usb: xhci-plat: properly handle probe deferral for devm_clk_get()

commit de95c40d upstream.

On some platforms, the clocks might be registered by a platform
driver. When this is the case, the clock platform driver may very well
be probed after xhci-plat, in which case the first probe() invocation
of xhci-plat will receive -EPROBE_DEFER as the return value of
devm_clk_get().

The current code handles that as a normal error, and simply assumes
that this means that the system doesn't have a clock for the XHCI
controller, and continues probing without calling
clk_prepare_enable(). Unfortunately, this doesn't work on systems
where the XHCI controller does have a clock, but that clock is
provided by another platform driver. In order to fix this situation,
we handle the -EPROBE_DEFER error condition specially, and abort the
XHCI controller probe(). It will be retried later automatically, the
clock will be available, devm_clk_get() will succeed, and the probe()
will continue with the clock prepared and enabled as expected.

In practice, such issue is seen on the ARM64 Marvell 7K/8K platform,
where the clocks are registered by a platform driver.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.16: correct error label here is unmap_registers]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/host/xhci-plat.c

@@ -154,6 +154,9 @@ static int xhci_plat_probe(struct platform_device *pdev)
 		ret = clk_prepare_enable(clk);
 		if (ret)
 			goto unmap_registers;
+	} else if (PTR_ERR(clk) == -EPROBE_DEFER) {
+		ret = -EPROBE_DEFER;
+		goto unmap_registers;
 	}
 
 	if (of_device_is_compatible(pdev->dev.of_node,