tty: serial: qcom_geni_serial: Remove interrupt storm

Disable M_TX_FIFO_WATERMARK_EN after we've sent all data for a given
transaction so we don't continue to receive a flurry of free space
interrupts while waiting for the M_CMD_DONE notification. Re-enable the
watermark when establishing the next transaction.

Also clear the watermark interrupt after filling the FIFO so we do not
receive notification again prior to actually having free space.

Signed-off-by: Ryan Case <[e-mail]>
Reviewed-by: Douglas Anderson <[e-mail]>
Tested-by: Douglas Anderson <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/qcom_geni_serial.c

···
 	size_t pending;
 	int i;
 	u32 status;
+	u32 irq_en;
 	unsigned int chunk;
 	int tail;
 
···
 	if (!port->tx_remaining) {
 		qcom_geni_serial_setup_tx(uport, pending);
 		port->tx_remaining = pending;
+
+		irq_en = readl_relaxed(uport->membase + SE_GENI_M_IRQ_EN);
+		if (!(irq_en & M_TX_FIFO_WATERMARK_EN))
+			writel_relaxed(irq_en | M_TX_FIFO_WATERMARK_EN,
+					uport->membase + SE_GENI_M_IRQ_EN);
 	}
 
 	remaining = chunk;
···
 	}
 
 	xmit->tail = tail & (UART_XMIT_SIZE - 1);
+
+	/*
+	 * The tx fifo watermark is level triggered and latched. Though we had
+	 * cleared it in qcom_geni_serial_isr it will have already reasserted
+	 * so we must clear it again here after our writes.
+	 */
+	writel_relaxed(M_TX_FIFO_WATERMARK_EN,
+			uport->membase + SE_GENI_M_IRQ_CLEAR);
+
 out_write_wakeup:
+	if (!port->tx_remaining) {
+		irq_en = readl_relaxed(uport->membase + SE_GENI_M_IRQ_EN);
+		if (irq_en & M_TX_FIFO_WATERMARK_EN)
+			writel_relaxed(irq_en & ~M_TX_FIFO_WATERMARK_EN,
+					uport->membase + SE_GENI_M_IRQ_EN);
+	}
+
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
 		uart_write_wakeup(uport);
 }
···
 		tty_insert_flip_char(tport, 0, TTY_OVERRUN);
 	}
 
-	if (m_irq_status & (M_TX_FIFO_WATERMARK_EN | M_CMD_DONE_EN) &&
-	    m_irq_en & (M_TX_FIFO_WATERMARK_EN | M_CMD_DONE_EN))
+	if (m_irq_status & m_irq_en & (M_TX_FIFO_WATERMARK_EN | M_CMD_DONE_EN))
 		qcom_geni_serial_handle_tx(uport, m_irq_status & M_CMD_DONE_EN,
 					geni_status & M_GENI_CMD_ACTIVE);