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SubjectRe: [PATCH net-next 3/3] net: stmmac: Introducing support for Page Pool
On Wed,  3 Jul 2019 12:37:50 +0200
Jose Abreu <Jose.Abreu@synopsys.com> wrote:

> @@ -3547,6 +3456,9 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
>
> napi_gro_receive(&ch->rx_napi, skb);
>
> + page_pool_recycle_direct(rx_q->page_pool, buf->page);

This doesn't look correct.

The page_pool DMA mapping cannot be "kept" when page traveling into the
network stack attached to an SKB. (Ilias and I have a long term plan[1]
to allow this, but you cannot do it ATM).

You will have to call:
page_pool_release_page(rx_q->page_pool, buf->page);

This will do a DMA-unmap, and you will likely loose your performance
gain :-(


> + buf->page = NULL;
> +
> priv->dev->stats.rx_packets++;
> priv->dev->stats.rx_bytes += frame_len;
> }

Also remember that the page_pool requires you driver to do the DMA-sync
operation. I see a dma_sync_single_for_cpu(), but I didn't see a
dma_sync_single_for_device() (well, I noticed one getting removed).
(For some HW Ilias tells me that the dma_sync_single_for_device can be
elided, so maybe this can still be correct for you).


[1] https://github.com/xdp-project/xdp-project/blob/master/areas/mem/page_pool02_SKB_return_callback.org
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer

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