Adds the ns16550a driver.

Signed-off-by: Amy Ringo <me@remexre.com>

Author: remexre

src/kernel/drivers/include.mak

@@ -0,0 +1,5 @@
+# SPDX-FileCopyrightText: 2025 ukoOS Contributors
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+kernel-objs-c += drivers/ns16550a

src/kernel/drivers/ns16550a.c

@@ -0,0 +1,209 @@
+/*
+ * SPDX-FileCopyrightText: 2025 ukoOS Contributors
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#include <devices/uart.h>
+#include <devicetree.h>
+#include <mm/paging.h>
+#include <physical.h>
+
+/**
+ * The bits of the Interrupt Enable Register.
+ */
+struct ns16550a_ier {
+  /**
+   * Enable Receiver Buffer Full Interrupt.
+   */
+  bool erbfi : 1;
+
+  /**
+   * Enable Transmitter Buffer Empty Interrupt.
+   */
+  bool etbei : 1;
+
+  /**
+   * Enable (Receiver) Line Status Interrupt.
+   */
+  bool elsi : 1;
+
+  /**
+   * Enable Modem Status Interrupt.
+   */
+  bool edssi : 1;
+
+  /**
+   * Reserved, should be zero.
+   *
+   * Wikipedia suggests that the 16750 uses some of these bits.
+   */
+  u8 rsvd : 4;
+};
+
+/**
+ * The bits of the Interrupt Identification Register.
+ */
+struct ns16550a_iir {
+  /**
+   * A bit that becomes unset when an interrupt is pending.
+   */
+  bool interrupt_pending : 1;
+
+  /**
+   * The index of the bit in `ns16550a_ier` corresponding to the pending
+   * interrupt.
+   */
+  u8 interrupt_id : 3;
+
+  /**
+   * Reserved, should be zero.
+   *
+   * Wikipedia suggests that the 16750 uses some of these bits.
+   */
+  u8 rsvd : 2;
+
+  /**
+   * A flag for whether the FIFOs are currently functioning.
+   */
+  bool fifo_working : 1;
+
+  /**
+   * A flag for whether the FIFOs are enabled.
+   */
+  bool fifo_enabled : 1;
+};
+
+/**
+ * The UART's registers.
+ */
+struct ns16550a_regs {
+  union {
+    /**
+     * The Receiver Buffer Register. Read-only. Only valid when DLAB is unset.
+     */
+    u8 rbr;
+
+    /**
+     * The Transmitter Holding Register. Write-only. Only valid when DLAB is
+     * unset.
+     */
+    u8 thr;
+
+    /**
+     * The Divisor Latch Low byte. Only valid when DLAB is set.
+     */
+    u8 dll;
+  };
+
+  /**
+   * When DLAB is unset, this is the Interrupt Enable Register. When DLAB is
+   * set, this is.
+   */
+  union {
+    /**
+     * The Interrupt Enable Register. Only valid when DLAB is unset.
+     */
+    struct ns16550a_ier ier;
+
+    /**
+     * The Divisor Latch High byte. Only valid when DLAB is set.
+     */
+    u8 dlh;
+  };
+
+  union {
+    /**
+     * The Interrupt Identification Register. Read-only.
+     */
+    struct ns16550a_iir iir;
+
+    /**
+     * The FIFO Control Register. Write-only.
+     */
+    u8 fcr;
+  };
+
+  /**
+   * The Line Control Register.
+   */
+  u8 lcr;
+
+  /**
+   * The Modem Control Register.
+   */
+  u8 mcr;
+
+  /**
+   * The Line Status Register.
+   */
+  u8 lsr;
+
+  /**
+   * The Modem Status Register.
+   */
+  u8 msr;
+
+  /**
+   * The Scratch Register.
+   */
+  u8 sr;
+};
+
+static_assert(sizeof(struct ns16550a_regs) == 8);
+
+struct ns16550a {
+  struct device device;
+  struct uart uart;
+  struct ns16550a_regs *regs;
+};
+
+static void ns16550a_write_byte(struct uart *uart, u8 byte) { TODO(); }
+
+static const struct uart_ops ns16550a_uart_ops = {
+    .write_byte = ns16550a_write_byte,
+};
+
+static struct device *ns16550a_enumerate_dt(struct devicetree_node *node) {
+  struct ns16550a *device = nullptr;
+  paddr reg_addr;
+  usize reg_size;
+
+  if (!devicetree_reg(node, 0, &reg_addr, &reg_size))
+    goto fail;
+  if (reg_size < sizeof(struct ns16550a_regs))
+    goto fail;
+
+  device = alloc(sizeof(struct ns16550a));
+  if (!device)
+    goto fail;
+
+  *device = (struct ns16550a){
+      .device = DEVICE_INIT(device->device, "ns16550a@{paddr}", reg_addr),
+      .uart =
+          {
+              .list = LIST_INIT(device->uart.list),
+              .ops = &ns16550a_uart_ops,
+              .device = &device->device,
+          },
+      .regs = iomem_map(reg_addr, reg_size),
+  };
+  if (!device->device.name || !device->regs)
+    goto fail;
+
+  list_push(&uarts, &device->uart.list);
+
+  return &device->device;
+
+fail:
+  if (device) {
+    free(device->device.name);
+    iomem_unmap(device->regs, reg_size);
+  }
+  free(device);
+  return nullptr;
+}
+
+DEFINE_INIT(INIT_REGISTER_DRIVERS) {
+  devicetree_register("ns16550a", ns16550a_enumerate_dt);
+}

src/kernel/include.mak

@@ -49,4 +49,8 @@ kernel-objs-c += random
 kernel-objs-c += selftest
 kernel-objs-c += swar_test
 kernel-objs-c += symbolicate
+include $(srcdir)/src/kernel/drivers/include.mak
+
+# The architecture-specific file needs to be last, since it calls
+# compute_component_variables for the kernel.
 include $(srcdir)/src/kernel/arch/$(arch)/include.mak