From 0ec8a3a52a9f9886b99cf9f8d809a9f623bae22f Mon Sep 17 00:00:00 2001
From: Kevin Foucault <k.r.foucault@gmail.com>
Date: Sun, 27 Sep 2026 23:07:59 +0200
Subject: [PATCH] Fix ESP32 UART reader thread spinning at 100% CPU

pthread_recv_esp32() initialised its select() timeout once, before the
for(;;) loop. On Linux, select() overwrites the timeval with the time
remaining, so after the first 100 ms with no byte from the backpack the
timeout is {0, 0} and every subsequent select() returns immediately.
The thread then spins forever, burning a whole Cortex-A7 core whenever
the ELRS backpack option is enabled (the backpack only talks on events,
so the first timeout happens right after boot).

Measured on HDZero Goggles 2 (Linux 4.9, idle main menu, elrs.enable=1):

  - reader thread: 98% of one core, 85% in kernel, 0 voluntary sleeps
  - whole app: 1.45 cores -> 0.54 core after the fix
  - cpufreq: pinned at 1104 MHz 97% of the time -> 816 MHz 93% of the time

Re-arm the timeout on every iteration.

esp32_handler_timeout() fires the delayed DVR start/stop (record_time).
It only ran when select() timed out; the spinning loop hid that, as it
ran the handler continuously. With the thread sleeping again, a backpack
sending bytes less than 100 ms apart would hold the DVR command off, so
call the handler on every wake-up. It only compares record_time with
time().
---
 src/driver/esp32.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/src/driver/esp32.c b/src/driver/esp32.c
index aefe17212..8638baa8e 100644
--- a/src/driver/esp32.c
+++ b/src/driver/esp32.c
@@ -36,7 +36,7 @@ static void *pthread_recv_esp32(void *arg) {
     int i, len = 0;
     uint8_t buffer[128];
     fd_set rd;
-    struct timeval tv = {0, 100000};
+    struct timeval tv;
 
     // Init UART buffers
     uart3_rptr = 0;
@@ -48,6 +48,11 @@ static void *pthread_recv_esp32(void *arg) {
     for (;;) {
         FD_ZERO(&rd);
         FD_SET(fd_esp32, &rd);
+        // On Linux, select() overwrites tv with the time left; once it has
+        // timed out it stays {0, 0} and select() returns immediately, so the
+        // timeout has to be re-armed on every iteration.
+        tv.tv_sec = 0;
+        tv.tv_usec = 100000;
         if (stopping) {
             LOGI("UART3:stopping, exiting thread.");
             return NULL;
@@ -71,8 +76,10 @@ static void *pthread_recv_esp32(void *arg) {
             }
             if (len)
                 esp32_rx();
-        } else
-            esp32_handler_timeout();
+        }
+        // Delayed DVR command: check it on every wake-up, not only on timeout,
+        // or a backpack talking more often than every 100 ms would hold it off.
+        esp32_handler_timeout();
     }
     return NULL;
 }