netdev - Re: [PATCH v3 net-next 5/5] net: fec: enable the Jumbo frame support for i.MX8QM

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Message-ID: <fd9af170-fb59-43fa-9eea-ff147f4a84a7@lunn.ch>
Date: Sat, 23 Aug 2025 21:25:39 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Shenwei Wang <shenwei.wang@....com>
Cc: Wei Fang <wei.fang@....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>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	Clark Wang <xiaoning.wang@....com>,
	Stanislav Fomichev <sdf@...ichev.me>, imx@...ts.linux.dev,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-imx@....com
Subject: Re: [PATCH v3 net-next 5/5] net: fec: enable the Jumbo frame support
 for i.MX8QM

> @@ -1278,8 +1280,16 @@ fec_restart(struct net_device *ndev)
>  	if (fep->quirks & FEC_QUIRK_ENET_MAC) {
>  		/* enable ENET endian swap */
>  		ecntl |= FEC_ECR_BYTESWP;
> -		/* enable ENET store and forward mode */
> -		writel(FEC_TXWMRK_STRFWD, fep->hwp + FEC_X_WMRK);
> +
> +		/* When Jumbo Frame is enabled, the FIFO may not be large enough
> +		 * to hold an entire frame. In this case, configure the interface
> +		 * to operate in cut-through mode, triggered by the FIFO threshold.
> +		 * Otherwise, enable the ENET store-and-forward mode.
> +		 */
> +		if (fep->quirks & FEC_QUIRK_JUMBO_FRAME)
> +			writel(0xF, fep->hwp + FEC_X_WMRK);

The quirk indicates the hardware is capable of jumbo frames, not that
jumbo frames are enabled. Don't you need to compare the mtu with
ETH_FRAME_LEN + ETH_FCS_LEN to say jumbo is enabled?

Is there a counter or other indication that the FIFO experienced an
underflow?

	Andrew