linux-kernel - Re: [PATCH v2 1/6] device property: Helper to match multiple connections

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Date:   Mon, 21 Feb 2022 11:08:28 -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 v2 1/6] device property: Helper to match multiple
 connections

On Mon 21 Feb 09:19 PST 2022, Andy Shevchenko wrote:

> On Sun, Feb 20, 2022 at 08:55:10PM -0800, Bjorn Andersson wrote:
> > On Sun 20 Feb 03:16 PST 2022, Andy Shevchenko wrote:
> > > On Fri, Feb 18, 2022 at 11:00:45AM -0800, Bjorn Andersson wrote:
> > > > On Wed 09 Feb 04:30 PST 2022, Andy Shevchenko wrote:
> > > > > On Mon, Feb 07, 2022 at 07:19:39PM -0800, Bjorn Andersson wrote:
> 
> ...
> 
> > > > > > +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;
> > > > > > +
> > > > > > +	if (!fwnode || !match || !matches)
> > > > > 
> > > > > !matches case may be still useful to get the count and allocate memory by
> > > > > caller. Please, consider this case.
> > > > 
> > > > As discussed in previous version, and described in the commit message,
> > > > the returned value of "match" is a opaque pointer to something which
> > > > has to be passed back to the caller in order to be cleaned up.
> > > > 
> > > > E.g. the typec mux code returns a pointer to a typec_mux/switch object
> > > > with a refcounted struct device within, or an ERR_PTR().
> > > > 
> > > > So unfortunately we can must gather the results into matches and pass it
> > > > back to the caller to take consume or clean up.
> > > 
> > > It's fine. You have **matches, means pointer of an opaque pointer.
> > > What I'm talking about is memory allocation for and array of _pointers_.
> > > That's what caller very much aware of and can allocate on heap. So, please
> > > consider this case.
> > 
> > I'm sorry, but I'm not sure what you're looking for.
> > 
> > 
> > I still interpret your comment as that it would be nice to be able to do
> > something like:
> > 
> > count = fwnode_connection_find_matches(fwnode, "orientation-switch",
> > 				       NULL, typec_switch_match, NULL, 0);
> > 
> > based on the returned value the caller could allocate an array of
> > "count" pointers and then call the function again to actually fill out
> > the count elements.
> 
> Yes, that's what I want from the generic fwnode APIs.
> (Keyword: generic)
> 
> > The problem with this is that, typec_switch_match() does:
> 
> As you stated, the problem is in the typec_switch_match(). So, it's not related
> to the fwnode, but how you are using it.
> 
> > void *typec_switch_match(fwnode, id, data) {
> > 	struct device *dev = find_struct_device(fwnode, id);
> > 	if (!dev)
> > 		return NULL;
> > 	get_device(dev);
> > 	return container_of(dev, struct typec_switch, dev);
> > }
> > 
> > So if we call the match function and if that finds a "dev" it will
> > return a struct typec_switch with a refcounted struct device within.
> 
> fwnode (as being an abstraction on top of the others) has no knowledge
> about this. And more important should not know that.
> 
> > We can see if that's NULL or not and will be able to return a "count",
> > but we have no way of releasing the reference acquired - we must return
> > the void pointer back to the client, so that it can release it.
> 
> The caller (if it wants to!) may create different callbacks for count and real
> matching, no?
> 

Ahh, yeah you're right, we can shift this responsibility onto the caller
and thereby allow them to implement the count as well. Makes sense!

Thanks,
Bjorn

> > My claim is that this is not a problem, because this works fine with any
> > reasonable size of fwnode graphs we might run into - and the client will
> > in general have a sense of the worst case number of matches (in this
> > series its 3, as there's 3 types of lanes that can be switched/muxed
> > coming out of a USB connector).
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>