linux-kernel - Re: [PATCH net-next 3/3] net: stmmac: Introducing support for Page Pool

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Message-ID: <20190704120018.4523a119@carbon>
Date:   Thu, 4 Jul 2019 12:00:18 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Jose Abreu <Jose.Abreu@...opsys.com>
Cc:     brouer@...hat.com, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        "David S . Miller" <davem@...emloft.net>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>
Subject: Re: [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@...opsys.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