logging: Use UART directly if available

Some SBIs don't implement the Debug Console extension. Using the UART is one solution, supporting the SBI Legacy extension might work too if it exists.

1 files changed, 58 insertions, 0 deletions

diff --git a/src/uart.zig b/src/uart.zig
new file mode 100644
index 0000000..3a15a6c
--- /dev/null
+++ b/src/uart.zig
@@ -0,0 +1,58 @@
+// SPDX-FileCopyrightText: 2024 Himbeer <himbeer@disroot.org>
+//
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+const std = @import("std");
+
+const fdt = @import("fdt.zig");
+const paging = @import("paging.zig");
+
+pub var default: ?Port = null;
+
+pub const Error = error{
+    NoSocNode,
+};
+
+pub const Port = struct {
+    reg: []volatile u8,
+    reg_shift: u6,
+
+    const lsr_temt = 1 << 5;
+
+    pub const Writer = std.io.Writer(Port, error{}, write);
+
+    fn isWriteReady(self: Port) bool {
+        return self.reg[@as(usize, 5) << self.reg_shift] & lsr_temt != 0;
+    }
+
+    pub fn write(self: Port, bytes: []const u8) !usize {
+        for (bytes) |byte| {
+            while (!self.isWriteReady()) {}
+            self.reg[0] = byte;
+        }
+
+        return bytes.len;
+    }
+
+    pub fn writer(port: Port) Writer {
+        return .{ .context = port };
+    }
+};
+
+pub fn init(kmem: *paging.Table, dt: *const fdt.Tree, allocator: std.mem.Allocator) !void {
+    var soc = fdt.findPathExact(dt, "/soc") orelse return Error.NoSocNode;
+
+    const compatible = fdt.findCompatible(&soc, &[_][]const u8{ "ns16550", "snps,dw-apb-uart" }, 1);
+
+    const node = if (compatible.len > 0) compatible[0] else return;
+
+    const regs = try node.reg(allocator);
+    defer regs.deinit();
+
+    try kmem.mapDevice(regs.items[0]);
+
+    const reg_shift_bytes = node.props.get("reg-shift");
+    const reg_shift = if (reg_shift_bytes) |bytes| std.mem.readInt(u32, bytes[0..4], .big) else 0;
+
+    default = .{ .reg = regs.items[0].slice(u8), .reg_shift = @intCast(reg_shift) };
+}