[PATCH 2/2] usb: dwc3: host: Disable USB3 ports if maximum-speed doesn't permit USB3

[Sam Edwards]

The DWC3 core can be configured (during IP instantiation, and/or via
configuration signals) not to have any USB3 ports. Some SoCs, however,
may have USB3 interfaces enabled that do not have USB3 PHYs driving
them. This can be due to a few circumstances, including:
a) The hardware designer didn't include USB3 PHYs and neglected to
   disable the DWC3 core's USB ports (I know of no instance where this
   has actually happened, however, and it seems pretty unlikely).
b) The USB3 PHYs are present but powered off. Perhaps a driver to enable
   the PHYs has not yet been written or merged, or USB3 capability is
   unneeded in the system and the system designer would like to conserve
   power.
c) The USB3 PHYs are muxed to a different controller. This can happen if
   the PHYs support non-USB protocols and one of these alternate
   functions is needed instead.

In these circumstances, if the DWC3 does not receive clear link status
indication on an enabled USB3 port, the DWC3 may not allow even USB2
to function: in host mode, the DWC3 generates an endless barrage of
PORT_CSC status on the accompanying USB2 port, and the xHCI driver is
unable to bring the USB2 port to a functioning state.

Fix this by first checking if the maximum-speed property in the DT
permits USB3. If not, pass the new `disable-usb3;` property to the
virtual xHCI device, causing the xHCI driver not to enable the USB3
ports. This allows USB2 to function even with USB3 PHYs
missing/misbehaving, and may be useful even when the USB3 PHYs are
well-behaved: a DT author may know that USB3 support is intact, but
disconnected (not exposed off-board) and choose to lower the
maximum-speed property to avoid an unusable USB3 rhub showing up in
sysfs/lsusb where it may mislead end-users.

Signed-off-by: Sam Edwards <CFSworks@xxxxxxxxx>
---
 drivers/usb/dwc3/host.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/dwc3/host.c b/drivers/usb/dwc3/host.c
index 61f57fe5bb78..29f170927e70 100644
--- a/drivers/usb/dwc3/host.c
+++ b/drivers/usb/dwc3/host.c
@@ -61,7 +61,7 @@ static int dwc3_host_get_irq(struct dwc3 *dwc)
 
 int dwc3_host_init(struct dwc3 *dwc)
 {
-	struct property_entry	props[4];
+	struct property_entry	props[5];
 	struct platform_device	*xhci;
 	int			ret, irq;
 	int			prop_idx = 0;
@@ -95,6 +95,9 @@ int dwc3_host_init(struct dwc3 *dwc)
 	if (dwc->usb2_lpm_disable)
 		props[prop_idx++] = PROPERTY_ENTRY_BOOL("usb2-lpm-disable");
 
+	if (dwc->maximum_speed == USB_SPEED_FULL || dwc->maximum_speed == USB_SPEED_HIGH)
+		props[prop_idx++] = PROPERTY_ENTRY_BOOL("disable-usb3");
+
 	/**
 	 * WORKAROUND: dwc3 revisions <=3.00a have a limitation
 	 * where Port Disable command doesn't work.
-- 
2.41.0