[PATCH v6 1/2] ns16550: use poll mode if INTERRUPT_LINE is 0xff

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

Intel LPSS has INTERRUPT_LINE set to 0xff by default, that is declared
by the PCI Local Bus Specification Revision 3.0 (from 2004) as
"unknown"/"no connection". Fallback to poll mode in this case.
The 0xff handling is x86-specific, the surrounding code is guarded with
CONFIG_X86 anyway.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes in v6:
 - wrap the check in additional CONFIG_X86, with appropriate comment
Changes in v5:
 - drop IRQ 0 from the log message
Changes in v4:
 - adjust log message, change it from WARNING to INFO
 - re-add x86 reference in the commit message
Changes in v3:
 - change back to checking 0xff explicitly
 - adjust commit message, include spec reference
 - change warning to match the above
Changes in v2:
 - add log message
 - extend commit message
 - code style fix
---
 xen/drivers/char/ns16550.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index fb75cee4a13a..b37f67dc7430 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -1238,6 +1238,17 @@ pci_uart_config(struct ns16550 *uart, bool_t skip_amt, 
unsigned int idx)
                             pci_conf_read8(PCI_SBDF(0, b, d, f),
                                            PCI_INTERRUPT_LINE) : 0;
 
+#ifdef CONFIG_X86
+                /* PCI Local Bus Specification Revision 3.0 defines 0xff value
+                 * as special only for X86 */
+                if ( uart->irq == 0xff )
+                    uart->irq = 0;
+#endif
+                if ( !uart->irq )
+                    printk(XENLOG_INFO
+                           "ns16550: %pp no legacy IRQ, using poll mode\n",
+                           &PCI_SBDF(0, b, d, f));
+
                 return 0;
             }
         }
-- 
2.35.1