usb: dwc3: workaround: U1/U2 -> U0 transiton

RTL revisions <1.83a have an issue where, depending
on the link partner, the USB link might do multiple
entry/exit of low power states before a transfer
takes place causing degraded throughput.

The suggested workaround is to clear bits
12:9 of DCTL register if we see a transition
from U1|U2 to U0 and only re-enable that on
a transfer complete IRQ and we have no pending
transfers on any of the enabled endpoints.

Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/core.h

@@ -569,6 +569,7 @@ struct dwc3_hwparams {
  * @regs_size: address space size
  * @irq: IRQ number
  * @num_event_buffers: calculated number of event buffers
+ * @u1u2: only used on revisions <1.83a for workaround
  * @maximum_speed: maximum speed requested (mainly for testing purposes)
  * @revision: revision register contents
  * @mode: mode of operation
@@ -614,6 +615,7 @@ struct dwc3 {
 	int			irq;
 
 	u32			num_event_buffers;
+	u32			u1u2;
 	u32			maximum_speed;
 	u32			revision;
 	u32			mode;

drivers/usb/dwc3/gadget.c

@@ -1376,6 +1376,31 @@ static void dwc3_endpoint_transfer_complete(struct dwc3 *dwc,
 		dep->flags &= ~DWC3_EP_BUSY;
 		dep->res_trans_idx = 0;
 	}
+
+	/*
+	 * WORKAROUND: This is the 2nd half of U1/U2 -> U0 workaround.
+	 * See dwc3_gadget_linksts_change_interrupt() for 1st half.
+	 */
+	if (dwc->revision < DWC3_REVISION_183A) {
+		u32		reg;
+		int		i;
+
+		for (i = 0; i < DWC3_ENDPOINTS_NUM; i++) {
+			struct dwc3_ep	*dep = dwc->eps[i];
+
+			if (!(dep->flags & DWC3_EP_ENABLED))
+				continue;
+
+			if (!list_empty(&dep->req_queued))
+				return;
+		}
+
+		reg = dwc3_readl(dwc->regs, DWC3_DCTL);
+		reg |= dwc->u1u2;
+		dwc3_writel(dwc->regs, DWC3_DCTL, reg);
+
+		dwc->u1u2 = 0;
+	}
 }
 
 static void dwc3_gadget_start_isoc(struct dwc3 *dwc,
@@ -1794,8 +1819,55 @@ static void dwc3_gadget_wakeup_interrupt(struct dwc3 *dwc)
 static void dwc3_gadget_linksts_change_interrupt(struct dwc3 *dwc,
 		unsigned int evtinfo)
 {
-	/*  The fith bit says SuperSpeed yes or no. */
-	dwc->link_state = evtinfo & DWC3_LINK_STATE_MASK;
+	enum dwc3_link_state	next = evtinfo & DWC3_LINK_STATE_MASK;
+
+	/*
+	 * WORKAROUND: DWC3 Revisions <1.83a have an issue which, depending
+	 * on the link partner, the USB session might do multiple entry/exit
+	 * of low power states before a transfer takes place.
+	 *
+	 * Due to this problem, we might experience lower throughput. The
+	 * suggested workaround is to disable DCTL[12:9] bits if we're
+	 * transitioning from U1/U2 to U0 and enable those bits again
+	 * after a transfer completes and there are no pending transfers
+	 * on any of the enabled endpoints.
+	 *
+	 * This is the first half of that workaround.
+	 *
+	 * Refers to:
+	 *
+	 * STAR#9000446952: RTL: Device SS : if U1/U2 ->U0 takes >128us
+	 * core send LGO_Ux entering U0
+	 */
+	if (dwc->revision < DWC3_REVISION_183A) {
+		if (next == DWC3_LINK_STATE_U0) {
+			u32	u1u2;
+			u32	reg;
+
+			switch (dwc->link_state) {
+			case DWC3_LINK_STATE_U1:
+			case DWC3_LINK_STATE_U2:
+				reg = dwc3_readl(dwc->regs, DWC3_DCTL);
+				u1u2 = reg & (DWC3_DCTL_INITU2ENA
+						| DWC3_DCTL_ACCEPTU2ENA
+						| DWC3_DCTL_INITU1ENA
+						| DWC3_DCTL_ACCEPTU1ENA);
+
+				if (!dwc->u1u2)
+					dwc->u1u2 = reg & u1u2;
+
+				reg &= ~u1u2;
+
+				dwc3_writel(dwc->regs, DWC3_DCTL, reg);
+				break;
+			default:
+				/* do nothing */
+				break;
+			}
+		}
+	}
+
+	dwc->link_state = next;
 
 	dev_vdbg(dwc->dev, "%s link %d\n", __func__, dwc->link_state);
 }