usb: dwc3: Allow disabling of metastability workaround

Some platforms (e.g. TI's DRA7 USB2 instance) have more trouble
with the metastability workaround as it supports only
a High-Speed PHY and the PHY can enter into an Erratic state [1]
when the controller is set in SuperSpeed mode as part of
the metastability workaround.

This causes upto 2 seconds delay in enumeration on DRA7's USB2
instance in gadget mode.

If these platforms can be better off without the workaround,
provide a device tree property to suggest that so the workaround
is avoided.

[1] Device mode enumeration trace showing PHY Erratic Error.
     irq/90-dwc3-969   [000] d...    52.323145: dwc3_event: event (00000901): Erratic Error [U0]
     irq/90-dwc3-969   [000] d...    52.560646: dwc3_event: event (00000901): Erratic Error [U0]
     irq/90-dwc3-969   [000] d...    52.798144: dwc3_event: event (00000901): Erratic Error [U0]

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Documentation/devicetree/bindings/usb/dwc3.txt

@@ -47,6 +47,8 @@ Optional properties:
 			from P0 to P1/P2/P3 without delay.
  - snps,dis-tx-ipgap-linecheck-quirk: when set, disable u2mac linestate check
 			during HS transmit.
+ - snps,dis_metastability_quirk: when set, disable metastability workaround.
+			CAUTION: use only if you are absolutely sure of it.
  - snps,is-utmi-l1-suspend: true when DWC3 asserts output signal
 			utmi_l1_suspend_n, false when asserts utmi_sleep_n
  - snps,hird-threshold: HIRD threshold

drivers/usb/dwc3/core.c

@@ -1062,6 +1062,9 @@ static void dwc3_get_properties(struct dwc3 *dwc)
 	device_property_read_u32(dev, "snps,quirk-frame-length-adjustment",
 				 &dwc->fladj);
 
+	dwc->dis_metastability_quirk = device_property_read_bool(dev,
+				"snps,dis_metastability_quirk");
+
 	dwc->lpm_nyet_threshold = lpm_nyet_threshold;
 	dwc->tx_de_emphasis = tx_de_emphasis;
 

drivers/usb/dwc3/core.h

@@ -856,6 +856,7 @@ struct dwc3_scratchpad_array {
  * 	1	- -3.5dB de-emphasis
  * 	2	- No de-emphasis
  * 	3	- Reserved
+ * @dis_metastability_quirk: set to disable metastability quirk.
  * @imod_interval: set the interrupt moderation interval in 250ns
  *                 increments or 0 to disable.
  */
@@ -1008,6 +1009,8 @@ struct dwc3 {
 	unsigned		tx_de_emphasis_quirk:1;
 	unsigned		tx_de_emphasis:2;
 
+	unsigned		dis_metastability_quirk:1;
+
 	u16			imod_interval;
 };
 

drivers/usb/dwc3/gadget.c

@@ -2005,7 +2005,8 @@ static void dwc3_gadget_set_speed(struct usb_gadget *g,
 	 * STAR#9000525659: Clock Domain Crossing on DCTL in
 	 * USB 2.0 Mode
 	 */
-	if (dwc->revision < DWC3_REVISION_220A) {
+	if (dwc->revision < DWC3_REVISION_220A &&
+	    !dwc->dis_metastability_quirk) {
 		reg |= DWC3_DCFG_SUPERSPEED;
 	} else {
 		switch (speed) {
@@ -3224,7 +3225,8 @@ int dwc3_gadget_init(struct dwc3 *dwc)
 	 * is less than super speed because we don't have means, yet, to tell
 	 * composite.c that we are USB 2.0 + LPM ECN.
 	 */
-	if (dwc->revision < DWC3_REVISION_220A)
+	if (dwc->revision < DWC3_REVISION_220A &&
+	    !dwc->dis_metastability_quirk)
 		dev_info(dwc->dev, "changing max_speed on rev %08x\n",
 				dwc->revision);