More frequent optimistic_yield looks safe to me here.

dok-net · dok-net · commit 3cb53ba04fa1 · 2022-12-23T10:24:03.000+01:00
diff --git a/cores/esp8266/uart.cpp b/cores/esp8266/uart.cpp
@@ -510,7 +510,10 @@ uart_tx_fifo_full(const int uart_nr)
 static void
 uart_do_write_char(const int uart_nr, char c)
 {
-    while(uart_tx_fifo_full(uart_nr));
+    while(uart_tx_fifo_full(uart_nr))
+    {
+        optimistic_yield(1000UL);
+    }
 
     USF(uart_nr) = c;
 }
@@ -544,7 +547,6 @@ uart_write(uart_t* uart, const char* buf, size_t size)
     const int uart_nr = uart->uart_nr;
     while (size--) {
         uart_do_write_char(uart_nr, pgm_read_byte(buf++));
-        optimistic_yield(10000UL);
     }
 
     return ret;
diff --git a/libraries/ESP8266WiFi/src/WiFiUdp.cpp b/libraries/ESP8266WiFi/src/WiFiUdp.cpp
@@ -194,7 +194,7 @@ int WiFiUDP::parsePacket()
         return 0;
 
     if (!_ctx->next()) {
-        optimistic_yield(10000);
+        optimistic_yield(1000);
         return 0;
     }
 

Original file line number	Diff line number	Diff line change
`@@ -510,7 +510,10 @@ uart_tx_fifo_full(const int uart_nr)`
`510`	`510`	`static void`
`511`	`511`	`uart_do_write_char(const int uart_nr, char c)`
`512`	`512`	`{`
`513`		`- while(uart_tx_fifo_full(uart_nr));`
	`513`	`+ while(uart_tx_fifo_full(uart_nr))`
	`514`	`+ {`
	`515`	`+ optimistic_yield(1000UL);`
	`516`	`+ }`
`514`	`517`
`515`	`518`	`USF(uart_nr) = c;`
`516`	`519`	`}`
`@@ -544,7 +547,6 @@ uart_write(uart_t* uart, const char* buf, size_t size)`
`544`	`547`	`const int uart_nr = uart->uart_nr;`
`545`	`548`	`while (size--) {`
`546`	`549`	`uart_do_write_char(uart_nr, pgm_read_byte(buf++));`
`547`		`- optimistic_yield(10000UL);`
`548`	`550`	`}`
`549`	`551`
`550`	`552`	`return ret;`