stm32: Fix usbfs spurious USB packet transmit on startup

Commit cd8d57c added USB double buffering mode on transmits.
However, when enabling double buffering mode, the hardware seems to
always send at least two packets.  Spurious transmissions could cause
the Linux gs_usb driver to get confused, which could lead to the can0
device becoming unavailable on restarts.  Fix by waiting for two USB
packets to be available before enabling the endpoint.

Signed-off-by: Kevin O'Connor <[email protected]>

src/stm32/usbfs.c

@@ -236,7 +236,8 @@ usb_read_bulk_out(void *data, uint_fast8_t max_len)
     return count;
 }
 
-static uint32_t bulk_in_push_count, bulk_in_pop_flag;
+static uint32_t bulk_in_push_pos, bulk_in_pop_flag;
+#define BI_START 2
 
 int_fast8_t
 usb_send_bulk_in(void *data, uint_fast8_t len)
@@ -245,16 +246,26 @@ usb_send_bulk_in(void *data, uint_fast8_t len)
         // No buffer space available
         return -1;
     uint32_t ep = USB_CDC_EP_BULK_IN;
-    int bufnum = bulk_in_push_count & 1;
-    bulk_in_push_count++;
+    uint32_t bipp = bulk_in_push_pos, bufnum = bipp & 1;
+    bulk_in_push_pos = bipp ^ 1;
     btable_write_packet(ep, bufnum, data, len);
     writel(&bulk_in_pop_flag, USB_EP_DTOG_RX);
 
     // Check if hardware needs to be notified
     uint32_t epr = USB_EPR[ep];
     if (epr_is_dbuf_blocking(epr) && readl(&bulk_in_pop_flag)) {
         writel(&bulk_in_pop_flag, 0);
-        USB_EPR[ep] = calc_epr_bits(epr, 0, 0) | USB_EP_DTOG_RX;
+        if (bipp & BI_START) {
+            // Two packets are always sent when starting in double
+            // buffering mode, so wait for second packet before starting.
+            if (bipp == (BI_START | 1)) {
+                bulk_in_push_pos = 0;
+                USB_EPR[ep] = calc_epr_bits(epr, USB_EPTX_STAT
+                                            , USB_EP_TX_VALID);
+            }
+        } else {
+            USB_EPR[ep] = calc_epr_bits(epr, 0, 0) | USB_EP_DTOG_RX;
+        }
     }
 
     return len;
@@ -319,9 +330,8 @@ usb_set_configure(void)
     USB_EPR[ep] = calc_epr_bits(USB_EPR[ep], USB_EPRX_STAT, USB_EP_RX_VALID);
 
     ep = USB_CDC_EP_BULK_IN;
-    bulk_in_push_count = 0;
+    bulk_in_push_pos = BI_START;
     writel(&bulk_in_pop_flag, 0);
-    USB_EPR[ep] = calc_epr_bits(USB_EPR[ep], USB_EPTX_STAT, USB_EP_TX_VALID);
 }