All commands switching from TRANSFER state to (sending)DATA
do the same: send stream of data on the DAT lines. Instead
of duplicating the same code many times, introduce 2 helpers:
- sd_cmd_to_sendingdata() on the I/O line setup the data to
  be transferred,
- sd_generic_read_byte() on the DAT lines to fetch the data.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
 hw/sd/sd.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/hw/sd/sd.c b/hw/sd/sd.c
index d85b2906f4..1a8d06804d 100644
--- a/hw/sd/sd.c
+++ b/hw/sd/sd.c
@@ -142,8 +142,10 @@ struct SDState {
      */
     bool expecting_acmd;
     uint32_t blk_written;
+
     uint64_t data_start;
     uint32_t data_offset;
+    size_t data_size;
     uint8_t data[512];
     uint8_t vendor_data[512];
 
@@ -1083,6 +1085,29 @@ static sd_rsp_type_t sd_cmd_unimplemented(SDState *sd, SDRequest req)
     return sd_illegal;
 }
 
+/* Configure fields for following sd_generic_read_byte() calls */
+__attribute__((unused))
+static sd_rsp_type_t sd_cmd_to_sendingdata(SDState *sd, SDRequest req,
+                                           uint64_t start,
+                                           const void *data, size_t size)
+{
+    if (sd->state != sd_transfer_state) {
+        sd_invalid_state_for_cmd(sd, req);
+    }
+
+    sd->state = sd_sendingdata_state;
+    sd->data_start = start;
+    sd->data_offset = 0;
+    if (data) {
+        assert(size);
+        memcpy(sd->data, data, size);
+    }
+    if (size) {
+        sd->data_size = size;
+    }
+    return sd_r1;
+}
+
 /* CMD0 */
 static sd_rsp_type_t sd_cmd_GO_IDLE_STATE(SDState *sd, SDRequest req)
 {
@@ -1920,6 +1945,20 @@ send_response:
     return rsplen;
 }
 
+/* Return true when buffer is consumed. Configured by sd_cmd_to_sendingdata() */
+__attribute__((unused))
+static bool sd_generic_read_byte(SDState *sd, uint8_t *value)
+{
+    *value = sd->data[sd->data_offset];
+
+    if (++sd->data_offset >= sd->data_size) {
+        sd->state = sd_transfer_state;
+        return true;
+    }
+
+    return false;
+}
+
 void sd_write_byte(SDState *sd, uint8_t value)
 {
     int i;
-- 
2.41.0