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

[Jose Abreu]

From: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>

> 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() ...