linux-kernel - Re: [PATCH v2 13/28] thunderbolt: Add helper function to iterate from one port to another

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Message-ID: <20190211061600.6ty733qfagk7f6fp@wunner.de>
Date:   Mon, 11 Feb 2019 07:16:00 +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 13/28] thunderbolt: Add helper function to iterate
 from one port to another

On Wed, Feb 06, 2019 at 04:17:23PM +0300, Mika Westerberg wrote:
> We need to be able to walk from one port to another when we are creating
> paths where there are multiple switches between two ports. For this
> reason introduce a new function tb_port_get_next() and a new macro
> tb_for_each_port().

These names seem fairly generic, they might as well refer to the next port
on a switch or iterate over the ports on a switch.  E.g. I've proposed a
tb_sw_for_each_port() macro in this patch:

https://lore.kernel.org/patchwork/patch/983863/

I'd suggest renaming tb_port_get_next() to something like
tb_next_port_on_path() or tb_path_next_port() or tb_path_walk().

And I'd suggest dropping tb_for_each_port() because there are only
two occurrences where it's used, one calculates the path length,
and I think that's simply the route string length plus 2, and the
other one in patch 17 isn't even interested in the ports along a path,
but rather in the switches between the root switch and the end of a path.
It seems simpler to just iterate from the switch at the end upwards to
the root switch by following the parent pointer in the switch's
struct device, or alternatively by bytewise iterating over the route
string and calling get_switch_at_route() each time.

> +/**
> + * tb_port_get_next() - Return next port for given port
> + * @start: Start port of the walk
> + * @end: End port of the walk
> + * @prev: Previous port (%NULL if this is the first)
> + *
> + * This function can be used to walk from one port to another if they
> + * are connected through zero or more switches. If the @prev is dual
> + * link port, the function follows that link and returns another end on
> + * that same link.
> + *
> + * If the walk cannot be continued, returns %NULL.

This sounds as if NULL is returned if an error occurs but that doesn't
seem to be what the function does.  I'd suggest:

"If the @end port has been reached, return %NULL."

> +struct tb_port *tb_port_get_next(struct tb_port *start, struct tb_port *end,
> +				 struct tb_port *prev)
> +{
> +	struct tb_port *port, *next;
> +
> +	if (!prev)
> +		return start;
> +
> +	if (prev->sw == end->sw) {
> +		if (prev != end)
> +			return end;
> +		return NULL;
> +	}
> +
> +	/* Switch back to use primary links for walking */

"Switch back" requires that you switched to something else before,
which you didn't.  I'd suggest something like:

"use primary link to discover next port"

Why is it necessary to use the primary link anyway?  Is the
->remote member not set on the secondary link port?  The reason
should probably be spelled out in the code comment.

> +	if (prev->dual_link_port && prev->link_nr)
> +		port = prev->dual_link_port;
> +	else
> +		port = prev;
> +
> +	if (start->sw->config.depth < end->sw->config.depth) {
> +		if (port->remote &&
> +		    port->remote->sw->config.depth > port->sw->config.depth)

Can we use "if (!tb_is_upstream_port(port))" for consistency with the
if-clause below?

> +			next = port->remote;
> +		else
> +			next = tb_port_at(tb_route(end->sw), port->sw);
> +	} else if (start->sw->config.depth > end->sw->config.depth) {
> +		if (tb_is_upstream_port(port))
> +			next = port->remote;
> +		else
> +			next = tb_upstream_port(port->sw);
> +	} else {
> +		/* Must be the same switch then */
> +		if (start->sw != end->sw)
> +			return NULL;
> +		return end;
> +	}

The else-clause here appears to be dead code, you've already checked
further up whether prev and end are on the same switch.

> +
> +	/* If prev was dual link return another end of that link then */

*Here* a "switch back" comment would be appropriate.  Nit: Please either
end code comments with a period or don't start them with an upper case
letter.

> +	if (next->dual_link_port && next->link_nr != prev->link_nr)
> +		return next->dual_link_port;
> +
> +	return next;
> +}

Thanks,

Lukas