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

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 4 Jul 2019 19:39:44 +0200

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.

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