dcd/stm32_fsdev: make CH32 ep0_set_type read-modify-write atomic

ep0_set_type() read the EP0 register outside any critical section and
only ep_write() masked the USB IRQ around the store. A USB interrupt
landing between the read and the write (e.g. a new SETUP whose handler
installs the BULK gate) was silently undone when the task resumed and
wrote back its pre-interrupt snapshot with the type forced to CONTROL,
re-exposing the unsolicited EP0 OUT ACK the workaround blocks.

Bracket the whole read-modify-write with fsdev_int_disable/enable when
called with need_exclusive (task context). ISR-context callers pass
false and are unaffected (the ISR cannot preempt itself).

Co-Authored-By: Claude Fable 5 <[email protected]>

1 files changed, 9 insertions, 2 deletions

diff --git a/src/portable/st/stm32_fsdev/dcd_stm32_fsdev.c b/src/portable/st/stm32_fsdev/dcd_stm32_fsdev.c
index 957795e09..9c3431c20 100644
--- a/src/portable/st/stm32_fsdev/dcd_stm32_fsdev.c
+++ b/src/portable/st/stm32_fsdev/dcd_stm32_fsdev.c
@@ -159,12 +159,19 @@ TU_ATTR_ALWAYS_INLINE static inline xfer_ctl_t *xfer_ctl_ptr(uint8_t epnum, uint
 }
 
 #if defined(TUP_USBIP_FSDEV_CH32)
-// CH32 FSDEV workaround: gate EP0 handshakes by switching type between CONTROL and BULK.
+// CH32 EP0 workaround: gate handshakes by switching EP0 type CONTROL<->BULK.
+// need_exclusive brackets the read-modify-write so the USB ISR can't race it.
 TU_ATTR_ALWAYS_INLINE static inline void ep0_set_type(uint32_t ep_type, bool need_exclusive) {
+  if (need_exclusive) {
+    fsdev_int_disable(0);
+  }
   uint32_t ep_reg = ep_read(0) | U_EP_CTR_TX | U_EP_CTR_RX;
   ep_reg &= U_EPREG_MASK;
   ep_reg = (ep_reg & ~U_EP_T_FIELD) | ep_type;
-  ep_write(0, ep_reg, need_exclusive);
+  ep_write(0, ep_reg, false);
+  if (need_exclusive) {
+    fsdev_int_enable(0);
+  }
 }
 #endif