netdev - Re: [PATCH net 2/5] mlxsw: pci: Sync Rx buffers for CPU

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Message-ID: <151a8cbe-3b97-495d-849e-a5a2574c9457@intel.com>
Date: Fri, 25 Oct 2024 17:00:21 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Petr Machata <petrm@...dia.com>, Amit Cohen <amcohen@...dia.com>
CC: <netdev@...r.kernel.org>, Andrew Lunn <andrew+netdev@...n.ch>, "David S.
 Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, "Jakub
 Kicinski" <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman
	<horms@...nel.org>, Danielle Ratson <danieller@...dia.com>, Ido Schimmel
	<idosch@...dia.com>, <mlxsw@...dia.com>, Jiri Pirko <jiri@...nulli.us>
Subject: Re: [PATCH net 2/5] mlxsw: pci: Sync Rx buffers for CPU

From: Petr Machata <petrm@...dia.com>
Date: Fri, 25 Oct 2024 16:26:26 +0200

> From: Amit Cohen <amcohen@...dia.com>
> 
> When Rx packet is received, drivers should sync the pages for CPU, to
> ensure the CPU reads the data written by the device and not stale
> data from its cache.

[...]

> -static struct sk_buff *mlxsw_pci_rdq_build_skb(struct page *pages[],
> +static struct sk_buff *mlxsw_pci_rdq_build_skb(struct mlxsw_pci_queue *q,
> +					       struct page *pages[],
>  					       u16 byte_count)
>  {
> +	struct mlxsw_pci_queue *cq = q->u.rdq.cq;
>  	unsigned int linear_data_size;
> +	struct page_pool *page_pool;
>  	struct sk_buff *skb;
>  	int page_index = 0;
>  	bool linear_only;
>  	void *data;
>  
> +	linear_only = byte_count + MLXSW_PCI_RX_BUF_SW_OVERHEAD <= PAGE_SIZE;
> +	linear_data_size = linear_only ? byte_count :
> +					 PAGE_SIZE -
> +					 MLXSW_PCI_RX_BUF_SW_OVERHEAD;

Maybe reformat the line while at it?

	linear_data_size = linear_only ? byte_count :
			   PAGE_SIZE - MLXSW_PCI_RX_BUF_SW_OVERHEAD;

> +
> +	page_pool = cq->u.cq.page_pool;
> +	page_pool_dma_sync_for_cpu(page_pool, pages[page_index],
> +				   MLXSW_PCI_SKB_HEADROOM, linear_data_size);

page_pool_dma_sync_for_cpu() already skips the headroom:

	dma_sync_single_range_for_cpu(pool->p.dev,
				      offset + pool->p.offset, ...

Since your pool->p.offset is MLXSW_PCI_SKB_HEADROOM, I believe you need
to pass 0 here.

> +
>  	data = page_address(pages[page_index]);
>  	net_prefetch(data);

Thanks,
Olek