linux-kernel - Re: [net-next PATCH 1/4] drivers: net: dsa: qca8k: drop MTU tracking from qca8k

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Date:   Tue, 22 Mar 2022 15:55:35 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Ansuel Smith <ansuelsmth@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [net-next PATCH 1/4] drivers: net: dsa: qca8k: drop MTU tracking
 from qca8k_priv

On Tue, Mar 22, 2022 at 02:38:08PM +0100, Ansuel Smith wrote:
> On Tue, Mar 22, 2022 at 01:58:12PM +0200, Vladimir Oltean wrote:
> > On Tue, Mar 22, 2022 at 02:45:03AM +0100, Ansuel Smith wrote:
> > > Drop the MTU array from qca8k_priv and use slave net dev to get the max
> > > MTU across all user port. CPU port can be skipped as DSA already make
> > > sure CPU port are set to the max MTU across all ports.
> > > 
> > > Signed-off-by: Ansuel Smith <ansuelsmth@...il.com>
> > > ---
> > 
> > I hardly find this to be an improvement and I would rather not see such
> > unjustified complexity in a device driver. What are the concrete
> > benefits, size wise?
> >
> 
> The main idea here is, if the value is already present and accessible,
> why should we duplicate it? Tracking the MTU in this custom way already
> caused some bugs (check the comment i'm removing). We both use standard
> way to track ports MTU and we save some additional space. At the cost of
> 2 additional checks are are not that much of a problem.

Where is the bug?

> Also from this I discovered that (at least on ipq806x that use stmmac)
> when master needs to change MTU, stmmac complains that the interface is
> up and it must be put down. Wonder if that's common across other drivers
> or it's only specific to stmmac.

I never had the pleasure of dealing with such DSA masters. I wonder why
can't stmmac_change_mtu() check if netif_running(), call dev_close and
set a bool, and at the end, if the bool was set, call dev_open back?