linux-kernel - Re: [net-next v19 4/7] net: mtip: Add net_device

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Message-ID: <20250907183854.06771a13@wsk>
Date: Sun, 7 Sep 2025 18:38:54 +0200
From: Łukasz Majewski <lukasz.majewski@...lbox.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Andrew Lunn <andrew+netdev@...n.ch>, davem@...emloft.net, Eric Dumazet
 <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Rob Herring
 <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Shawn Guo <shawnguo@...nel.org>, Sascha Hauer
 <s.hauer@...gutronix.de>, Pengutronix Kernel Team <kernel@...gutronix.de>,
 Fabio Estevam <festevam@...il.com>, Richard Cochran
 <richardcochran@...il.com>, netdev@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org, Stefan Wahren
 <wahrenst@....net>, Simon Horman <horms@...nel.org>
Subject: Re: [net-next v19 4/7] net: mtip: Add net_device_ops functions to
 the L2 switch driver

Hi Jakub,

Sorry for late reply.

> On Mon, 25 Aug 2025 00:07:33 +0200 Lukasz Majewski wrote:
> > +	/* Set buffer length and buffer pointer */
> > +	bufaddr = skb->data;  
> 
> You can't write (swap) skb->data if the skb is a clone..

I do use skb = buld_skb() which, "builds" the SKB around the memory
page (from pool).

Then, I "pass" this data (and swap it) to upper layer of the network
stack.

The same approach is used in the fec_main.c driver:
https://elixir.bootlin.com/linux/v6.17-rc3/source/drivers/net/ethernet/freescale/fec_main.c#L1853

> 
> > +	bdp->cbd_datlen = skb->len;
> > +
> > +	/* On some FEC implementations data must be aligned on
> > +	 * 4-byte boundaries. Use bounce buffers to copy data
> > +	 * and get it aligned.spin
> > +	 */
> > +	if ((unsigned long)bufaddr & MTIP_ALIGNMENT) {  
> 
> add 
> 	.. ||
> 	(fep->quirks & FEC_QUIRK_SWAP_FRAME && skb_cloned(skb))
> 
> here to switch to the local buffer for clones ?

Please see the above comment.

> 
> > +		unsigned int index;
> > +
> > +		index = bdp - fep->tx_bd_base;
> > +		memcpy(fep->tx_bounce[index], skb->data, skb->len);
> > +		bufaddr = fep->tx_bounce[index];
> > +	}
> > +
> > +	if (fep->quirks & FEC_QUIRK_SWAP_FRAME)
> > +		swap_buffer(bufaddr, skb->len);  
> 
> > +	if (unlikely(dma_mapping_error(&fep->pdev->dev,
> > bdp->cbd_bufaddr))) {
> > +		dev_err(&fep->pdev->dev,
> > +			"Failed to map descriptor tx buffer\n");  
> 
> All per-packet prints must be rate limited

Ok. I will update it globally.

> 
> > +		/* Since we have freed up a buffer, the ring is no
> > longer
> > +		 * full.
> > +		 */
> > +		if (fep->tx_full) {
> > +			fep->tx_full = 0;
> > +			if (netif_queue_stopped(dev))
> > +				netif_wake_queue(dev);
> > +		}  
> 
> I must say I'm still quite confused by the netdev management in this
> driver. You seem to have 2 netdevs, one per port.

Yes.

> There's one
> set of queues and one NAPI.

Yes.

> Whichever netdev gets up first gets the
> NAPI.

Yes.

What I'm trying to do - is to model the HW which I do have...

When switch is enabled I do have ONE uDMA0 which works for both eth
ports (lan0 and lan1).

That is why I do have only one NAPI queue.

> What makes my head spin is that you seem to record which
> netdev/port was doing Rx _last_ and then pass that netdev to
> mtip_switch_tx(). Why?

You may have port == 1 || port == 2 when you receive packet from ingres
ports.
You may also have port == 0xFF when you first time encounter the SA on
the port and port == 0 when you send/receive data from the "host"
interface.

When port 1/2 is "detected" then the net dev for this particular port
is used. In other cases the one for NAPI is used (which is one of those
two - please see comment above).

This was the approach from original NXP (Freescale) driver. It in some
way prevents from "starvation" from net devices when L2 switch is
disabled and I need to provide port separation.

(port separation in fact is achieved by programming L2 switch registers
and is realized in HW).

> Isn't the dev that we're completing Tx for is
> best read from skb->dev packet by packet?

It may be worth to try.... I think that the code, which we do have now,
tries to reuse some kind of "locality".

> Also this wake up logic
> looks like it will wake up _one_ netdev's queue and then set tx_full
> = 0, so presumably it will not wake the other port if both ports
> queues were stopped. Why keep tx_full state in the first place? Just
> check if the queues is stopped..?

As I said - we do have only ONE queue, which corresponds to uDMA0 when
the switch is enabled. This single queue is responsible for handling
transmission for both ports (this is how the HW is designed).

-- 
Best regards,

Łukasz Majewski