netdev - Re: net: stmmac: weirdness in stmmac_hw

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Message-ID: <j4y47hdsavbxg5loszsq5ykr5v3lky7t46nj77zcxptuzlbsx7@dgqi5fj42573>
Date: Tue, 25 Feb 2025 19:33:18 +0100
From: Thierry Reding <treding@...dia.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>, 
	Alexandre Torgue <alexandre.torgue@...s.st.com>, Andrew Lunn <andrew+netdev@...n.ch>, 
	"David S. Miller" <davem@...emloft.net>, Emil Renner Berthing <kernel@...il.dk>, 
	Eric Dumazet <edumazet@...gle.com>, Fabio Estevam <festevam@...il.com>, imx@...ts.linux.dev, 
	Inochi Amaoto <inochiama@...il.com>, Jakub Kicinski <kuba@...nel.org>, 
	Jan Petrous <jan.petrous@....nxp.com>, Jon Hunter <jonathanh@...dia.com>, 
	linux-arm-kernel@...ts.infradead.org, linux-stm32@...md-mailman.stormreply.com, 
	Maxime Coquelin <mcoquelin.stm32@...il.com>, Minda Chen <minda.chen@...rfivetech.com>, netdev@...r.kernel.org, 
	NXP S32 Linux Team <s32@....com>, Paolo Abeni <pabeni@...hat.com>, 
	Pengutronix Kernel Team <kernel@...gutronix.de>, Sascha Hauer <s.hauer@...gutronix.de>, 
	Shawn Guo <shawnguo@...nel.org>
Subject: Re: net: stmmac: weirdness in stmmac_hw_setup()

On Thu, Feb 20, 2025 at 04:17:43PM +0000, Russell King (Oracle) wrote:
> While looking through the stmmac driver, I've come across some
> weirdness in stmmac_hw_setup() which looks completely barmy to me.
> 
> It seems that it follows the initialisation suggested by Synopsys
> (as best I can determine from the iMX8MP documentation), even going
> as far as to *enable* transmit and receive *before* the network
> device has been administratively brought up. stmmac_hw_setup() does
> this:
> 
>         /* Enable the MAC Rx/Tx */
>         stmmac_mac_set(priv, priv->ioaddr, true);
> 
> which sets the TE and RE bits in the MAC configuration register.
> 
> This means that if the network link is active, packets will start
> to be received and will be placed into the receive descriptors.
> 
> We won't transmit anything because we won't be placing packets in
> the transmit descriptors to be transmitted.
> 
> However, this in stmmac_hw_setup() is just wrong. Can it be deleted
> as per the below?
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index c2913f003fe6..d6e492f523f5 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -3493,9 +3493,6 @@ static int stmmac_hw_setup(struct net_device *dev, bool ptp_register)
>  		priv->hw->rx_csum = 0;
>  	}
>  
> -	/* Enable the MAC Rx/Tx */
> -	stmmac_mac_set(priv, priv->ioaddr, true);
> -
>  	/* Set the HW DMA mode and the COE */
>  	stmmac_dma_operation_mode(priv);
>  

Tested this on Jetson TX2 and couldn't detect a change in behaviour.
Booted the device using NFS and ran a few iterations of iperf. Same
as before, so:

Tested-by: Thierry Reding <treding@...dia.com>

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