[PATCH v4 1/7] hw/char: Add dw8250 UART

Add the dw8250 uart support. This hardware is a widely used 16550A
derivative that includes additional registers.

Without this specific device support, the Linux 8250_dw driver fails to
probe the extended registers (UCV, CPR, etc.), which are essential for
correct feature detection.

Signed-off-by: Kuan-Wei Chiu <[email protected]>
Reviewed-by: Chao Liu <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
---
 hw/char/Kconfig          |   4 ++
 hw/char/dw8250.c         | 131 +++++++++++++++++++++++++++++++++++++++
 hw/char/meson.build      |   1 +
 include/hw/char/dw8250.h |  27 ++++++++
 4 files changed, 163 insertions(+)
 create mode 100644 hw/char/dw8250.c
 create mode 100644 include/hw/char/dw8250.h

diff --git a/hw/char/Kconfig b/hw/char/Kconfig
index 020c0a84bb..418d99b757 100644
--- a/hw/char/Kconfig
+++ b/hw/char/Kconfig
@@ -95,3 +95,7 @@ config IP_OCTAL_232
     bool
     default y
     depends on IPACK
+
+config DW8250
+    bool
+    select SERIAL
diff --git a/hw/char/dw8250.c b/hw/char/dw8250.c
new file mode 100644
index 0000000000..ccf05dd9ad
--- /dev/null
+++ b/hw/char/dw8250.c
@@ -0,0 +1,131 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Synopsys DesignWare APB UART (DW 8250)
+ *
+ * Copyright (c) 2026 Kuan-Wei Chiu <[email protected]>
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/log.h"
+#include "qapi/error.h"
+#include "hw/char/dw8250.h"
+#include "hw/core/qdev-properties.h"
+#include "hw/core/qdev-properties-system.h"
+
+#define DW_UART_REGION_SIZE 0x100
+
+#define DW_UART_RE_EN (0xB4 >> 2) /* Receiver Output Enable Register */
+#define DW_UART_DLF   (0xC0 >> 2) /* Divisor Latch Fraction Register */
+#define DW_UART_CPR   (0xF4 >> 2) /* Component Parameter Register */
+#define DW_UART_UCV   (0xF8 >> 2) /* UART Component Version */
+#define DW_UART_CTR   (0xFC >> 2) /* Component Type Register */
+
+#define DW_UART_UCV_VALUE 0x3332332A /* "323*" -> v3.23a */
+#define DW_UART_CTR_VALUE 0x44570110 /* "DW" */
+
+static uint64_t dw8250_ext_read(void *opaque, hwaddr addr, unsigned int size)
+{
+    DW8250State *s = opaque;
+
+    switch (addr >> s->regshift) {
+    case DW_UART_UCV:
+        return DW_UART_UCV_VALUE;
+    case DW_UART_CPR:
+        return 0x00000000; /* No advanced features (DMA, extra FIFOs) */
+    case DW_UART_CTR:
+        return DW_UART_CTR_VALUE;
+
+    case DW_UART_RE_EN:
+    case DW_UART_DLF:
+        /*
+         * Return 0 to indicate these optional features
+         * (RS485 and Fractional Divisor) are not implemented.
+         */
+        return 0x00000000;
+
+    default:
+        return 0;
+    }
+}
+
+static void dw8250_ext_write(void *opaque, hwaddr addr, uint64_t val, unsigned int size)
+{
+    qemu_log_mask(LOG_UNIMP, "%s: Unimplemented write to address 0x%" HWADDR_PRIx "\n",
+                  __func__, addr);
+}
+
+static const MemoryRegionOps dw8250_ext_ops = {
+    .read = dw8250_ext_read,
+    .write = dw8250_ext_write,
+    .endianness = DEVICE_LITTLE_ENDIAN,
+    .impl = {
+        .min_access_size = 4,
+        .max_access_size = 4,
+    },
+    .valid = {
+        .min_access_size = 1,
+        .max_access_size = 4,
+    },
+};
+
+static void dw8250_instance_init(Object *obj)
+{
+    DW8250State *s = DW8250(obj);
+
