EXI: Add USB Gecko support

This third-party device exposes a serial console which can communicate
with another device over USB, and is thus very helpful for debugging.

luma_core/src/exi/mod.rs

@@ -4,6 +4,8 @@
 
 use crate::io::{read32, write32};
 
+pub mod usb_gecko;
+
 // Are we really on a GameCube in Dolphin?
 // TODO: Use 0xcd006800 instead when on a Wii.
 const EXI: u32 = 0xcc006800;

luma_core/src/exi/usb_gecko.rs

@@ -0,0 +1,109 @@
+//! Contains functions for accessing the USB Gecko.
+
+use crate::exi::{Channel, Clock, Cr, Exi};
+use alloc::vec::Vec;
+
+#[derive(Debug)]
+pub enum Error {
+    UsbGeckoNotFound,
+    SendImpossible,
+    ReceiveImpossible,
+}
+
+/// Struct which represents an usable USB Gecko.
+pub struct UsbGecko<'a> {
+    channel: Channel<'a>,
+}
+
+impl<'a> UsbGecko<'a> {
+    /// Try accessing a plugged in USB Gecko, returns Error::UsbGeckoNotFound if none is currently
+    /// plugged in.
+    pub fn new(exi: &'a Exi) -> Result<UsbGecko<'a>, Error> {
+        let channel = exi.get_channel(1);
+        let gecko = UsbGecko { channel };
+        if !gecko.check_usb_gecko() {
+            return Err(Error::UsbGeckoNotFound);
+        }
+        Ok(gecko)
+    }
+
+    fn query_usb_gecko(&self, query: u16) -> u16 {
+        self.channel.select_device(0, Clock::CLOCK_32MHZ);
+        self.channel.write_data((query as u32) << 16);
+        self.channel
+            .write_cr(Cr::TLEN16 | Cr::RW | Cr::IMMEDIATE | Cr::TSTART);
+        self.channel.wait_for_completion();
+        (self.channel.read_data() >> 16) as u16
+    }
+
+    fn check_usb_gecko(&self) -> bool {
+        let query = 0x9000;
+        let ret = self.query_usb_gecko(query);
+        ret == 0x0470
+    }
+
+    fn receive_byte(&self) -> Result<u8, Error> {
+        let query = 0xa000;
+        let ret = self.query_usb_gecko(query);
+        if ret & 0x0800 == 0 {
+            return Err(Error::ReceiveImpossible);
+        }
+        Ok((ret & 0xff) as u8)
+    }
+
+    fn send_byte(&self, byte: u8) -> Result<(), Error> {
+        let query = 0xb000 | ((byte as u16) << 4);
+        let ret = self.query_usb_gecko(query);
+        if ret & 0x0400 == 0 {
+            return Err(Error::SendImpossible);
+        }
+        Ok(())
+    }
+
+    fn can_send(&self) -> bool {
+        let query = 0xc000;
+        let ret = self.query_usb_gecko(query);
+        ret & 0x0400 != 0
+    }
+
+    fn can_receive(&self) -> bool {
+        let query = 0xd000;
+        let ret = self.query_usb_gecko(query);
+        ret & 0x0400 != 0
+    }
+
+    /// Receive one or more bytes from the USB Gecko and returns them in a Vec.  This function
+    /// returns Error::ReceiveImpossible if no bytes are available.
+    pub fn receive(&self) -> Result<Vec<u8>, Error> {
+        let mut data = Vec::new();
+        loop {
+            if !self.can_receive() {
+                break;
+            }
+            let byte = self.receive_byte()?;
+            data.push(byte);
+        }
+        if data.is_empty() {
+            return Err(Error::ReceiveImpossible);
+        }
+        Ok(data)
+    }
+
+    /// Send a slice of bytes to the USB Gecko.  This function returns Error::SendImpossible if no
+    /// byte could be transmitted despite the device saying we could, it otherwise blocks until all
+    /// bytes from the slice have been transmitted.
+    pub fn send(&self, mut data: &[u8]) -> Result<(), Error> {
+        loop {
+            if data.is_empty() {
+                break;
+            }
+            if !self.can_send() {
+                continue;
+            }
+            let byte = data[0];
+            data = &data[1..];
+            self.send_byte(byte)?;
+        }
+        Ok(())
+    }
+}