stm32: performance improvement for spi on stm32f0

The stm32 has a small queue for spi tx/rx. The current code only uses the
spi with a single byte buffer, effectively waiting for each byte to complete
before starting the next transfer.
This patch changes the structure of spi_transfer() to make use of the queue
and achieve back-to-back transfer of bytes on spi.

Signed-off-by: Arne Jansen <arne@die-jansens.de>

1 files changed, 8 insertions, 4 deletions

diff --git a/src/stm32/spi.c b/src/stm32/spi.c
index 376e11d0..f3d83e83 100644
--- a/src/stm32/spi.c
+++ b/src/stm32/spi.c
@@ -84,10 +84,14 @@ spi_transfer(struct spi_config config, uint8_t receive_data,
              uint8_t len, uint8_t *data)
 {
     SPI_TypeDef *spi = config.spi;
-    while (len--) {
-        writeb((void *)&spi->DR, *data);
-        while (!(spi->SR & SPI_SR_RXNE))
-            ;
+    uint8_t *wptr = data;
+    uint8_t *end = data + len;
+
+    while (data < end) {
+        if (spi->SR & SPI_SR_TXE && wptr < end)
+            writeb((void *)&spi->DR, *wptr++);
+        if (!(spi->SR & SPI_SR_RXNE))
+            continue;
         uint8_t rdata = readb((void *)&spi->DR);
         if (receive_data)
             *data = rdata;