fix(spid): DIF SPID to write full word to TPM Rd FIFO

Revise dif_spi_device to write full word to RdFIFO and unittest too

Signed-off-by: Eunchan Kim <eunchan@opentitan.org>

sw/device/lib/dif/dif_spi_device.c

diff --git a/sw/device/lib/dif/dif_spi_device.c b/sw/device/lib/dif/dif_spi_device.c
index d8a33aa..2366cf1 100644
--- a/sw/device/lib/dif/dif_spi_device.c
+++ b/sw/device/lib/dif/dif_spi_device.c
@@ -1542,15 +1542,28 @@
   }
   dif_spi_device_tpm_data_status_t status;
   dif_result_t result = dif_spi_device_tpm_get_data_status(spi, &status);
+  uint8_t offset = length & 0x3;  // lower two bits of length
+  uint32_t rdfifo_wdata;
+
   if (result != kDifOk) {
     return result;
   }
   if (DIF_SPI_DEVICE_TPM_FIFO_DEPTH - status.read_fifo_occupancy < length) {
     return kDifOutOfRange;
   }
-  for (int i = 0; i < length; i++) {
+  for (int i = 0; i < length; i += 4) {
+    if (i + 4 > length) {
+      // Send partial
+      rdfifo_wdata = 0;
+      for (int j = 0; j <= offset; j++) {
+        rdfifo_wdata |= buf[i + j] << (8 * j);
+      }
+    } else {
+      // Type casting to uint32_t then fetch
+      rdfifo_wdata = *((uint32_t *)buf + (i >> 2));
+    }
     mmio_region_write32(spi->dev.base_addr, SPI_DEVICE_TPM_READ_FIFO_REG_OFFSET,
-                        buf[i]);
+                        rdfifo_wdata);
   }
   return kDifOk;
 }

sw/device/lib/dif/dif_spi_device_unittest.cc

diff --git a/sw/device/lib/dif/dif_spi_device_unittest.cc b/sw/device/lib/dif/dif_spi_device_unittest.cc
index 29e3bb8..7a3c0b0 100644
--- a/sw/device/lib/dif/dif_spi_device_unittest.cc
+++ b/sw/device/lib/dif/dif_spi_device_unittest.cc
@@ -1815,9 +1815,9 @@
                     {SPI_DEVICE_TPM_STATUS_RDFIFO_DEPTH_OFFSET, 1},
                     {SPI_DEVICE_TPM_STATUS_WRFIFO_DEPTH_OFFSET, 4},
                 });
-  for (int i = 0; i < 3; i++) {
-    EXPECT_WRITE32(SPI_DEVICE_TPM_READ_FIFO_REG_OFFSET, data[i]);
-  }
+  EXPECT_WRITE32(SPI_DEVICE_TPM_READ_FIFO_REG_OFFSET,
+                 (data[3] << 24) | (data[2] << 16) | (data[1] << 8) | data[0]);
+
   EXPECT_DIF_OK(dif_spi_device_tpm_write_data(&spi_, /*length=*/3, data));
 
   EXPECT_READ32(SPI_DEVICE_TPM_STATUS_REG_OFFSET,