From 3912ede2cf5b9eb5884ec09641986e037a9263d1 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Tue, 14 Apr 2026 20:42:25 -1000
Subject: [PATCH] safer

---
 esphome/components/esp8266/core.cpp | 36 +++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 10 deletions(-)

diff --git a/esphome/components/esp8266/core.cpp b/esphome/components/esp8266/core.cpp
index 7551ecc3a7..1cca79f104 100644
--- a/esphome/components/esp8266/core.cpp
+++ b/esphome/components/esp8266/core.cpp
@@ -34,9 +34,10 @@ void HOT yield() { ::yield(); }
 // that Arduino library code and ISR handlers (e.g. Wiegand, ZyAura) calling
 // ::millis() directly also get the fast version. Interrupts are briefly disabled
 // to protect the static state from concurrent ISR access. The critical section
-// is bounded: the common path (delta < 10 ms) runs at most 10 subtract-and-
-// compare iterations (~100 ns). Large gaps (WiFi scan, boot) fall back to a
-// constant-time multiply-by-reciprocal (~2.5 μs, rare).
+// is bounded on *both* paths: the common path (delta < 10 ms) runs at most 10
+// subtract-and-compare iterations (~100 ns). The rare path drops out of the
+// critical section before doing the expensive /1000, so unrelated peripherals
+// and the WiFi MAC are not blocked while the divide runs.
 // Threshold above which we use constant-time /1000 instead of the while loop.
 // 10 ms means the while loop runs at most 10 iterations (~100 ns) on the
 // common path, well within the WiFi stack's ~10 μs interrupt latency budget.
@@ -56,20 +57,35 @@ uint32_t IRAM_ATTR HOT millis() {
   uint32_t delta = now_us - state.last_us;
   state.last_us = now_us;
   state.remainder += delta;
-  if (state.remainder >= MILLIS_RARE_PATH_THRESHOLD_US) {
-    // Rare path: large gap (WiFi scan, boot, long block). Constant-time
-    // conversion keeps the critical section bounded.
-    uint32_t ms = state.remainder / US_PER_MS;
-    state.cache += ms;
-    state.remainder -= ms * US_PER_MS;
-  } else {
+  if (state.remainder < MILLIS_RARE_PATH_THRESHOLD_US) {
     // Common path: small gap. Loop runs at most
     // MILLIS_RARE_PATH_THRESHOLD_US / US_PER_MS iterations.
     while (state.remainder >= US_PER_MS) {
       state.cache++;
       state.remainder -= US_PER_MS;
     }
+    uint32_t result = state.cache;
+    xt_wsr_ps(ps);
+    return result;
   }
+  // Rare path: large gap (WiFi scan, boot, long block). Take the accumulated
+  // μs out of the shared state, drop the critical section, then divide. This
+  // keeps interrupts unmasked during the ~2.5 μs __umulsidi3 — the rare path
+  // fires precisely when the system was already blocked, which is also when
+  // WiFi/peripheral ISRs most need to run on time.
+  //
+  // Concurrency on single-core ESP8266: only an ISR can preempt us. An ISR
+  // that calls millis() during the divide sees remainder=0 and a tiny delta,
+  // takes the common path, and returns the pre-commit cache. Monotonic — and
+  // any μs it adds to remainder are preserved by the `+=` on commit.
+  uint32_t pending_us = state.remainder;
+  state.remainder = 0;
+  xt_wsr_ps(ps);
+  uint32_t ms_to_add = pending_us / US_PER_MS;
+  uint32_t us_left = pending_us - ms_to_add * US_PER_MS;
+  ps = xt_rsil(15);
+  state.cache += ms_to_add;
+  state.remainder += us_left;
   uint32_t result = state.cache;
   xt_wsr_ps(ps);
   return result;