linux-kernel - Re: [PATCH 13/13] driver-core: probe dependencies before probing

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Date:	Wed, 17 Jun 2015 19:13:59 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Alexander Holler <holler@...oftware.de>,
	Alexandre Courbot <gnurou@...il.com>,
	Andrzej Hajda <a.hajda@...sung.com>,
	Arnd Bergmann <arnd@...db.de>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Grant Likely <grant.likely@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Kumar Gala <galak@...eaurora.org>, Len Brown <lenb@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	linux-kernel@...r.kernel.org, Lv Zheng <lv.zheng@...el.com>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Robert Moore <robert.moore@...el.com>,
	Rob Herring <robh+dt@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	Stephen Warren <swarren@...dotorg.org>,
	Terje Bergström <tbergstrom@...dia.com>,
	Thierry Reding <thierry.reding@...il.com>
Subject: Re: [PATCH 13/13] driver-core: probe dependencies before probing

On Wed, Jun 17, 2015 at 03:42:23PM +0200, Tomeu Vizoso wrote:
> Before actually probing a device, find out what dependencies it has and
> do our best to ensure that they are available at this point.

> This is accomplished by finding out what platform devices need to be
> probed so the dependencies are available.

...and then trying to probe them first.

> If any dependencies are still unavailable after that (most probably a
> missing driver or an error in the HW description from the firmware), we
> print a nice error message so that people don't have to add a zillion of
> printks to find out why a device asked for its probe to be deferred.

So, I think I like this approach though I've not done a full pass
through and I'm not sure how expensive it gets (there's definitely room
for optimisation as the patch notes).  I'm not 100% sure I see what
prints this error message you're referring to (I'm just seeing debug
prints).

> +static struct fwnode_handle *get_enclosing_platform_dev(
> +						struct fwnode_handle *fwnode)

Only platform devices?

> +static void check_dependencies_per_class(struct class *class, void *data)
> +{
> +	struct fwnode_handle *fwnode = data;
> +	struct list_head *deps;
> +	struct fwnode_dependency *dep, *tmp;
> +
> +	if (!class->get_dependencies)
> +		return;
> +
> +	deps = class->get_dependencies(fwnode);
> +	if (!deps)
> +		return;
> +
> +	list_for_each_entry_safe(dep, tmp, deps, dependency) {
> +		if (!check_dependency(dep->fwnode))
> +			pr_debug("Dependency '%s' not available\n",
> +				 fwnode_get_name(dep->fwnode));
> +
> +		list_del(&dep->dependency);
> +		kfree(dep);
> +	}
> +
> +	kfree(deps);

OK, so the caller is responsible for freeing everything and the class
must allocate - this definitely suggests that 

I'm not sure there's any benefit in having deps be dynamically allocated
here, just put it on the stack and iterate through the list - the
iteration is going to be cheap if we get nothing back (probably the
common case) and probably cheaper than the alloc/free.

One thing here is that I was under the impression classes were supposed
to be going away...

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