netdev - Re: [PATCH v2 17/28] thunderbolt: Add support for full PCIe daisy chains

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Date:   Mon, 25 Mar 2019 12:16:16 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>,
        Andreas Noever <andreas.noever@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2 17/28] thunderbolt: Add support for full PCIe daisy
 chains

On Mon, Mar 25, 2019 at 11:57:33AM +0200, Mika Westerberg wrote:
> On Sun, Mar 24, 2019 at 12:31:44PM +0100, Lukas Wunner wrote:
> > On Wed, Feb 06, 2019 at 04:17:27PM +0300, Mika Westerberg wrote:
> > > -static void tb_activate_pcie_devices(struct tb *tb)
> > > +static int tb_tunnel_pci(struct tb *tb, struct tb_switch *sw)
> > >  {
> > [...]
> > > +	/*
> > > +	 * Look up available down port. Since we are chaining, it is
> > > +	 * typically found right above this switch.
> > > +	 */
> > > +	down = NULL;
> > > +	parent_sw = tb_to_switch(sw->dev.parent);
> > > +	while (parent_sw) {
> > > +		down = tb_find_unused_down_port(parent_sw);
> > > +		if (down)
> > > +			break;
> > > +		parent_sw = tb_to_switch(parent_sw->dev.parent);
> > > +	}
> > 
> > The problem I see here is that there's no guarantee that the switch
> > on which you're selecting a down port is actually itself connected
> > with a PCI tunnel.  E.g., allocation of a tunnel to that parent
> > switch may have failed.  In that case you end up establishing a
> > tunnel between that parent switch and the newly connected switch
> > but the tunnel is of no use.
> 
> Since this is going through tb_domain_approve_switch() it does not allow
> PCIe tunnel creation if the parent is not authorized first.

Yes, but my point is that it doesn't make much sense to establish a tunnel
between a switch and one of its parent switches unless that parent switch
is reachable from the root switch over a PCI tunnel or a series of PCI
tunnels.

It may be worth checking that condition, or, if new tunnels are established
top-down in the daisy-chain and tunnel establishment has failed for a
switch, to not establish tunnels for switches beyond that one.

Thanks,

Lukas