3 files changed, 27 insertions, 5 deletions
diff --git a/atmel-samd/common-hal/busio/SPI.c b/atmel-samd/common-hal/busio/SPI.c
index c1b13cec8..e5a945df2 100644
--- a/atmel-samd/common-hal/busio/SPI.c
+++ b/atmel-samd/common-hal/busio/SPI.c
@@ -257,3 +257,16 @@ bool common_hal_busio_spi_read(busio_spi_obj_t *self,
     }
     return status == STATUS_OK;
 }
+
+bool common_hal_busio_spi_transfer(busio_spi_obj_t *self, uint8_t *data_out, uint8_t *data_in, size_t len) {
+    if (len == 0) {
+        return true;
+    }
+    enum status_code status;
+    if (len >= 16) {
+        status = shared_dma_transfer(self->spi_master_instance.hw, data_out, data_in, len, 0 /*ignored*/);
+    } else {
+        status = spi_transceive_buffer_wait(&self->spi_master_instance, data_out, data_in, len);
+    }
+    return status == STATUS_OK;
+}
diff --git a/atmel-samd/shared_dma.c b/atmel-samd/shared_dma.c
index 461798366..c0f0d4672 100644
--- a/atmel-samd/shared_dma.c
+++ b/atmel-samd/shared_dma.c
@@ -103,7 +103,7 @@ enum status_code shared_dma_write(Sercom* sercom, const uint8_t* buffer, uint32_
     }
     dma_configure(general_dma_tx.channel_id, sercom_index(sercom) * 2 + 2, false);
 
-    // Set up TX second.
+    // Set up TX. There is no RX job.
     struct dma_descriptor_config descriptor_config;
     dma_descriptor_get_config_defaults(&descriptor_config);
     descriptor_config.beat_size = DMA_BEAT_SIZE_BYTE;
@@ -137,6 +137,12 @@ enum status_code shared_dma_write(Sercom* sercom, const uint8_t* buffer, uint32_
 }
 
 enum status_code shared_dma_read(Sercom* sercom, uint8_t* buffer, uint32_t length, uint8_t tx) {
+    return shared_dma_transfer(sercom, NULL, buffer, length, tx);
+}
+
+// Do write and read simultaneously. If buffer_out is NULL, write the tx byte over and over.
+// If buffer_out is a real buffer, ignore tx.
+enum status_code shared_dma_transfer(Sercom* sercom, uint8_t* buffer_out, uint8_t* buffer_in, uint32_t length, uint8_t tx) {
     if (general_dma_tx.job_status != STATUS_OK) {
         return general_dma_tx.job_status;
     }
@@ -153,17 +159,19 @@ enum status_code shared_dma_read(Sercom* sercom, uint8_t* buffer, uint32_t lengt
     descriptor_config.block_transfer_count = length;
     // DATA register is consistently addressed across all SERCOM modes.
     descriptor_config.source_address = ((uint32_t)&sercom->SPI.DATA.reg);
-    descriptor_config.destination_address = ((uint32_t)buffer + length);
+    descriptor_config.destination_address = ((uint32_t)buffer_in + length);
 
     dma_descriptor_create(general_dma_rx.descriptor, &descriptor_config);
 
-    // Set up TX to retransmit the same byte over and over.
+    // Set up TX second.
     dma_descriptor_get_config_defaults(&descriptor_config);
     descriptor_config.beat_size = DMA_BEAT_SIZE_BYTE;
-    descriptor_config.src_increment_enable = false;
+    // Increment write address only if we have a real buffer.
+    descriptor_config.src_increment_enable = buffer_out != NULL;
     descriptor_config.dst_increment_enable = false;
     descriptor_config.block_transfer_count = length;
-    descriptor_config.source_address = ((uint32_t)&tx);
+    // 
+    descriptor_config.source_address = ((uint32_t) (buffer_out != NULL ? buffer_out + length : &tx));
     // DATA register is consistently addressed across all SERCOM modes.
     descriptor_config.destination_address = ((uint32_t)&sercom->SPI.DATA.reg);
 
diff --git a/atmel-samd/shared_dma.h b/atmel-samd/shared_dma.h
index 7aa45547d..251b8357b 100644
--- a/atmel-samd/shared_dma.h
+++ b/atmel-samd/shared_dma.h
@@ -39,6 +39,7 @@ void init_shared_dma(void);
 
 enum status_code shared_dma_write(Sercom* sercom, const uint8_t* buffer, uint32_t length);
 enum status_code shared_dma_read(Sercom* sercom, uint8_t* buffer, uint32_t length, uint8_t tx);
+enum status_code shared_dma_transfer(Sercom* sercom, uint8_t* buffer_out, uint8_t* buffer_in, uint32_t length, uint8_t tx);
 
 // Allocate a counter to track how far along we are in a DMA double buffer.
 bool allocate_block_counter(void);