usb: ehci-platform: add optional reset controller retrieval

On the Allwinner's A31 SoC the reset line connected to the EHCI IP has to
be deasserted for the EHCI block to be usable.

Add support for an optional reset controller that will be deasserted on
power off and asserted on power on.

Signed-off-by: Boris BREZILLON <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>
Reviewed-by: Hans de Goede <[e-mail]>
Acked-by: Alan Stern <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/usb/usb-ehci.txt

···
  - clocks : a list of phandle + clock specifier pairs
  - phys : phandle + phy specifier pair
  - phy-names : "usb"
+ - resets : phandle + reset specifier pair
 
 Example (Sequoia 440EPx):
     ehci@e0000300 {

drivers/usb/host/ehci-platform.c

···
 #include <linux/of.h>
 #include <linux/phy/phy.h>
 #include <linux/platform_device.h>
+#include <linux/reset.h>
 #include <linux/usb.h>
 #include <linux/usb/hcd.h>
 #include <linux/usb/ehci_pdriver.h>
···
 
 struct ehci_platform_priv {
 	struct clk *clks[EHCI_MAX_CLKS];
+	struct reset_control *rst;
 	struct phy *phy;
 };
 
···
 		}
 	}
 
+	priv->rst = devm_reset_control_get_optional(&dev->dev, NULL);
+	if (IS_ERR(priv->rst)) {
+		err = PTR_ERR(priv->rst);
+		if (err == -EPROBE_DEFER)
+			goto err_put_clks;
+		priv->rst = NULL;
+	} else {
+		err = reset_control_deassert(priv->rst);
+		if (err)
+			goto err_put_clks;
+	}
+
 	if (pdata->big_endian_desc)
 		ehci->big_endian_desc = 1;
 	if (pdata->big_endian_mmio)
···
 		dev_err(&dev->dev,
 			"Error: CONFIG_USB_EHCI_BIG_ENDIAN_MMIO not set\n");
 		err = -EINVAL;
-		goto err_put_clks;
+		goto err_reset;
 	}
 #endif
 #ifndef CONFIG_USB_EHCI_BIG_ENDIAN_DESC
···
 		dev_err(&dev->dev,
 			"Error: CONFIG_USB_EHCI_BIG_ENDIAN_DESC not set\n");
 		err = -EINVAL;
-		goto err_put_clks;
+		goto err_reset;
 	}
 #endif
 
 	if (pdata->power_on) {
 		err = pdata->power_on(dev);
 		if (err < 0)
-			goto err_put_clks;
+			goto err_reset;
 	}
 
 	hcd->rsrc_start = res_mem->start;
···
 err_power:
 	if (pdata->power_off)
 		pdata->power_off(dev);
+err_reset:
+	if (priv->rst)
+		reset_control_assert(priv->rst);
 err_put_clks:
 	while (--clk >= 0)
 		clk_put(priv->clks[clk]);
···
 
 	if (pdata->power_off)
 		pdata->power_off(dev);
+
+	if (priv->rst)
+		reset_control_assert(priv->rst);
 
 	for (clk = 0; clk < EHCI_MAX_CLKS && priv->clks[clk]; clk++)
 		clk_put(priv->clks[clk]);