linux-kernel - Re: [PATCH v3 1/6] device property: Helper to match multiple connections

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Date:   Sun, 6 Mar 2022 20:09:39 -0600
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Daniel Scally <djrscally@...il.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        linux-usb@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Subject: Re: [PATCH v3 1/6] device property: Helper to match multiple
 connections

On Fri 04 Mar 06:54 CST 2022, Andy Shevchenko wrote:

> On Thu, Mar 03, 2022 at 02:33:46PM -0800, Bjorn Andersson wrote:
> > In some cases multiple connections with the same connection id
> > needs to be resolved from a fwnode graph.
> > 
> > One such example is when separate hardware is used for performing muxing
> > and/or orientation switching of the SuperSpeed and SBU lines in a USB
> > Type-C connector. In this case the connector needs to belong to a graph
> > with multiple matching remote endpoints, and the Type-C controller needs
> > to be able to resolve them both.
> > 
> > Add a new API that allows this kind of lookup.
> 
> ...
> 
> > +static unsigned int fwnode_graph_devcon_matches(struct fwnode_handle *fwnode,
> > +						const char *con_id, void *data,
> > +						devcon_match_fn_t match,
> > +						void **matches,
> > +						unsigned int matches_len)
> > +{
> > +	struct fwnode_handle *node;
> > +	struct fwnode_handle *ep;
> > +	unsigned int count = 0;
> > +	void *ret;
> > +
> > +	fwnode_graph_for_each_endpoint(fwnode, ep) {
> 
> > +		if (count >= matches_len && matches) {
> > +			fwnode_handle_put(ep);
> > +			return count;
> > +		}
> 
> Wouldn't be the same as
> 
> 		if (count >= matches_len) {

This would cause the return value to be at most matches_len, seems
relevant to ignore (or perhaps require it to be 0) when matches is NULL.

But flipping the order of the expression seems better to me, now that
this has been sitting for a while.

> 			fwnode_handle_put(ep);
> 			break;

Right, this isn't an "early return", so nicer to have a single return at
the bottom.

> 		}
> 
> ?
> 
> > +		node = fwnode_graph_get_remote_port_parent(ep);
> > +		if (!fwnode_device_is_available(node)) {
> > +			fwnode_handle_put(node);
> > +			continue;
> > +		}
> > +
> > +		ret = match(node, con_id, data);
> > +		fwnode_handle_put(node);
> > +		if (ret) {
> > +			if (matches)
> > +				matches[count] = ret;
> > +			count++;
> > +		}
> > +	}
> > +	return count;
> > +}
> 
> ...
> 
> > +static unsigned int fwnode_devcon_matches(struct fwnode_handle *fwnode,
> > +					  const char *con_id, void *data,
> > +					  devcon_match_fn_t match,
> > +					  void **matches,
> > +					  unsigned int matches_len)
> > +{
> > +	struct fwnode_handle *node;
> > +	unsigned int count = 0;
> > +	unsigned int i;
> > +	void *ret;
> > +
> > +	for (i = 0; ; i++) {
> 
> > +		if (count >= matches_len && matches)
> > +			return count;
> 
> Ditto.
> 
> > +		node = fwnode_find_reference(fwnode, con_id, i);
> > +		if (IS_ERR(node))
> > +			break;
> > +
> > +		ret = match(node, NULL, data);
> > +		fwnode_handle_put(node);
> > +		if (ret) {
> > +			if (matches)
> > +				matches[count] = ret;
> > +			count++;
> > +		}
> > +	}
> > +
> > +	return count;
> > +}
> 
> ...
> 
> > +int fwnode_connection_find_matches(struct fwnode_handle *fwnode,
> > +				   const char *con_id, void *data,
> > +				   devcon_match_fn_t match,
> > +				   void **matches, unsigned int matches_len)
> > +{
> > +	unsigned int count_graph;
> > +	unsigned int count_ref;
> > +
> > +	if (!fwnode || !match)
> > +		return -EINVAL;
> 
> > +	count_graph = fwnode_graph_devcon_matches(fwnode, con_id, data, match,
> > +						  matches, matches_len);
> 
> > +	matches += count_graph;
> > +	matches_len -= count_graph;
> 
> No, won't work when matches == NULL.
> 

Sorry about that, you're obviously correct.

> Also, matches_len is expected to be 0 in that case (or at least being ignored,
> check with vsnprintf() behaviour in similar case).
> 
> So, something like this, perhaps
> 
> 	if (matches && matches_len) {
> 		matches += count_graph;
> 		matches_len -= count_graph;
> 	}

Seems reasonable.

Thanks,
Bjorn

> 
> > +	count_ref = fwnode_devcon_matches(fwnode, con_id, data, match,
> > +					  matches, matches_len);
> > +
> > +	return count_graph + count_ref;
> > +}
> 
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>