Make it non-fatal to perform SPI transfers from interrupt context.

apps/drivers/device/spi.cpp

@@ -35,8 +35,16 @@
  * @file Base class for devices connected via SPI.
  *
  * @todo Work out if caching the mode/frequency would save any time.
+ *
+ * @todo A separate bus/device abstraction would allow for mixed interrupt-mode
+ * and non-interrupt-mode clients to arbitrate for the bus.  As things stand,
+ * a bus shared between clients of both kinds is vulnerable to races between
+ * the two, where an interrupt-mode client will ignore the lock held by the
+ * non-interrupt-mode client.
  */
 
+#include <nuttx/arch.h>
+
 #include "spi.h"
 
 #ifndef CONFIG_SPI_EXCHANGE
@@ -124,7 +132,8 @@ SPI::transfer(uint8_t *send, uint8_t *recv, unsigned len)
 		return -EINVAL;
 
 	/* do common setup */
-	SPI_LOCK(_dev, true);
+	if (!up_interrupt_context())
+		SPI_LOCK(_dev, true);
 	SPI_SETFREQUENCY(_dev, _frequency);
 	SPI_SETMODE(_dev, _mode);
 	SPI_SETBITS(_dev, 8);
@@ -135,7 +144,8 @@ SPI::transfer(uint8_t *send, uint8_t *recv, unsigned len)
 
 	/* and clean up */
 	SPI_SELECT(_dev, _device, false);
-	SPI_LOCK(_dev, false);
+	if (!up_interrupt_context())
+		SPI_LOCK(_dev, false);
 
 	return OK;
 }

apps/drivers/device/spi.h

@@ -81,6 +81,12 @@ protected:
 	/**
 	 * Perform a SPI transfer.
 	 *
+	 * If called from interrupt context, this interface does not lock
+	 * the bus and may interfere with non-interrupt-context callers.
+	 *
+	 * Clients in a mixed interrupt/non-interrupt configuration must 
+	 * ensure appropriate interlocking.
+	 *
 	 * At least one of send or recv must be non-null.
 	 *
 	 * @param send		Bytes to send to the device, or nullptr if