cpu/stm32/periph_spi: Fix /CS handling

Previously, the /CS signal was performed by enabling / disabling the
SPI peripheral. This had the disadvantage that clock polarity settings
where not applied starting with `spi_acquire()`, as assumed by e.g.
the SPI SD card driver, but only just before transmitting data.

Now the SPI peripheral is enabled on `spi_acquire()` and only disabled
when calling `spi_release()`, and the `SPI_CR2_SSOE` bit in the `CR2`
register is used for hardware /CS handling (as supposed to).

Author: maribu

cpu/stm32/periph/spi.c

@@ -29,7 +29,6 @@
 #include <assert.h>
 
 #include "bitarithm.h"
-#include "cpu.h"
 #include "mutex.h"
 #include "periph/gpio.h"
 #include "periph/spi.h"
@@ -246,7 +245,7 @@ void spi_acquire(spi_t bus, spi_cs_t cs, spi_mode_t mode, spi_clk_t clk)
           periph_apb_clk(spi_config[bus].apbbus)/(1 << (br + 1)),
           br);
 
-    uint16_t cr1_settings = ((br << BR_SHIFT) | mode | SPI_CR1_MSTR);
+    uint16_t cr1_settings = ((br << BR_SHIFT) | mode | SPI_CR1_MSTR | SPI_CR1_SPE);
     /* Settings to add to CR2 in addition to SPI_CR2_SETTINGS */
     uint16_t cr2_extra_settings = 0;
     if (cs != SPI_HWCS_MASK) {
@@ -393,10 +392,12 @@ void spi_transfer_bytes(spi_t bus, spi_cs_t cs, bool cont,
     assert(out || in);
 
     /* active the given chip select line */
-    dev(bus)->CR1 |= (SPI_CR1_SPE);     /* this pulls the HW CS line low */
     if ((cs != SPI_HWCS_MASK) && gpio_is_valid(cs)) {
         gpio_clear((gpio_t)cs);
     }
+    else {
+        dev(bus)->CR2 |= SPI_CR2_SSOE;
+    }
 
 #ifdef MODULE_PERIPH_DMA
     if (_use_dma(&spi_config[bus])) {
@@ -411,9 +412,11 @@ void spi_transfer_bytes(spi_t bus, spi_cs_t cs, bool cont,
 
     /* release the chip select if not specified differently */
     if ((!cont) && gpio_is_valid(cs)) {
-        dev(bus)->CR1 &= ~(SPI_CR1_SPE);    /* pull HW CS line high */
         if (cs != SPI_HWCS_MASK) {
             gpio_set((gpio_t)cs);
         }
+        else {
+            dev(bus)->CR2 &= ~(SPI_CR2_SSOE);
+        }
     }
 }