stmhal/spi: Increase SPI transfer timeout, proportional to num bytes.

With the existing timeout of 100ms the transfer would end prematurely if
the baudrate was low and the number of bytes to send was high.  This patch
fixes the problem by making the timeout proportional to the number of bytes
that are being transferred.

stmhal/spi.c

@@ -403,6 +403,11 @@ STATIC HAL_StatusTypeDef spi_wait_dma_finished(SPI_HandleTypeDef *spi, uint32_t
     return HAL_OK;
 }
 
+// A transfer of "len" bytes should take len*8*1000/baudrate milliseconds.
+// To simplify the calculation we assume the baudrate is never less than 8kHz
+// and use that value for the baudrate in the formula, plus a small constant.
+#define SPI_TRANSFER_TIMEOUT(len) ((len) + 100)
+
 STATIC void spi_transfer(const pyb_spi_obj_t *self, size_t len, const uint8_t *src, uint8_t *dest, uint32_t timeout) {
     // Note: there seems to be a problem sending 1 byte using DMA the first
     // time directly after the SPI/DMA is initialised.  The cause of this is
@@ -815,7 +820,7 @@ STATIC const mp_map_elem_t pyb_spi_locals_dict_table[] = {
 STATIC MP_DEFINE_CONST_DICT(pyb_spi_locals_dict, pyb_spi_locals_dict_table);
 
 STATIC void spi_transfer_machine(mp_obj_base_t *self_in, size_t len, const uint8_t *src, uint8_t *dest) {
-    spi_transfer((pyb_spi_obj_t*)self_in, len, src, dest, 100);
+    spi_transfer((pyb_spi_obj_t*)self_in, len, src, dest, SPI_TRANSFER_TIMEOUT(len));
 }
 
 STATIC const mp_machine_spi_p_t pyb_spi_p = {
@@ -932,7 +937,7 @@ STATIC void machine_hard_spi_deinit(mp_obj_base_t *self_in) {
 
 STATIC void machine_hard_spi_transfer(mp_obj_base_t *self_in, size_t len, const uint8_t *src, uint8_t *dest) {
     machine_hard_spi_obj_t *self = (machine_hard_spi_obj_t*)self_in;
-    spi_transfer(self->pyb, len, src, dest, 100);
+    spi_transfer(self->pyb, len, src, dest, SPI_TRANSFER_TIMEOUT(len));
 }
 
 STATIC const mp_machine_spi_p_t machine_hard_spi_p = {