netdev - Re: [PATCH net 4/4] net: ethernet: cortina: Handle large frames

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Date: Sat, 4 Nov 2023 15:56:59 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Linus Walleij <linus.walleij@...aro.org>
Cc: Hans Ulli Kroll <ulli.kroll@...glemail.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Michał Mirosław <mirq-linux@...e.qmqm.pl>,
	Vladimir Oltean <olteanv@...il.com>,
	linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 4/4] net: ethernet: cortina: Handle large frames

> @@ -1170,7 +1171,14 @@ static int gmac_map_tx_bufs(struct net_device *netdev, struct sk_buff *skb,
>  		word3 |= mtu;
>  	}
>  
> -	if (skb->ip_summed != CHECKSUM_NONE) {
> +	if (skb->len >= ETH_FRAME_LEN) {
> +		/* Hardware offloaded checksumming isn't working on frames
> +		 * bigger than 1514 bytes. Perhaps the buffer is only 1518
> +		 * bytes fitting mach a normal frame and a checksum?

mach ?

> +		 * Just bypass on bigger frames.
> +		 */
> +		word1 |= TSS_BYPASS_BIT;
> +	} else if (skb->ip_summed != CHECKSUM_NONE) {

I've never looked at how the network stack does checksums. But looking
at this patch, it made me wounder, how do you tell the stack it needs
to do a software checksum because the hardware cannot? Or for this
driver, is it always calculating a checksum, which is then ignored?
Maybe you can improve performance a little but disabling software
checksum when it is not needed?

	 Andrew