fix!: addresses pygamer BSP issues discussed in PR atsamd-rs#777

* Changes the `DisplayError` to an enum with variants to provide more detail about why the display driver initialization failed.
* Exposes the `dma` and `max-channels` HAL features in the BSP for convenience.
* Adds a `cortex_m::asm::wfi()` call inside the terminal infinite loop in the examples to address the `empty_loop` clippy lint.

boards/pygamer/Cargo.toml

@@ -47,6 +47,8 @@ usbd-serial = "0.2"
 [features]
 # ask the HAL to enable atsamd51j support
 default = ["rt", "atsamd-hal/samd51j"]
+dma = ["atsamd-hal/dma"]
+max-channels = ["dma", "atsamd-hal/max-channels"]
 panic_led = []
 rt = ["cortex-m-rt", "atsamd-hal/samd51j-rt"]
 usb = ["atsamd-hal/usb", "usb-device"]

boards/pygamer/examples/clock_out.rs

@@ -30,5 +30,7 @@ fn main() -> ! {
         .configure_gclk_divider_and_source(Gclk2, 40, Dpll0, false)
         .unwrap();
     let _clock_out_pin: GclkOut = pins.d5.into();
-    loop {}
+    loop {
+        cortex_m::asm::wfi();
+    }
 }

boards/pygamer/examples/ferris_img.rs

@@ -59,5 +59,7 @@ fn main() -> ! {
     let ferris = Image::new(&raw_image, Point::new(32, 32));
 
     ferris.draw(&mut display).unwrap();
-    loop {}
+    loop {
+        cortex_m::asm::wfi();
+    }
 }

boards/pygamer/examples/qspi.rs

@@ -99,7 +99,9 @@ fn main() -> ! {
     flash.read_memory(0, &mut read_buf);
     assert_eq!(read_buf, write_buf);
 
-    loop {}
+    loop {
+        cortex_m::asm::wfi();
+    }
 }
 
 /// Wait for the write-in-progress and suspended write/erase.

boards/pygamer/src/pins.rs

@@ -582,13 +582,22 @@ pub struct Display {
     pub tft_backlight: TftBacklightReset,
 }
 
-/// This empty error occurs if there is an issue setting up the on-board
-/// display.
+/// Error that can occur when initializing the display.
 #[derive(Debug)]
-pub struct DisplayError;
+pub enum DisplayError {
+    /// Could not configure the SERCOM4 clock.
+    SercomClock,
+    /// Could not configure the SPI port to drive the display.
+    Spi,
+    /// Could not setup the ST7735 display driver.
+    Driver,
+    /// Could not configure the TC2/TC3 clock for PWM control of the backlight.
+    Tc2Tc3Clock,
+}
 impl From<()> for DisplayError {
+    #[inline]
     fn from(_value: ()) -> Self {
-        DisplayError
+        Self::Driver
     }
 }
 
@@ -615,7 +624,9 @@ impl Display {
         delay: &mut hal::delay::Delay,
     ) -> Result<(DisplayDriver, Pwm2<PA01>), DisplayError> {
         let gclk0 = clocks.gclk0();
-        let clock = &clocks.sercom4_core(&gclk0).ok_or(DisplayError)?;
+        let clock = &clocks
+            .sercom4_core(&gclk0)
+            .ok_or(DisplayError::SercomClock)?;
         let pads = spi::Pads::default()
             .sclk(self.tft_sclk)
             .data_out(self.tft_mosi);
@@ -629,7 +640,7 @@ impl Display {
                 .into_panic_on_read(),
             tft_cs,
         )
-        .map_err(|_| DisplayError)?;
+        .map_err(|_| DisplayError::Spi)?;
         let mut display = st7735_lcd::ST7735::new(
             tft_spi,
             self.tft_dc.into(),