 5544a722543f..ada7626bf3a2 100644
> > --- a/drivers/net/ethernet/socionext/netsec.c
> > +++ b/drivers/net/ethernet/socionext/netsec.c
> > @@ -727,21 +727,26 @@ static void *netsec_alloc_rx_data(struct netsec_priv *priv,
> >  {
> >  
> >  	struct netsec_desc_ring *dring = &priv->desc_ring[NETSEC_RING_RX];
> > +	enum dma_data_direction dma_dir;
> > +	dma_addr_t dma_start;
> >  	struct page *page;
> >  
> >  	page = page_pool_dev_alloc_pages(dring->page_pool);
> >  	if (!page)
> >  		return NULL;
> >  
> > +	dma_start = page_pool_get_dma_addr(page);
> >  	/* We allocate the same buffer length for XDP and non-XDP cases.
> >  	 * page_pool API will map the whole page, skip what's needed for
> >  	 * network payloads and/or XDP
> >  	 */
> > -	*dma_handle = page_pool_get_dma_addr(page) + NETSEC_RXBUF_HEADROOM;
> > +	*dma_handle = dma_start + NETSEC_RXBUF_HEADROOM;
> >  	/* Make sure the incoming payload fits in the page for XDP and non-XDP
> >  	 * cases and reserve enough space for headroom + skb_shared_info
> >  	 */
> >  	*desc_len = PAGE_SIZE - NETSEC_RX_BUF_NON_DATA;
> > +	dma_dir = page_pool_get_dma_dir(dring->page_pool);
> > +	dma_sync_single_for_device(priv->dev, dma_start, PAGE_SIZE, dma_dir);
> 
> It's it costly to sync_for_device the entire page size?
> 
> E.g. we already know that the head-room is not touched by device.  And
> we actually want this head-room cache-hot for e.g. xdp_frame, thus it
> would be unfortunate if the head-room is explicitly evicted from the
> cache here.
> 
> Even smarter, the driver could do the sync for_device, when it
> release/recycle page, as it likely know the exact length that was used
> by the packet.
It does sync for device when recycling takes place in XDP_TX with the correct
size. 
I guess i can explicitly sync on the xdp_return_buff cases, and 
netsec_setup_rx_dring() instead of the generic buffer allocation

I'll send a V3

Thanks!
/Ilias