netdev - RE: [PATCH net-next 3/3] net: stmmac: Introducing support for Page Pool

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Message-ID: <BN8PR12MB3266BC5322AADFAC49D9BAFAD3FA0@BN8PR12MB3266.namprd12.prod.outlook.com>
Date:   Thu, 4 Jul 2019 10:13:37 +0000
From:   Jose Abreu <Jose.Abreu@...opsys.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Jose Abreu <Jose.Abreu@...opsys.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <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

From: Jesper Dangaard Brouer <brouer@...hat.com>

> 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).

The reason I recycle the page is this previous call to:

	skb_copy_to_linear_data()

So, technically, I'm syncing to CPU the page(s) and then memcpy to a 
previously allocated SKB ... So it's safe to just recycle the mapping I 
think.

Its kind of using bounce buffers and I do see performance gain in this 
(I think the reason is because my setup uses swiotlb for DMA mapping).

Anyway, I'm open to some suggestions on how to improve this ...

> 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).

My HW just needs descriptors refilled which are in different coherent 
region so I don't see any reason for dma_sync_single_for_device() ...