serial: 8250_dw: proper support for UARTs without busy functionality

If the DW_apb_uart is configured with UART_16550_COMPATIBLE
configuration parameter set, then the Busy Functionality is
not available. These UARTs will never generate the Busy
detect indication interrupt, and therefore don't need
handling for it.

This creates a small optimization for the DW_apb_uarts
configured without the busy functionality, but more
importantly, it removes the small but real risk of hitting
potential issues caused by busy functionality handling when
no busy functionality exist.

Signed-off-by: Heikki Krogerus <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/snps-dw-apb-uart.txt

···
 	Required elements: "baudclk", "apb_pclk"
 
 Optional properties:
+- snps,uart-16550-compatible : reflects the value of UART_16550_COMPATIBLE
+  configuration parameter. Define this if your UART does not implement the busy
+  functionality.
 - resets : phandle to the parent reset controller.
 - reg-shift : quantity to shift the register offsets by.  If this property is
   not present then the register offsets are not shifted.

drivers/tty/serial/8250/8250_dw.c

···
 	struct uart_8250_dma	dma;
 
 	unsigned int		skip_autocfg:1;
+	unsigned int		uart_16550_compatible:1;
 };
 
 #define BYT_PRV_CLK			0x800
···
 		p->iotype = UPIO_MEM32;
 		p->regshift = 2;
 		p->serial_in = dw8250_serial_in32;
-		p->serial_out = dw8250_serial_out32;
 		p->set_termios = dw8250_set_termios;
+		/* So far none of there implement the Busy Functionality */
+		data->uart_16550_compatible = true;
 	}
 
 	/* Platforms with iDMA */
···
 
 	data->usr_reg = DW_UART_USR;
 	p->private_data = data;
+
+	data->uart_16550_compatible = device_property_read_bool(p->dev,
+						"snps,uart-16550-compatible");
 
 	err = device_property_read_u32(p->dev, "reg-shift", &val);
 	if (!err)
···
 	data->dma.fn = dw8250_dma_filter;
 
 	dw8250_quirks(p, data);
+
+	/* If the Busy Functionality is not implemented, don't handle it */
+	if (data->uart_16550_compatible) {
+		p->serial_out = NULL;
+		p->handle_irq = NULL;
+	}
 
 	if (!data->skip_autocfg)
 		dw8250_setup_port(&uart);