+    s->serial_mm = qdev_new("serial-mm");
+    object_property_add_child(obj, "serial-mm", OBJECT(s->serial_mm));
+    object_property_add_alias(obj, "chardev", OBJECT(s->serial_mm), "chardev");
+}
+
+static void dw8250_realize(DeviceState *dev, Error **errp)
+{
+    DW8250State *s = DW8250(dev);
+    SysBusDevice *sbd = SYS_BUS_DEVICE(dev);
+    SysBusDevice *serial_sbd = SYS_BUS_DEVICE(s->serial_mm);
+
+    memory_region_init(&s->container, OBJECT(dev), "dw8250-container",
+                       DW_UART_REGION_SIZE);
+    sysbus_init_mmio(sbd, &s->container);
+
+    qdev_prop_set_uint8(s->serial_mm, "regshift", s->regshift);
+    qdev_prop_set_uint8(s->serial_mm, "endianness", DEVICE_LITTLE_ENDIAN);
+    if (!sysbus_realize(serial_sbd, errp)) {
+        return;
+    }
+
+    memory_region_init_io(&s->ext_iomem, OBJECT(dev), &dw8250_ext_ops, s,
+                          "dw8250-ext", DW_UART_REGION_SIZE);
+    memory_region_add_subregion(&s->container, 0, &s->ext_iomem);
+
+    memory_region_add_subregion_overlap(&s->container, 0,
+                                        sysbus_mmio_get_region(serial_sbd, 0), 1);
+
+    sysbus_pass_irq(sbd, serial_sbd);
+}
+
+static const Property dw8250_properties[] = {
+    DEFINE_PROP_UINT8("regshift", DW8250State, regshift, 2),
+};
+
+static void dw8250_class_init(ObjectClass *klass, const void *data)
+{
+    DeviceClass *dc = DEVICE_CLASS(klass);
+
+    dc->realize = dw8250_realize;
+    device_class_set_props(dc, dw8250_properties);
+}
+
+static const TypeInfo dw8250_info = {
+    .name          = TYPE_DW8250,
+    .parent        = TYPE_SYS_BUS_DEVICE,
+    .instance_size = sizeof(DW8250State),
+    .instance_init = dw8250_instance_init,
+    .class_init    = dw8250_class_init,
+};
+
+static void dw8250_register_types(void)
+{
+    type_register_static(&dw8250_info);
+}
+
+type_init(dw8250_register_types)
diff --git a/hw/char/meson.build b/hw/char/meson.build
index fc3d7ee506..b2250ee6ae 100644
--- a/hw/char/meson.build
+++ b/hw/char/meson.build
@@ -38,6 +38,7 @@ system_ss.add(when: 'CONFIG_STM32L4X5_USART', if_true: files('stm32l4x5_usart.c'
 system_ss.add(when: 'CONFIG_MCHP_PFSOC_MMUART', if_true: files('mchp_pfsoc_mmuart.c'))
 system_ss.add(when: 'CONFIG_HTIF', if_true: files('riscv_htif.c'))
 system_ss.add(when: 'CONFIG_GOLDFISH_TTY', if_true: files('goldfish_tty.c'))
+system_ss.add(when: 'CONFIG_DW8250', if_true: files('dw8250.c'))
 
 specific_ss.add(when: 'CONFIG_TERMINAL3270', if_true: files('terminal3270.c'))
 specific_ss.add(when: 'CONFIG_PSERIES', if_true: files('spapr_vty.c'))
diff --git a/include/hw/char/dw8250.h b/include/hw/char/dw8250.h
new file mode 100644
index 0000000000..59396ad202
--- /dev/null
+++ b/include/hw/char/dw8250.h
@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Synopsys DesignWare APB UART (DW 8250)
+ *
+ * Copyright (c) 2026 Kuan-Wei Chiu <[email protected]>
+ */
+
+#ifndef HW_CHAR_DW8250_H
+#define HW_CHAR_DW8250_H
+
+#include "hw/core/sysbus.h"
+#include "qom/object.h"
+
+#define TYPE_DW8250 "dw8250"
+OBJECT_DECLARE_SIMPLE_TYPE(DW8250State, DW8250)
+
+struct DW8250State {
+    SysBusDevice parent_obj;
+
+    MemoryRegion container;
+    MemoryRegion ext_iomem;
+    DeviceState *serial_mm;
+
+    uint8_t regshift;
+};
+
+#endif
-- 
2.55.0.766.g2966f0265a-goog