netdev - RE: [PATCH net-next v7 3/3] net: axienet: Introduce dmaengine support

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Date: Mon, 9 Oct 2023 19:41:35 +0000
From: "Pandey, Radhey Shyam" <radhey.shyam.pandey@....com>
To: Jakub Kicinski <kuba@...nel.org>
CC: "davem@...emloft.net" <davem@...emloft.net>, "edumazet@...gle.com"
	<edumazet@...gle.com>, "pabeni@...hat.com" <pabeni@...hat.com>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	"krzysztof.kozlowski+dt@...aro.org" <krzysztof.kozlowski+dt@...aro.org>,
	"conor+dt@...nel.org" <conor+dt@...nel.org>, "Simek, Michal"
	<michal.simek@....com>, "linux@...linux.org.uk" <linux@...linux.org.uk>,
	"f.fainelli@...il.com" <f.fainelli@...il.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "devicetree@...r.kernel.org"
	<devicetree@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "git (AMD-Xilinx)" <git@....com>
Subject: RE: [PATCH net-next v7 3/3] net: axienet: Introduce dmaengine support

> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: Saturday, October 7, 2023 3:03 AM
> To: Pandey, Radhey Shyam <radhey.shyam.pandey@....com>
> Cc: davem@...emloft.net; edumazet@...gle.com; pabeni@...hat.com;
> robh+dt@...nel.org; krzysztof.kozlowski+dt@...aro.org;
> conor+dt@...nel.org; Simek, Michal <michal.simek@....com>;
> linux@...linux.org.uk; f.fainelli@...il.com; netdev@...r.kernel.org;
> devicetree@...r.kernel.org; linux-kernel@...r.kernel.org; linux-arm-
> kernel@...ts.infradead.org; git (AMD-Xilinx) <git@....com>
> Subject: Re: [PATCH net-next v7 3/3] net: axienet: Introduce dmaengine
> support
> 
> On Fri, 6 Oct 2023 19:04:29 +0000 Pandey, Radhey Shyam wrote:
> > > > +		netif_stop_queue(ndev);
> > > > +		if (net_ratelimit())
> > > > +			netdev_warn(ndev, "TX ring unexpectedly full\n");
> > >
> > > I don't see you stopping the queue when the ring gets full, am I not
> > > looking into the right place? Otherwise this is sort of expected to
> > > occasionally hapen
> >
> > We are calling stop_queue if TX ring is full. Is that what you meant?
> > netif_stop_queue(ndev);
> 
> But somewhere else or just here after printing the warning?

There is call to netif_stop_queue(ndev) just before this netdev_warn.
I copied below for quick reference.

+	sg_len = skb_shinfo(skb)->nr_frags + 1;
+	if (!CIRC_SPACE(lp->tx_ring_head, lp->tx_ring_tail, TX_BD_NUM_MAX)) {
+		netif_stop_queue(ndev);
+		if (net_ratelimit())
+			netdev_warn(ndev, "TX ring unexpectedly full\n");
+		return NETDEV_TX_BUSY;
+	}

However, I noticed above check doesn't account for all frags - so will 
modify the if check to see if available space is less than sg_len. If yes, then 
stop the queue and return busy , else continue with xmit.

> 
> The recommended flow is to stop the queue after enqueuing a packet
> successfully, if the remaining space is not enough to hold an skb with all frags
> populated. That avoids ever returning BUSY.