linux-kernel - Re: [PATCH net-next v2 4/4] net: lan96x: Use page

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Date:   Mon, 7 Nov 2022 22:35:21 +0100
From:   Horatiu Vultur <horatiu.vultur@...rochip.com>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
CC:     <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>, <davem@...emloft.net>,
        <edumazet@...gle.com>, <kuba@...nel.org>, <pabeni@...hat.com>,
        <ast@...nel.org>, <daniel@...earbox.net>, <hawk@...nel.org>,
        <john.fastabend@...il.com>, <linux@...linux.org.uk>
Subject: Re: [PATCH net-next v2 4/4] net: lan96x: Use page_pool API

The 11/07/2022 17:40, Alexander Lobakin wrote:

Hi Olek,

> 
> From: Horatiu Vultur <horatiu.vultur@...rochip.com>
> 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@...rochip.com>
> > ---
> >  .../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.

> 
> > +     };
> > +
> > +     rx->page_pool = page_pool_create(&pp_params);
> > +     if (IS_ERR(rx->page_pool))
> > +             return PTR_ERR(rx->page_pool);
> > +
> > +     return 0;
> 
>         return PTR_ERR_OR_ZERO(rx->page_pool);

Yes, I will use this.

> 
> > +}
> > +
> >  static int lan966x_fdma_rx_alloc(struct lan966x_rx *rx)
> >  {
> >       struct lan966x *lan966x = rx->lan966x;
> 
> [...]
> 
> > --
> > 2.38.0
> 
> Thanks,
> Olek

-- 
/Horatiu