gimlet-seq-server: start driving fault pin.

This pulls the fault pin low (active) shortly after the sequencer task
first runs, and then sets it high (inactive) only once we get up to the
IPC loop.

Failures in the sequencer startup tend to cause panics, which will have
the effect of holding the pin low as the task is repeatedly restarted.
This also means we will be able to externally observe any other restart
of the task.

drv/gimlet-seq-server/src/main.rs

@@ -115,6 +115,16 @@ fn main() -> ! {
     let jefe = Jefe::from(JEFE.get_task_id());
     let hf = hf_api::HostFlash::from(HF.get_task_id());
 
+    // Ensure the SP fault pin is configured as an open-drain output, and pull
+    // it low to make the sequencer restart externally visible.
+    sys.gpio_configure_output(
+        FAULT_PIN_L,
+        sys_api::OutputType::OpenDrain,
+        sys_api::Speed::Low,
+        sys_api::Pull::None,
+    );
+    sys.gpio_reset(FAULT_PIN_L);
+
     // Turn off the chassis LED, in case this is a task restart (and not a
     // full chip restart, which would leave the GPIO unconfigured).
     sys.gpio_configure_output(
@@ -403,6 +413,10 @@ fn main() -> ! {
         sys_api::Pull::None,
     );
 
+    // Clear the external fault now that we're about to start serving messages
+    // and fewer things can go wrong.
+    sys.gpio_set(FAULT_PIN_L);
+
     loop {
         idol_runtime::dispatch_n(&mut buffer, &mut server);
     }
@@ -1135,6 +1149,8 @@ cfg_if::cfg_if! {
 
         // SP_STATUS_LED
         const CHASSIS_LED: sys_api::PinSet = sys_api::Port::A.pin(3);
+        // SP_TO_IGNIT_FAULT_L
+        const FAULT_PIN_L: sys_api::PinSet = sys_api::Port::A.pin(15);
 
         // Gimlet provides external pullups.
         const PGS_PULL: sys_api::Pull = sys_api::Pull::None;