[PATCH v1] usb: xhci: allow imod-interval to be configurable

The xHCI driver currently has the IMOD set to 160, which
translates to an IMOD interval of 40,000ns (160 * 250)ns

Commit 0cbd4b34cda9 ("xhci: mediatek: support MTK xHCI host controller")
introduced a QUIRK for the MTK platform to adjust this interval to 20,
which translates to an IMOD interval of 5,000ns (20 * 250)ns. This is
due to the fact that the MTK controller IMOD interval is 8 times
as much as defined in xHCI spec.

Instead of adding more quirk bits for additional platforms, this patch
introduces the ability for vendors to set the IMOD_INTERVAL as is
optimal for their platform. By using device_property_read_u32() on
"imod-interval", the IMOD INTERVAL can be specified in nano seconds. If
no interval is specified, the default of 40,000ns (IMOD=160) will be
used.

No bounds checking has been implemented due to the fact that a vendor
may have violated the spec and would need to specify a value outside of
the max 8,000 IRQs/second limit specified in the xHCI spec.

Backwards compatibility is maintained for MTK_HOSTS through the quirk
bit, however, imod_interval should be pushed into device tree at a
future point and this quirk should be removed from xhci_plat_probe

Signed-off-by: Adam Wallis <redacted>
---
 Documentation/devicetree/bindings/usb/usb-xhci.txt |  1 +
 drivers/usb/host/xhci-plat.c                       | 15 +++++++++++++++
 drivers/usb/host/xhci.c                            |  7 ++-----
 drivers/usb/host/xhci.h                            |  2 ++
 4 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/Documentation/devicetree/bindings/usb/usb-xhci.txt b/Documentation/devicetree/bindings/usb/usb-xhci.txt
index ae6e484..3998459 100644
--- a/Documentation/devicetree/bindings/usb/usb-xhci.txt
+++ b/Documentation/devicetree/bindings/usb/usb-xhci.txt

@@ -29,6 +29,7 @@ Optional properties:
   - usb2-lpm-disable: indicate if we don't want to enable USB2 HW LPM
   - usb3-lpm-capable: determines if platform is USB3 LPM capable
   - quirk-broken-port-ped: set if the controller has broken port disable mechanism
+  - imod-interval: IMOD_INTERVAL in nano-seconds. Default is 40000
 
 Example:
 	usb at f0931000 {

diff --git a/drivers/usb/host/xhci-plat.c b/drivers/usb/host/xhci-plat.c
index 09f164f..f7730c8 100644
--- a/drivers/usb/host/xhci-plat.c
+++ b/drivers/usb/host/xhci-plat.c

@@ -23,6 +23,7 @@
 #include "xhci-plat.h"
 #include "xhci-mvebu.h"
 #include "xhci-rcar.h"
+#include "xhci-mtk.h"
 
 static struct hc_driver __read_mostly xhci_plat_hc_driver;
 

@@ -269,6 +270,20 @@ static int xhci_plat_probe(struct platform_device *pdev)
 	if (device_property_read_bool(&pdev->dev, "quirk-broken-port-ped"))
 		xhci->quirks |= XHCI_BROKEN_PORT_PED;
 
+	/* imod interval in nanoseconds */
+	if (device_property_read_u32(sysdev,
+		"imod-interval", &xhci->imod_interval))
+		xhci->imod_interval = 40000;