From 2cb5cf5d66d5df8f310a335032d836ad5f0dc977 Mon Sep 17 00:00:00 2001
From: Andrew Waterman <andrew@sifive.com>
Date: Wed, 11 Jan 2023 15:38:00 -0800
Subject: [PATCH] For NS16550 UART, poll stdin less often

On my Mac Mini, calling `poll()` on stdin takes around 10 us, and we
are invoking it every 20 us or so.  Reduce the frequency of polling
by 16x when not actively receiving data, thereby reducing the fraction
of time spent in `poll()` to a trivial amount.
---
 riscv/devices.h  |  3 +++
 riscv/ns16550.cc | 10 +++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/riscv/devices.h b/riscv/devices.h
index 0030da8a..040fe3ee 100644
--- a/riscv/devices.h
+++ b/riscv/devices.h
@@ -148,6 +148,9 @@ class ns16550_t : public abstract_device_t {
   void update_interrupt(void);
   uint8_t rx_byte(void);
   void tx_byte(uint8_t val);
+
+  int backoff_counter;
+  static const int MAX_BACKOFF = 16;
 };
 
 class mmio_plugin_device_t : public abstract_device_t {
diff --git a/riscv/ns16550.cc b/riscv/ns16550.cc
index 7bc1d208..8d7e4de2 100644
--- a/riscv/ns16550.cc
+++ b/riscv/ns16550.cc
@@ -71,7 +71,7 @@
 
 ns16550_t::ns16550_t(class bus_t *bus, abstract_interrupt_controller_t *intctrl,
                      uint32_t interrupt_id, uint32_t reg_shift, uint32_t reg_io_width)
-  : bus(bus), intctrl(intctrl), interrupt_id(interrupt_id), reg_shift(reg_shift), reg_io_width(reg_io_width)
+  : bus(bus), intctrl(intctrl), interrupt_id(interrupt_id), reg_shift(reg_shift), reg_io_width(reg_io_width), backoff_counter(0)
 {
   ier = 0;
   iir = UART_IIR_NO_INT;
@@ -300,11 +300,19 @@ void ns16550_t::tick(void)
     return;
   }
 
+  if (backoff_counter > 0 && backoff_counter < MAX_BACKOFF) {
+    backoff_counter++;
+    return;
+  }
+
   int rc = canonical_terminal_t::read();
   if (rc < 0) {
+    backoff_counter = 1;
     return;
   }
 
+  backoff_counter = 0;
+
   rx_queue.push((uint8_t)rc);
   lsr |= UART_LSR_DR;
   update_interrupt();