serial: 8250_omap: workaround errata around idling UART after using DMA

AM335x, AM437x and DRA7x SoCs have an errata[1] due to which UART
cannot be idled after it has been used with DMA.

OMAP3 has a similar sounding errata which has been worked around
in a2fc36613ac1af2e9 ("ARM: OMAP3: Use manual idle for UARTs
because of DMA errata"). But the workaround used there does not
apply to AM335x, AM437x SoCs.

After using DMA, the UART module on these SoCs must be soft reset
to go to idle.

This patch implements that errata workaround. It is expected that
UART will be used with DMA so no explicit check for DMA usage
has been added for errata applicability.

MDR1 register needs to be restored right after soft-reset because
"UART mode" must be set in that register for module wake-up on AM335x
to work. As a result, SCR register is now used to determine if
context was lost during sleep.

[1] See Advisory 21 in AM437x errata SPRZ408B, updated April 2015.
    http://www.ti.com/lit/er/sprz408b/sprz408b.pdf

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_omap.c

@@ -33,6 +33,11 @@
 #define UART_ERRATA_i202_MDR1_ACCESS	(1 << 0)
 #define OMAP_UART_WER_HAS_TX_WAKEUP	(1 << 1)
 #define OMAP_DMA_TX_KICK		(1 << 2)
+/*
+ * See Advisory 21 in AM437x errata SPRZ408B, updated April 2015.
+ * The same errata is applicable to AM335x and DRA7x processors too.
+ */
+#define UART_ERRATA_CLOCK_DISABLE	(1 << 3)
 
 #define OMAP_UART_FCR_RX_TRIG		6
 #define OMAP_UART_FCR_TX_TRIG		4
@@ -54,6 +59,12 @@
 #define OMAP_UART_MVR_MAJ_SHIFT		8
 #define OMAP_UART_MVR_MIN_MASK		0x3f
 
+/* SYSC register bitmasks */
+#define OMAP_UART_SYSC_SOFTRESET	(1 << 1)
+
+/* SYSS register bitmasks */
+#define OMAP_UART_SYSS_RESETDONE	(1 << 0)
+
 #define UART_TI752_TLR_TX	0
 #define UART_TI752_TLR_RX	4
 
@@ -1062,13 +1073,15 @@ static int omap8250_no_handle_irq(struct uart_port *port)
 	return 0;
 }
 
-static const u8 am3352_habit = OMAP_DMA_TX_KICK;
+static const u8 am3352_habit = OMAP_DMA_TX_KICK | UART_ERRATA_CLOCK_DISABLE;
+static const u8 am4372_habit = UART_ERRATA_CLOCK_DISABLE;
 
 static const struct of_device_id omap8250_dt_ids[] = {
 	{ .compatible = "ti,omap2-uart" },
 	{ .compatible = "ti,omap3-uart" },
 	{ .compatible = "ti,omap4-uart" },
 	{ .compatible = "ti,am3352-uart", .data = &am3352_habit, },
+	{ .compatible = "ti,am4372-uart", .data = &am4372_habit, },
 	{},
 };
 MODULE_DEVICE_TABLE(of, omap8250_dt_ids);
@@ -1282,17 +1295,46 @@ static int omap8250_lost_context(struct uart_8250_port *up)
 {
 	u32 val;
 
-	val = serial_in(up, UART_OMAP_MDR1);
+	val = serial_in(up, UART_OMAP_SCR);
 	/*
-	 * If we lose context, then MDR1 is set to its reset value which is
-	 * UART_OMAP_MDR1_DISABLE. After set_termios() we set it either to 13x
-	 * or 16x but never to disable again.
+	 * If we lose context, then SCR is set to its reset value of zero.
+	 * After set_termios() we set bit 3 of SCR (TX_EMPTY_CTL_IT) to 1,
+	 * among other bits, to never set the register back to zero again.
 	 */
-	if (val == UART_OMAP_MDR1_DISABLE)
+	if (!val)
 		return 1;
 	return 0;
 }
 
+/* TODO: in future, this should happen via API in drivers/reset/ */
+static int omap8250_soft_reset(struct device *dev)
+{
+	struct omap8250_priv *priv = dev_get_drvdata(dev);
+	struct uart_8250_port *up = serial8250_get_port(priv->line);
+	int timeout = 100;
+	int sysc;
+	int syss;
+
+	sysc = serial_in(up, UART_OMAP_SYSC);
+
+	/* softreset the UART */
+	sysc |= OMAP_UART_SYSC_SOFTRESET;
+	serial_out(up, UART_OMAP_SYSC, sysc);
+
+	/* By experiments, 1us enough for reset complete on AM335x */
+	do {
+		udelay(1);
+		syss = serial_in(up, UART_OMAP_SYSS);
+	} while (--timeout && !(syss & OMAP_UART_SYSS_RESETDONE));
+
+	if (!timeout) {
+		dev_err(dev, "timed out waiting for reset done\n");
+		return -ETIMEDOUT;
+	}
+
+	return 0;
+}
+
 static int omap8250_runtime_suspend(struct device *dev)
 {
 	struct omap8250_priv *priv = dev_get_drvdata(dev);
@@ -1310,6 +1352,17 @@ static int omap8250_runtime_suspend(struct device *dev)
 			return -EBUSY;
 	}
 
+	if (priv->habit & UART_ERRATA_CLOCK_DISABLE) {
+		int ret;
+
+		ret = omap8250_soft_reset(dev);
+		if (ret)
+			return ret;
+
+		/* Restore to UART mode after reset (for wakeup) */
+		omap8250_update_mdr1(up, priv);
+	}
+
 	if (up->dma && up->dma->rxchan)
 		omap_8250_rx_dma(up, UART_IIR_RX_TIMEOUT);