rcc: Fix I2S/SAI PLL configuration.

The PLLSRC needs to be set even if the main PLL is not enabled, because the
source is also used by the other PLLs.

The old code also did not correctly set M, destroying all other values in
the process.

Author: mgottschlag

src/rcc/pll.rs

@@ -19,6 +19,12 @@ impl MainPll {
     ) -> MainPll {
         let sysclk = pllsysclk.unwrap_or(pllsrcclk);
         if pllsysclk.is_none() && !pll48clk {
+            // Even if we do not use the main PLL, we still need to set the PLL source as that setting
+            // applies to the I2S and SAI PLLs as well.
+            unsafe { &*RCC::ptr() }
+                .pllcfgr
+                .write(|w| w.pllsrc().bit(use_hse));
+
             return MainPll {
                 use_pll: false,
                 pllsysclk: None,
@@ -307,7 +313,8 @@ impl I2sPll {
     fn apply_config(config: SingleOutputPll) {
         let rcc = unsafe { &*RCC::ptr() };
         // "M" may have been written before, but the value is identical.
-        rcc.pllcfgr.write(|w| unsafe { w.pllm().bits(config.m) });
+        rcc.pllcfgr
+            .modify(|_, w| unsafe { w.pllm().bits(config.m) });
         rcc.plli2scfgr
             .modify(|_, w| unsafe { w.plli2sn().bits(config.n).plli2sr().bits(config.outdiv) });
     }
@@ -429,16 +436,19 @@ impl SaiPll {
     #[cfg(not(feature = "stm32f446"))]
     fn apply_config(config: SingleOutputPll, saidiv: u32) {
         let rcc = unsafe { &*RCC::ptr() };
-        rcc.dckcfgr.modify(|_, w| w.pllsaidivq().bits(saidiv as u8));
+        rcc.dckcfgr
+            .modify(|_, w| w.pllsaidivq().bits(saidiv as u8 - 1));
         // "M" may have been written before, but the value is identical.
-        rcc.pllcfgr.write(|w| unsafe { w.pllm().bits(config.m) });
+        rcc.pllcfgr
+            .modify(|_, w| unsafe { w.pllm().bits(config.m) });
         rcc.pllsaicfgr
             .modify(|_, w| unsafe { w.pllsain().bits(config.n).pllsaiq().bits(config.outdiv) });
     }
     #[cfg(feature = "stm32f446")]
     fn apply_config(config: SingleOutputPll, saidiv: u32) {
         let rcc = unsafe { &*RCC::ptr() };
-        rcc.dckcfgr.modify(|_, w| w.pllsaidivq().bits(saidiv as u8));
+        rcc.dckcfgr
+            .modify(|_, w| w.pllsaidivq().bits(saidiv as u8 - 1));
         rcc.pllsaicfgr.modify(|_, w| unsafe {
             w.pllsaim()
                 .bits(config.m)