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    SubjectRe: [PATCH net-next 3/3] net: stmmac: Introducing support for Page Pool
    On Thu, 4 Jul 2019 10:13:37 +0000
    Jose Abreu <Jose.Abreu@synopsys.com> wrote:

    > From: Jesper Dangaard Brouer <brouer@redhat.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.

    I didn't notice the skb_copy_to_linear_data(), will copy the entire
    frame, thus leaving the page unused and avail for recycle.

    Then it looks like you are doing the correct thing. I will appreciate
    if you could add a comment above the call like:

    /* Data payload copied into SKB, page ready for recycle */
    page_pool_recycle_direct(rx_q->page_pool, buf->page);


    > 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 ...

    I was surprised to see page_pool being used outside the surrounding XDP
    APIs (included/net/xdp.h). For you use-case, where you "just" use
    page_pool as a driver-local fast recycle-allocator for RX-ring that
    keeps pages DMA mapped, it does make a lot of sense. It simplifies the
    driver a fair amount:

    3 files changed, 63 insertions(+), 144 deletions(-)

    Thanks for demonstrating a use-case for page_pool besides XDP, and for
    simplifying a driver with 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() ...

    For you use-case, given you are copying out the data, and not writing
    into it, then I don't think you need to do sync for device (before
    giving the device the page again for another RX-ring cycle).

    The way I understand the danger: if writing to the DMA memory region,
    and not doing the DMA-sync for-device, then the HW/coherency-system can
    write-back the memory later. Which creates a race with the DMA-device,
    if it is receiving a packet and is doing a write into same DMA memory
    region. Someone correct me if I misunderstood this...

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

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