On Thu, 4 Jul 2019 17:46:09 +0300
Ilias Apalodimas <ilias.apalodi...@linaro.org> 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 <ilias.apalodi...@linaro.org>
> ---
>
> 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
