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

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Date:   Wed, 22 Nov 2023 18:16:47 +0100
From:   Köry Maincent <kory.maincent@...tlin.com>
To:     Andrew Lunn <andrew@...n.ch>
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

On Wed, 22 Nov 2023 17:54:53 +0100
Andrew Lunn <andrew@...n.ch> wrote:

> > > > +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.

Ok I will try to refactor it to makes it more readable.

> > > > +	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.

I would say it never supposed to happen.
I never faced the issue playing with the controller. The first communication is
a simple i2c_master_recv of the controller status without entering the
pd692x0_sendrecv_msg function, therefore it won't be an issue.

Another solution could be to raise a flag if I enter in communication loss and
release the rtnlock. We would lock again the rtnl when the flags got disabled.
The controler won't be accessible until the flag is disabled.

Regards,
-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com