Re: lockdep: incorrect deadlock warning with two GPIO expanders

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Tue, Sep 13, 2016 at 02:29:24PM +0200, Linus Walleij wrote:
> On Mon, Sep 12, 2016 at 5:33 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Mon, Sep 12, 2016 at 05:16:14PM +0200, Bartosz Golaszewski wrote:
> 
> >> AFAIK there is no clean way to tell that a GPIO is used by an I2C
> >> multiplexer at probe time. Linus, Alexandre could you confirm?
> 
> Nominally, the GPIO descriptors are just abstract resources such
> as regulators or clocks, they can be used for a lot but just like
> a clock, regulator, dma channel etc does not know who is using
> it and for what, it does not know this, no.
> 
> > You cannot inspect the device tree while probing?
> 
> Of course it *can* but we would end up encoding a special
> case every time something like this happens, tied to just
> device tree, then another bolt-on for ACPI etc.
> 
> I have a hard time following the problem really, I'm
> afraid I'm simply just not smart enough :(

Why would this be DT or ACPI specific? Linux itself has a tree/graph of
all busses and devices right? That's what all this drivers/base/ stuff
is on about.

So can't you walk up that and see if you encounter the exact same driver
again?

Something like:

	for (nr = 0, parent = dev->parent; parent; parent = parent->parent) {
		if (parent->device_driver == &pca953x_driver.driver)
			nr++;
	}

Again, I have no clue how any of this works, but that seems like
something that ought to work.
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