linux-kernel - Re: [PATCH v4 1/7] device property: Helper to match multiple connections

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Date:   Mon, 7 Mar 2022 11:13:28 -0800
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 v4 1/7] device property: Helper to match multiple
 connections

On Mon 07 Mar 02:05 PST 2022, Andy Shevchenko wrote:

> On Sun, Mar 06, 2022 at 07:40:34PM -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.
> 
> Thanks for the update!
> 
> First of all, I have noticed that subject misses the verb, something like Add
> or Introduce.
> 

Will update accordingly.

> ...
> 
> > +/**
> > + * fwnode_connection_find_matches - Find connections from a device node
> > + * @fwnode: Device node with the connection
> > + * @con_id: Identifier for the connection
> > + * @data: Data for the match function
> > + * @match: Function to check and convert the connection description
> > + * @matches: Array of pointers to fill with matches
> 
> (Optional) array...
> 

Ditto.

> > + * @matches_len: Length of @matches
> > + *
> > + * Find up to @matches_len connections with unique identifier @con_id between
> > + * @fwnode and other device nodes. @match will be used to convert the
> > + * connection description to data the caller is expecting to be returned
> > + * through the @matches array.
> 
> > + * If @matches is NULL @matches_len is ignored and the total number of resolved
> > + * matches is returned.
> 
> I would require matches_len to be 0, see below.
> 
> > + * Return: Number of matches resolved, or negative errno.
> > + */
> > +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);
> 
> > +	if (matches) {
> > +		matches += count_graph;
> > +		matches_len -= count_graph;
> > +	}
> 
> So, the valid case is matches != NULL and matches_len == 0. For example, when
> we have run something previously on the buffer and it becomes full.
> 
> In this case we have carefully handle this case.
> 
> 	if (matches) {
> 		matches += count_graph;
> 		if (matches_len)
> 			matches_len -= count_graph;

When matches is non-NULL, both the sub-functions are limited by
matches_len and as such count_graph <= matches_len.

As such matches_len >= 0.

In the event that the originally passed matches_len was 0, then
count_graph will be 0 and matches_len will remain 0.

I therefor don't see that this additional check changes things.

> 	}
> 
> Seems it can be also
> 
> 	if (matches)
> 		matches += count_graph;
> 
> 	if (matches_len)
> 		matches_len -= count_graph;

We covered the case of matches && (matches_len || !matches_len) above.

For the case of !matches && matches_len, this added conditional would
cause matches_len to be extra ignored by keeping it at 0, but per
kernel-doc and implementation we ignore all other values already.


Note that this is in contrast from vsnprintf() where the code will
continue to produce results, only store the first "matches_len"
entires and return the final count.

Unfortunately we can't follow such semantics here, instead it is clearly
documented in the kernel-doc that @matches_len is ignored when @matches
is NULL.


So unless I'm missing something I don't see what you gain over keeping
the check on only matches.

> 
> That said, do we have a test cases for this?
> 

I looked briefly at adding some kunit tests for this, but was discourage
by the prospect of building up the graphs to run the tests against.

Regards,
Bjorn

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