linux-kernel - Re: [PATCH v4 net 3/4] net: enetc: disable Tx BD rings after they are empty

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Message-ID: <20241011144009.ydoyf5vlxqrdj2pu@skbuf>
Date: Fri, 11 Oct 2024 17:40:09 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: Wei Fang <wei.fang@....com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, claudiu.manoil@....com, ast@...nel.org,
	daniel@...earbox.net, hawk@...nel.org, john.fastabend@...il.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	bpf@...r.kernel.org, stable@...r.kernel.org, imx@...ts.linux.dev,
	rkannoth@...vell.com, maciej.fijalkowski@...el.com,
	sbhatta@...vell.com
Subject: Re: [PATCH v4 net 3/4] net: enetc: disable Tx BD rings after they
 are empty

On Thu, Oct 10, 2024 at 05:20:55PM +0800, Wei Fang wrote:
> The Tx BD rings are disabled first in enetc_stop() and the driver
> waits for them to become empty. This operation is not safe while
> the ring is actively transmitting frames, and will cause the ring
> to not be empty and hardware exception. As described in the NETC
> block guide, software should only disable an active Tx ring after
> all pending ring entries have been consumed (i.e. when PI = CI).
> Disabling a transmit ring that is actively processing BDs risks
> a HW-SW race hazard whereby a hardware resource becomes assigned
> to work on one or more ring entries only to have those entries be
> removed due to the ring becoming disabled.
> 
> When testing XDP_REDIRECT feautre, although all frames were blocked
> from being put into Tx rings during ring reconfiguration, the similar
> warning log was still encountered:
> 
> fsl_enetc 0000:00:00.2 eno2: timeout for tx ring #6 clear
> fsl_enetc 0000:00:00.2 eno2: timeout for tx ring #7 clear
> 
> The reason is that when there are still unsent frames in the Tx ring,
> disabling the Tx ring causes the remaining frames to be unable to be
> sent out. And the Tx ring cannot be restored, which means that even
> if the xdp program is uninstalled, the Tx frames cannot be sent out
> anymore. Therefore, correct the operation order in enect_start() and
> enect_stop().

Typos, no need to resend for this: enect -> enetc.

> 
> Fixes: ff58fda09096 ("net: enetc: prioritize ability to go down over packet processing")
> Cc: stable@...r.kernel.org
> Signed-off-by: Wei Fang <wei.fang@....com>
> ---

Reviewed-by: Vladimir Oltean <vladimir.oltean@....com>