netdev - Re: [PATCH net-next v3 1/4] net: stmmac: Switch to zero-copy in non-XDP RX path

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Message-ID: <Z5YeEVrI3zx4VOtF@shredder>
Date: Sun, 26 Jan 2025 13:35:45 +0200
From: Ido Schimmel <idosch@...sch.org>
To: Furong Xu <0x1207@...il.com>
Cc: Andrew Lunn <andrew@...n.ch>, Brad Griffis <bgriffis@...dia.com>,
	Jon Hunter <jonathanh@...dia.com>, netdev@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	Joe Damato <jdamato@...tly.com>,
	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>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>, xfr@...look.com,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH net-next v3 1/4] net: stmmac: Switch to zero-copy in
 non-XDP RX path

On Sun, Jan 26, 2025 at 06:37:14PM +0800, Furong Xu wrote:
> The "SPH feature" splits header into buf->page (non-zero offset) and
> splits payload into buf->sec_page (zero offset).
> 
> For buf->page, pp_params.max_len should be the size of L3/L4 header,
> and with a offset of NET_SKB_PAD.
> 
> For buf->sec_page, pp_params.max_len should be dma_conf->dma_buf_sz,
> and with a offset of 0.
> 
> This is always true:
> sizeof(L3/L4 header) + NET_SKB_PAD < dma_conf->dma_buf_sz + 0

Thanks, understood, but are there situations where the device is unable
to split a packet? For example, a large L2 packet. I am trying to
understand if there are situations where the device will write more than
"dma_conf->dma_buf_sz - NET_SKB_PAD" to the head buffer.