[sw/pinmux] Add DIF to set pinmux insel

Signed-off-by: Alexander Williams <awill@google.com>

sw/device/lib/dif/dif_pinmux.c

diff --git a/sw/device/lib/dif/dif_pinmux.c b/sw/device/lib/dif/dif_pinmux.c
index ef1343b..7bc798f 100644
--- a/sw/device/lib/dif/dif_pinmux.c
+++ b/sw/device/lib/dif/dif_pinmux.c
@@ -4,9 +4,26 @@
 
 #include "sw/device/lib/dif/dif_pinmux.h"
 
+#include "sw/device/lib/base/bitfield.h"
 #include "sw/device/lib/dif/dif_base.h"
 
 #include "pinmux_regs.h"  // Generated.
 
 // This just exists to check that the header compiles for now. The actual
 // implementation is work in progress.
+
+OT_WARN_UNUSED_RESULT
+dif_result_t dif_pinmux_input_select(const dif_pinmux_t *pinmux,
+                                     dif_pinmux_index_t peripheral_input,
+                                     dif_pinmux_index_t insel) {
+  if (pinmux == NULL || peripheral_input >= PINMUX_PARAM_N_MIO_PERIPH_IN ||
+      insel >= (2 + PINMUX_PARAM_N_MIO_PADS)) {
+    return kDifBadArg;
+  }
+  uint32_t reg_offset =
+      PINMUX_MIO_PERIPH_INSEL_0_REG_OFFSET + (peripheral_input << 2);
+  uint32_t reg_value =
+      bitfield_field32_write(0, PINMUX_MIO_PERIPH_INSEL_0_IN_0_FIELD, insel);
+  mmio_region_write32(pinmux->base_addr, reg_offset, reg_value);
+  return kDifOk;
+}