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

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 4 Jul 2019 13:54:14 +0200

On Thu, 4 Jul 2019 10:13:37 +0000
Jose Abreu <[email-masked]> wrote:

> From: Jesper Dangaard Brouer <[email-masked]>
> 
> > 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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