[PATCH v6 07/15] emul/ns16x50: implement LCR/LSR registers

[dmukhin@xxxxxxx]

From: Denis Mukhin <dmukhin@xxxxxxxx> 

Add LCR/LSR registers implementation to the I/O port handler.

Add implementation of ns16x50_dlab_get() and ns16x50_iir_check_lsi().

Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>
---
Changes since v5:
- Moved earlier in the series so follow on patches (THR/RBR) could make use of
  LSR bits more naturally
- Link to v5: 
https://lore.kernel.org/xen-devel/20250828235409.2835815-9-dmukhin@xxxxxxxx/
---
 xen/common/emul/vuart/ns16x50.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/xen/common/emul/vuart/ns16x50.c b/xen/common/emul/vuart/ns16x50.c
index a7429c84c36e..9d1fe0284362 100644
--- a/xen/common/emul/vuart/ns16x50.c
+++ b/xen/common/emul/vuart/ns16x50.c
@@ -88,12 +88,12 @@ struct vuart_ns16x50 {
 
 static uint8_t ns16x50_dlab_get(const struct vuart_ns16x50 *vdev)
 {
-    return 0;
+    return vdev->regs[UART_LCR] & UART_LCR_DLAB ? 1 : 0;
 }
 
 static bool cf_check ns16x50_iir_check_lsi(const struct vuart_ns16x50 *vdev)
 {
-    return false;
+    return vdev->regs[UART_LSR] & UART_LSR_MASK;
 }
 
 static bool cf_check ns16x50_iir_check_rda(const struct vuart_ns16x50 *vdev)
@@ -226,11 +226,16 @@ static int ns16x50_io_write8(
             regs[UART_IER] = val & UART_IER_MASK;
             break;
 
+        case UART_LCR:
+            regs[UART_LCR] = val;
+            break;
+
         /* NB: Firmware (e.g. OVMF) may rely on SCR presence. */
         case UART_SCR:
             regs[UART_SCR] = val;
             break;
 
+        case UART_LSR: /* RO */
         default:
             rc = -EINVAL;
             break;
@@ -312,6 +317,15 @@ static int ns16x50_io_read8(
             val = ns16x50_iir_get(vdev);
             break;
 
+        case UART_LCR:
+            val = regs[UART_LCR];
+            break;
+
+        case UART_LSR:
+            val = regs[UART_LSR] | UART_LSR_THRE | UART_LSR_TEMT;
+            regs[UART_LSR] = val & ~UART_LSR_MASK;
+            break;
+
         case UART_SCR:
             val = regs[UART_SCR];
             break;
-- 
2.51.0