[PATCHv4] drivers/amba: add reset control to amba bus probe

[Dinh Nguyen <dinguyen@xxxxxxxxxx>]

The primecell controller on some SoCs, i.e. SoCFPGA, is held in reset by
default. Until recently, the DMA controller was brought out of reset by the
bootloader(i.e. U-Boot). But a recent change in U-Boot, the peripherals
that are not used are held in reset and are left to Linux to bring them
out of reset.

Add a mechanism for getting the reset property and de-assert the primecell
module from reset if found. This is a not a hard fail if the reset properti
is not present in the device tree node, so the driver will continue to
probe.

Because there are different variants of the controller that may have
multiple reset signals, the code will find all reset(s) specified and
de-assert them.

Signed-off-by: Dinh Nguyen <dinguyen@xxxxxxxxxx>
Reviewed-by: Rob Herring <robh@xxxxxxxxxx>
---
v4: cleaned up indentation in loop
    fix up a few checkpatch warnings
    add Reviewed-by:
v3: add a reset_control_put()
    add error handling
v2: move reset control to bus code
    find all reset properties and de-assert them
---
 drivers/amba/bus.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/drivers/amba/bus.c b/drivers/amba/bus.c
index 100e798a5c82..76a1cd56a1ab 100644
--- a/drivers/amba/bus.c
+++ b/drivers/amba/bus.c
@@ -18,6 +18,7 @@
 #include <linux/limits.h>
 #include <linux/clk/clk-conf.h>
 #include <linux/platform_device.h>
+#include <linux/reset.h>
 
 #include <asm/irq.h>
 
@@ -401,6 +402,26 @@ static int amba_device_try_add(struct amba_device *dev, struct resource *parent)
 	ret = amba_get_enable_pclk(dev);
 	if (ret == 0) {
 		u32 pid, cid;
+		int count;
+		struct reset_control *rstc;
+
+		/*
+		 * Find reset control(s) of the amba bus and de-assert them.
+		 */
+		count = reset_control_get_count(&dev->dev);
+		while (count > 0) {
+			rstc = of_reset_control_get_shared_by_index(dev->dev.of_node, count - 1);
+			if (IS_ERR(rstc)) {
+				if (PTR_ERR(rstc) == -EPROBE_DEFER)
+					ret = -EPROBE_DEFER;
+				else
+					dev_err(&dev->dev, "Can't get amba reset!\n");
+				break;
+			}
+			reset_control_deassert(rstc);
+			reset_control_put(rstc);
+			count--;
+		}
 
 		/*
 		 * Read pid and cid based on size of resource
-- 
2.20.0