[PATCH] dt-bindings: usb: dwc3: Add snps,enable_guctl1_ipd_quirk

[Radhey Shyam Pandey]

From: Piyush Mehta <piyush.mehta@xxxxxxx>

SNPS controller when configured in HOST mode maintains Inter Packet
Delay (IPD) of ~380ns which works with most of the super-speed hubs
except VIA-LAB hubs. When IPD is ~380ns HOST controller fails to
enumerate FS/LS devices when connected behind VIA-LAB hubs.

To address the above issue, add 'snps,enable_guctl1_ipd_quirk' quirk,
This quirk set the bit 9 of GUCTL1 that enables the workaround in HW to
reduce the ULPI clock latency by 1 cycle, thus reducing the IPD (~360ns).

Signed-off-by: Piyush Mehta <piyush.mehta@xxxxxxx>
Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@xxxxxxx>
---
In the zynqmp public database GUCTL1 bit 9 is reserved but it is used to
enable a fix related to Inter Packet Delay in HW. The documentation team
is working to update GUCTL1 bit 9 description.
---
 Documentation/devicetree/bindings/usb/snps,dwc3.yaml | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/devicetree/bindings/usb/snps,dwc3.yaml b/Documentation/devicetree/bindings/usb/snps,dwc3.yaml
index 8f5d250070c7..b226457a6e50 100644
--- a/Documentation/devicetree/bindings/usb/snps,dwc3.yaml
+++ b/Documentation/devicetree/bindings/usb/snps,dwc3.yaml
@@ -280,6 +280,13 @@ properties:
       xhci reset. And the vbus will back to 5V automatically when reset done.
     type: boolean
 
+  snps,enable_guctl1_ipd_quirk:
+    description:
+      When set, HW reduce the ULPI clock latency by 1 cycle, thus reducing
+      the IPD (~360ns) and making controller enumerate FS/LS devices
+      connected behind via-hubs.
+    type: boolean
+
   snps,is-utmi-l1-suspend:
     description:
       True when DWC3 asserts output signal utmi_l1_suspend_n, false when
-- 
2.34.1