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

[Lukas Wunner <lukas@wunner.de>]

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