netdev - Re: [PATCH net-next 9/9] net: pse-pd: Add PD692x0 PSE controller driver

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Date: Wed, 22 Nov 2023 17:54:53 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Köry Maincent <kory.maincent@...tlin.com>
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Russ Weight <russ.weight@...ux.dev>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH net-next 9/9] net: pse-pd: Add PD692x0 PSE controller
 driver

> > > +static int pd692x0_sendrecv_msg(struct pd692x0_priv *priv,
> > > +				struct pd692x0_msg *msg,
> > > +				struct pd692x0_msg_content *buf)
> > > +{
> > > +	struct device *dev = &priv->client->dev;
> > > +	int ret;
> > > +
> > > +	ret = pd692x0_send_msg(priv, msg);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	ret = pd692x0_recv_msg(priv, msg, buf);  
> > 
> > So this function takes at least 10 seconds?
> 
> No, on normal communication it takes a bit more than 30ms.

So i think the first step is to refactor this code to make it clear
what the normal path is, and what the exception path is, and the
timing of each.

> > > +	msg.content.sub[2] = id;
> > > +	ret = pd692x0_sendrecv_msg(priv, &msg, &buf);  
> > 
> > So this is also 10 seconds? 
> > 
> > Given its name, it looks like this is called via ethtool? Is the
> > ethtool core holding RTNL? It is generally considered bad to hold RTNL for
> > that long.
> 
> Yes it is holding RTNL lock. Should I consider another behavior in case of
> communication loss to not holding RTNL lock so long?

How often does it happen? On the scale of its a theoretical
possibility, through to it happens every N calls? Also, does it happen
on cold boot and reboot?

If its never supposed to happen, i would keep holding RTNL, and add a
pr_warn() that the PSE has crashed and burned, waiting for it to
reboot. If this is likely to happen on the first communication with
the device, we might want to do a dummy transfer during probe to get
is synchronized before we start using it with the RTNL held.

   Andrew