[PATCH v6 04/15] emul/ns16x50: implement DLL/DLM registers

[dmukhin@xxxxxxx]

From: Denis Mukhin <dmukhin@xxxxxxxx> 

Add DLL/DLM registers emulation.

DLL/DLM registers report hardcoded 115200 baud rate to the guest OS.

Add stub for ns16x50_dlab_get() helper.

Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>
---
Changes since v5:
- dropped ns16x50_dlab_get() hunk (moved to emulator stub)
- Link to v5: 
https://lore.kernel.org/xen-devel/20250828235409.2835815-5-dmukhin@xxxxxxxx/
---
 xen/common/emul/vuart/ns16x50.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/xen/common/emul/vuart/ns16x50.c b/xen/common/emul/vuart/ns16x50.c
index 0462a961e785..7f479a5be4a2 100644
--- a/xen/common/emul/vuart/ns16x50.c
+++ b/xen/common/emul/vuart/ns16x50.c
@@ -97,8 +97,13 @@ static uint8_t ns16x50_dlab_get(const struct vuart_ns16x50 
*vdev)
 static int ns16x50_io_write8(
     struct vuart_ns16x50 *vdev, uint32_t reg, uint8_t *data)
 {
+    uint8_t *regs = vdev->regs;
+    uint8_t val = *data;
     int rc = 0;
 
+    if ( ns16x50_dlab_get(vdev) && (reg == UART_DLL || reg == UART_DLM) )
+        regs[NS16X50_REGS_NUM + reg] = val;
+
     return rc;
 }
 
@@ -109,8 +114,16 @@ static int ns16x50_io_write8(
 static int ns16x50_io_write16(
     struct vuart_ns16x50 *vdev, uint32_t reg, uint16_t *data)
 {
+    uint16_t val = *data;
     int rc = -EINVAL;
 
+    if ( ns16x50_dlab_get(vdev) && reg == UART_DLL )
+    {
+        vdev->regs[NS16X50_REGS_NUM + UART_DLL] = val & 0xff;
+        vdev->regs[NS16X50_REGS_NUM + UART_DLM] = (val >> 8) & 0xff;
+        rc = 0;
+    }
+
     return rc;
 }
 
@@ -146,9 +159,13 @@ static int ns16x50_io_write(
 static int ns16x50_io_read8(
     struct vuart_ns16x50 *vdev, uint32_t reg, uint8_t *data)
 {
+    uint8_t *regs = vdev->regs;
     uint8_t val = 0xff;
     int rc = 0;
 
+    if ( ns16x50_dlab_get(vdev) && (reg == UART_DLL || reg == UART_DLM) )
+        val = regs[NS16X50_REGS_NUM + reg];
+
     *data = val;
 
     return rc;
@@ -163,6 +180,13 @@ static int ns16x50_io_read16(
     uint16_t val = 0xffff;
     int rc = -EINVAL;
 
+    if ( ns16x50_dlab_get(vdev) && reg == UART_DLL )
+    {
+        val = vdev->regs[NS16X50_REGS_NUM + UART_DLM] << 8 |
+              vdev->regs[NS16X50_REGS_NUM + UART_DLL];
+        rc = 0;
+    }
+
     *data = val;
 
     return rc;
@@ -280,12 +304,17 @@ out:
 
 static int ns16x50_init(void *arg)
 {
+    const uint16_t divisor = (UART_CLOCK_HZ / 115200) >> 4;
     struct vuart_ns16x50 *vdev = arg;
     const struct vuart_info *info = vdev->info;
     struct domain *d = vdev->owner;
 
     ASSERT(vdev);
 
+    /* NB: report 115200 baud rate. */
+    vdev->regs[NS16X50_REGS_NUM + UART_DLL] = divisor & 0xff;
+    vdev->regs[NS16X50_REGS_NUM + UART_DLM] = (divisor >> 8) & 0xff;
+
     register_portio_handler(d, info->base_addr, info->size, ns16x50_io_handle);
 
     return 0;
-- 
2.51.0