From: Jesper Dangaard Brouer <brouer@redhat.com>
To: Jose Abreu <Jose.Abreu@synopsys.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
"linux-stm32@st-md-mailman.stormreply.com"
<linux-stm32@st-md-mailman.stormreply.com>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
Joao Pinto <Joao.Pinto@synopsys.com>,
"David S . Miller" <davem@davemloft.net>,
Giuseppe Cavallaro <peppe.cavallaro@st.com>,
Alexandre Torgue <alexandre.torgue@st.com>,
Maxime Coquelin <mcoquelin.stm32@gmail.com>,
Maxime Ripard <maxime.ripard@bootlin.com>,
Chen-Yu Tsai <wens@csie.org>,
Ilias Apalodimas <ilias.apalodimas@linaro.org>,
brouer@redhat.com
Subject: Re: [PATCH net-next 3/3] net: stmmac: Introducing support for Page Pool
Date: Thu, 4 Jul 2019 13:54:14 +0200 [thread overview]
Message-ID: <20190704135414.0dd5df76@carbon> (raw)
In-Reply-To: <BN8PR12MB3266BC5322AADFAC49D9BAFAD3FA0@BN8PR12MB3266.namprd12.prod.outlook.com>
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
next prev parent reply other threads:[~2019-07-04 11:54 UTC|newest]
Thread overview: 87+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-03 10:37 [PATCH net-next 0/3] net: stmmac: Some performance improvements and a fix Jose Abreu
2019-07-03 10:37 ` [PATCH net-next 1/3] net: stmmac: Implement RX Coalesce Frames setting Jose Abreu
2019-07-03 20:41 ` Jakub Kicinski
2019-07-03 10:37 ` [PATCH net-next 2/3] net: stmmac: Fix descriptors address being in > 32 bits address space Jose Abreu
2019-07-03 10:37 ` [PATCH net-next 3/3] net: stmmac: Introducing support for Page Pool Jose Abreu
2019-07-03 10:40 ` Jose Abreu
2019-07-04 9:39 ` Jesper Dangaard Brouer
2019-07-04 14:45 ` Jose Abreu
2019-07-04 15:09 ` Jesper Dangaard Brouer
2019-07-04 15:18 ` Jose Abreu
2019-07-04 15:33 ` Jesper Dangaard Brouer
2019-07-04 9:48 ` Jesper Dangaard Brouer
2019-07-04 10:00 ` Jesper Dangaard Brouer
2019-07-04 10:13 ` Jose Abreu
2019-07-04 11:11 ` Ilias Apalodimas
2019-07-04 11:54 ` Jesper Dangaard Brouer [this message]
2019-07-04 12:04 ` Ilias Apalodimas
2019-07-04 12:59 ` Jose Abreu
2019-07-04 13:06 ` Ilias Apalodimas
2019-07-04 10:30 ` Ilias Apalodimas
2019-07-04 12:14 ` Arnd Bergmann
2019-07-04 12:49 ` Ilias Apalodimas
2019-07-17 18:58 ` Jon Hunter
2019-07-18 7:29 ` Jose Abreu
2019-07-18 7:48 ` Jose Abreu
2019-07-18 9:16 ` Jon Hunter
2019-07-19 7:51 ` Jose Abreu
2019-07-19 8:37 ` Jon Hunter
2019-07-19 8:44 ` Jose Abreu
2019-07-19 8:49 ` Jon Hunter
2019-07-19 10:25 ` Jose Abreu
2019-07-19 12:28 ` Jose Abreu
2019-07-19 13:33 ` Jon Hunter
2019-07-19 12:30 ` Jon Hunter
2019-07-19 12:32 ` Jose Abreu
2019-07-19 13:35 ` Jon Hunter
2019-07-22 7:23 ` Jose Abreu
2019-07-22 9:37 ` Jon Hunter
2019-07-22 9:47 ` Jose Abreu
2019-07-22 9:57 ` Jose Abreu
2019-07-22 10:27 ` Jon Hunter
2019-07-22 10:18 ` Ilias Apalodimas
2019-07-22 11:11 ` Lars Persson
2019-07-22 11:39 ` Jose Abreu
2019-07-22 12:05 ` Jon Hunter
2019-07-22 14:04 ` Jose Abreu
2019-07-23 8:14 ` Jose Abreu
2019-07-23 10:01 ` Jon Hunter
2019-07-23 10:07 ` Jose Abreu
2019-07-23 10:29 ` Robin Murphy
2019-07-23 11:22 ` Jose Abreu
2019-07-23 12:09 ` Jon Hunter
2019-07-23 13:19 ` Robin Murphy
2019-07-23 21:39 ` Jon Hunter
2019-07-24 10:03 ` Robin Murphy
2019-07-23 18:51 ` David Miller
2019-07-24 8:54 ` Ilias Apalodimas
2019-07-24 9:43 ` Jose Abreu
2019-07-24 9:53 ` Ilias Apalodimas
2019-07-24 10:04 ` Jose Abreu
2019-07-24 11:10 ` Jon Hunter
2019-07-24 11:34 ` Jose Abreu
2019-07-24 11:58 ` Jon Hunter
2019-07-25 7:44 ` Jose Abreu
2019-07-25 9:45 ` Jon Hunter
2019-07-25 11:39 ` Ilias Apalodimas
2019-07-23 10:38 ` Jon Hunter
2019-07-23 10:49 ` Jose Abreu
2019-07-23 11:58 ` Jon Hunter
2019-07-23 12:51 ` Jose Abreu
2019-07-23 13:34 ` Jon Hunter
2019-07-29 9:45 ` Mikko Perttunen
2019-07-25 13:20 ` Jon Hunter
2019-07-25 13:26 ` Jose Abreu
2019-07-25 14:25 ` Jon Hunter
2019-07-25 15:12 ` Jose Abreu
2019-07-26 14:11 ` Jon Hunter
2019-07-27 15:56 ` Jose Abreu
2019-07-29 8:16 ` Jose Abreu
2019-07-29 10:55 ` Jon Hunter
2019-07-29 11:29 ` Jose Abreu
2019-07-29 11:52 ` Robin Murphy
2019-07-29 14:08 ` Jose Abreu
2019-07-29 21:33 ` Jon Hunter
2019-07-30 9:39 ` Jose Abreu
2019-07-30 13:36 ` Jon Hunter
2019-07-30 13:58 ` Jose Abreu
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