[PATCH v6 05/15] emul/ns16x50: implement SCR register

[dmukhin@xxxxxxx]

From: Denis Mukhin <dmukhin@xxxxxxxx> 

Add SCR register emulation to the I/O port handler.
Firmware (e.g. OVMF) may use SCR during the guest OS boot.

Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>
---
Changes since v5:
- moved earlier in the series to simplify I/O handler population in
  the follow on patches
- Link to v5: 
https://lore.kernel.org/xen-devel/20250828235409.2835815-11-dmukhin@xxxxxxxx/
---
 xen/common/emul/vuart/ns16x50.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/xen/common/emul/vuart/ns16x50.c b/xen/common/emul/vuart/ns16x50.c
index 7f479a5be4a2..51ec85e57627 100644
--- a/xen/common/emul/vuart/ns16x50.c
+++ b/xen/common/emul/vuart/ns16x50.c
@@ -103,6 +103,20 @@ static int ns16x50_io_write8(
 
     if ( ns16x50_dlab_get(vdev) && (reg == UART_DLL || reg == UART_DLM) )
         regs[NS16X50_REGS_NUM + reg] = val;
+    else
+    {
+        switch ( reg )
+        {
+        /* NB: Firmware (e.g. OVMF) may rely on SCR presence. */
+        case UART_SCR:
+            regs[UART_SCR] = val;
+            break;
+
+        default:
+            rc = -EINVAL;
+            break;
+        }
+    }
 
     return rc;
 }
@@ -165,6 +179,19 @@ static int ns16x50_io_read8(
 
     if ( ns16x50_dlab_get(vdev) && (reg == UART_DLL || reg == UART_DLM) )
         val = regs[NS16X50_REGS_NUM + reg];
+    else
+    {
+        switch ( reg )
+        {
+        case UART_SCR:
+            val = regs[UART_SCR];
+            break;
+
+        default:
+            rc = -EINVAL;
+            break;
+        }
+    }
 
     *data = val;
 
-- 
2.51.0