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

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Date:   Mon,  7 Nov 2022 17:40:55 +0100
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Horatiu Vultur <horatiu.vultur@...rochip.com>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        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

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).

> +	};
> +
> +	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);

> +}
> +
>  static int lan966x_fdma_rx_alloc(struct lan966x_rx *rx)
>  {
>  	struct lan966x *lan966x = rx->lan966x;

[...]

> -- 
> 2.38.0

Thanks,
Olek