linux-kernel - Re: [PATCH v2 14/28] thunderbolt: Extend tunnel creation to more than 2 adjacent switches

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Message-ID: <20190210153328.44ur6o5z2xjae42c@wunner.de>
Date:   Sun, 10 Feb 2019 16:33:28 +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>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v2 14/28] thunderbolt: Extend tunnel creation to more
 than 2 adjacent switches

On Wed, Feb 06, 2019 at 04:17:24PM +0300, Mika Westerberg wrote:
> Now that we can allocate hop IDs per port on a path, we can take
> advantage of this and create tunnels covering longer paths than just
> between two adjacent switches. PCIe actually does not need this as it is
> always a daisy chain between two adjacent switches but this way we do
> not need to hard-code creation of the tunnel.

That doesn't seem to be correct, at the bottom of this page there's
a figure showing a PCI tunnel between non-adjacent switches (blue line):

https://developer.apple.com/library/archive/documentation/HardwareDrivers/Conceptual/ThunderboltDevGuide/Basics/Basics.html

I'm not sure if there are advantages to such tunnels:  Reduced latency
perhaps because packets need not pass through PCIe adapters on the
in-between device?  Or maybe this allows for more fine-grained traffic
prioritization?

> +	i = 0;
> +	tb_for_each_port(in_port, src, dst)
> +		i++;

This looks more complicated than necessary.  Isn't the path length
always the length of the route string from in_port switch to out_port
switch, plus 2 for the adapter on each end?  Or do paths without
adapters exist?

> +	for (i = 0; i < num_hops; i++) {
> +		in_port = tb_port_get_next(src, dst, out_port);
> +
> +		if (in_port->dual_link_port && in_port->link_nr != link_nr)
> +			in_port = in_port->dual_link_port;
> +
> +		ret = tb_port_alloc_in_hopid(in_port, in_hopid, -1);
> +		if (ret < 0)
> +			goto err;
> +		in_hopid = ret;
> +
> +		out_port = tb_port_get_next(src, dst, in_port);
> +		if (!out_port)
> +			goto err;

There's a NULL pointer check here, but the invocation of tb_port_get_next()
further up to assign in_port lacks such a check.  Is it guaranteed to never
be NULL?

Thanks,

Lukas