Re: [PATCH net-next v2 4/4] net: lan96x: Use page_pool API

[Alexander Lobakin]

From: Horatiu Vultur <horatiu.vultur@xxxxxxxxxxxxx>
Date: Mon, 7 Nov 2022 22:35:21 +0100

> The 11/07/2022 17:40, Alexander Lobakin wrote:
> 
> Hi Olek,
> 
> > 
> > From: Horatiu Vultur <horatiu.vultur@xxxxxxxxxxxxx>
> > Date: Sun, 6 Nov 2022 22:11:54 +0100
> > 
> > > Use the page_pool API for allocation, freeing and DMA handling instead
> > > of dev_alloc_pages, __free_pages and dma_map_page.
> > >
> > > Signed-off-by: Horatiu Vultur <horatiu.vultur@xxxxxxxxxxxxx>
> > > ---
> > >  .../net/ethernet/microchip/lan966x/Kconfig    |  1 +
> > >  .../ethernet/microchip/lan966x/lan966x_fdma.c | 72 ++++++++++---------
> > >  .../ethernet/microchip/lan966x/lan966x_main.h |  3 +
> > >  3 files changed, 43 insertions(+), 33 deletions(-)
> > 
> > [...]
> > 
> > > @@ -84,6 +62,27 @@ static void lan966x_fdma_rx_add_dcb(struct lan966x_rx *rx,
> > >       rx->last_entry = dcb;
> > >  }
> > >
> > > +static int lan966x_fdma_rx_alloc_page_pool(struct lan966x_rx *rx)
> > > +{
> > > +     struct lan966x *lan966x = rx->lan966x;
> > > +     struct page_pool_params pp_params = {
> > > +             .order = rx->page_order,
> > > +             .flags = PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV,
> > > +             .pool_size = FDMA_DCB_MAX,
> > > +             .nid = NUMA_NO_NODE,
> > > +             .dev = lan966x->dev,
> > > +             .dma_dir = DMA_FROM_DEVICE,
> > > +             .offset = 0,
> > > +             .max_len = PAGE_SIZE << rx->page_order,
> > 
> > ::max_len's primary purpose is to save time on DMA syncs.
> > First of all, you can substract
> > `SKB_DATA_ALIGN(sizeof(struct skb_shared_info))`, your HW never
> > writes to those last couple hundred bytes.
> > But I suggest calculating ::max_len basing on your current MTU
> > value. Let's say you have 16k pages and MTU of 1500, that is a huge
> > difference (except your DMA is always coherent, but I assume that's
> > not the case).
> > 
> > In lan966x_fdma_change_mtu() you do:
> > 
> >         max_mtu = lan966x_fdma_get_max_mtu(lan966x);
> >         max_mtu += IFH_LEN_BYTES;
> >         max_mtu += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> >         max_mtu += VLAN_HLEN * 2;
> > 
> > `lan966x_fdma_get_max_mtu(lan966x) + IFH_LEN_BYTES + VLAN_HLEN * 2`
> > (ie 1536 for the MTU of 1500) is your max_len value actually, given
> > that you don't reserve any headroom (which is unfortunate, but I
> > guess you're working on this already, since XDP requires
> > %XDP_PACKET_HEADROOM).
> 
> Thanks for the suggestion. I will try it.
> Regarding XDP_PACKET_HEADROOM, for the XDP_DROP, I didn't see it to be
> needed. Once the support for XDP_TX or XDP_REDIRECT is added, then yes I
> need to reserve also the headroom.

Correct, since you're disabling metadata support in
xdp_prepare_buff(), headroom is not needed for pass and drop
actions.

It's always good to have at least %NET_SKB_PAD headroom inside an
skb, so that networking stack won't perform excessive reallocations,
and your code currently misses that -- if I understand currently,
after converting hardware-specific header to an Ethernet header you
have 28 - 14 = 14 bytes of headroom, which sometimes can be not
enough for example for forwarding cases. It's not related to XDP,
but I would do that as a prerequisite patch for Tx/redirect, since
you'll be adding headroom support anyway :)

> 
> > 
> > > +     };
> > > +
> > > +     rx->page_pool = page_pool_create(&pp_params);
> > > +     if (IS_ERR(rx->page_pool))
> > > +             return PTR_ERR(rx->page_pool);

[...]

> > > --
> > > 2.38.0
> > 
> > Thanks,
> > Olek
> 
> -- 
> /Horatiu

Thanks,
Olek