[PATCH v3] ns16550: add support for polling mode when device tree is used

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

RISC-V doesn't support interrupts for the time being, so it would be nice to
have polling mode.

The patch adds new option to console='...,polling' which forces driver to use
polling mode.

If there is no interrupt property in UART node, then polling will be used.    
According to 4.2.2 National Semiconductor 16450/16550 Compatible UART
Requirements from The Devicetree Specification v0.4
( https://www.devicetree.org/specifications/ ) the interrupt property is        
optional.

Also, it is possible that interrupt '0' can be used for some architectures as
an legal UART interrupt number ( according to dts files in Linux kernel ), so
the check of the return value of platform_get_irq() was updated in case of when
device tree is used for UART initialization.
For example:
https://github.com/torvalds/linux/blob/master/arch/powerpc/boot/dts/ebony.dts#L197

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
 xen/drivers/char/console.c |  2 ++
 xen/drivers/char/ns16550.c | 38 ++++++++++++++++++++++++++++----------
 2 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
index 0e410fa086..07e9610d77 100644
--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -993,6 +993,8 @@ void __init console_init_preirq(void)
 #endif
         else if ( !strncmp(p, "none", 4) )
             continue;
+        else if ( !strncmp(p, "polling", 7 )
+            continue;
         else if ( (sh = serial_parse_handle(p)) >= 0 )
         {
             sercon_handle = sh;
diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index 2aed6ec707..a6eb3b3997 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -58,7 +58,11 @@ static struct ns16550 {
     struct timer timer;
     struct timer resume_timer;
     unsigned int timeout_ms;
-    bool_t intr_works;
+    enum {
+        intr_off,
+        intr_on,
+        polling,
+    } intr_works;
     bool_t dw_usr_bsy;
 #ifdef NS16550_PCI
     /* PCI card parameters. */
@@ -181,7 +185,7 @@ static void cf_check ns16550_interrupt(
     struct serial_port *port = dev_id;
     struct ns16550 *uart = port->uart;
 
-    uart->intr_works = 1;
+    uart->intr_works = intr_on;
 
     while ( !(ns_read_reg(uart, UART_IIR) & UART_IIR_NOINT) )
     {
@@ -212,7 +216,7 @@ static void cf_check __ns16550_poll(struct cpu_user_regs 
*regs)
     struct serial_port *port = this_cpu(poll_port);
     struct ns16550 *uart = port->uart;
 
-    if ( uart->intr_works )
+    if ( uart->intr_works == intr_on )
         return; /* Interrupts work - no more polling */
 
     while ( ns_read_reg(uart, UART_LSR) & UART_LSR_DR )
@@ -305,7 +309,8 @@ static void ns16550_setup_preirq(struct ns16550 *uart)
     unsigned char lcr;
     unsigned int  divisor;
 
-    uart->intr_works = 0;
+    if ( uart->intr_works != polling )
+        uart->intr_works = intr_off;
 
     pci_serial_early_init(uart);
 
@@ -394,7 +399,7 @@ static void __init cf_check ns16550_init_irq(struct 
serial_port *port)
 
 static void ns16550_setup_postirq(struct ns16550 *uart)
 {
-    if ( uart->irq > 0 )
+    if ( uart->intr_works != polling )
     {
         /* Master interrupt enable; also keep DTR/RTS asserted. */
         ns_write_reg(uart,
@@ -473,6 +478,7 @@ static void __init cf_check ns16550_init_postirq(struct 
serial_port *port)
                 if ( rc )
                 {
                     uart->irq = 0;
+                    uart->intr_works = polling;
                     if ( msi_desc )
                         msi_free_irq(msi_desc);
                     else
@@ -488,7 +494,7 @@ static void __init cf_check ns16550_init_postirq(struct 
serial_port *port)
     }
 #endif
 
-    if ( uart->irq > 0 )
+    if ( uart->intr_works != polling )
     {
         uart->irqaction.handler = ns16550_interrupt;
         uart->irqaction.name    = "ns16550";
@@ -595,7 +601,9 @@ static void __init cf_check ns16550_endboot(struct 
serial_port *port)
 static int __init cf_check ns16550_irq(struct serial_port *port)
 {
     struct ns16550 *uart = port->uart;
-    return ((uart->irq > 0) ? uart->irq : -1);
+
+    return ( uart->intr_works != polling ) ?
+            uart->irq : -1;
 }
 
 static void cf_check ns16550_start_tx(struct serial_port *port)
@@ -654,6 +662,9 @@ static void ns16550_init_common(struct ns16550 *uart)
 
     /* Default lsr_mask = UART_LSR_THRE */
     uart->lsr_mask  = UART_LSR_THRE;
+
+    if ( console_has("polling") )
+        uart->intr_works = polling;
 }
 
 #ifdef CONFIG_X86
@@ -1330,9 +1341,12 @@ pci_uart_config(struct ns16550 *uart, bool_t skip_amt, 
unsigned int idx)
                  * as special only for X86.
                  */
                 if ( uart->irq == 0xff )
+                {
                     uart->irq = 0;
+                    uart->intr_works = polling;
+                }
 #endif
-                if ( !uart->irq )
+                if ( !uart->irq || (uart->intr_works == polling) )
                     printk(XENLOG_INFO
                            "ns16550: %pp: no legacy IRQ, using poll mode\n",
                            &PCI_SBDF(0, b, d, f));
@@ -1552,6 +1566,7 @@ static bool __init parse_positional(struct ns16550 *uart, 
char **str)
             conf += 3;
             uart->msi = true;
             uart->irq = 0;
+            uart->intr_works = polling;
         }
         else
 #endif
@@ -1791,8 +1806,11 @@ static int __init ns16550_uart_dt_init(struct 
dt_device_node *dev,
     }
 
     res = platform_get_irq(dev, 0);
-    if ( ! res )
-        return -EINVAL;
+    if ( res < 0 )
+    {
+        printk("there is no interrupt property, polling will be used\n");
+        uart->intr_works = polling;
+    }
     uart->irq = res;
 
     uart->dw_usr_bsy = dt_device_is_compatible(dev, "snps,dw-apb-uart");
-- 
2.41.0