diff --git a/platforms/platformio.esp32p4.ini b/platforms/platformio.esp32p4.ini
index 2381d0e..0ca70b3 100644
--- a/platforms/platformio.esp32p4.ini
+++ b/platforms/platformio.esp32p4.ini
@@ -19,9 +19,15 @@ board_build.partitions = huge_app.csv
 ; stalls during execution, silently corrupting LED protocol timing.
 ; See FastLED src/platforms/esp/32/drivers/parlio/ for the canonical chipset list.
 ;
-; Do NOT add -DARDUINO_USB_CDC_ON_BOOT=1 or -DARDUINO_USB_MODE=1 here. They
-; wire Serial to the USB-CDC interface, which only enumerates when a USB host
-; is connected — boards powered from a wall adapter then hang in setup().
+; Do NOT add -DARDUINO_USB_CDC_ON_BOOT=1 here. It wires Serial to the USB-CDC
+; interface, which only enumerates when a USB host is connected — sketches
+; that touch Serial in setup() (especially `while (!Serial) {}`) then hang
+; when the board is powered from a wall adapter instead of a PC.
+;
+; ARDUINO_USB_MODE selects HWCDC vs TinyUSB *when CDC-on-boot is enabled*;
+; it has no effect on Serial routing while CDC_ON_BOOT=0, so we leave it
+; unset here. P4's upstream default is 0 (TinyUSB); C3/C5/C6/H2 force it to
+; 1 (HWCDC) via platform.txt.
 build_flags =
   -DCONFIG_PARLIO_TX_ISR_HANDLER_IN_IRAM=1
   -DCONFIG_PARLIO_TX_ISR_CACHE_SAFE=1