freertos/xt_asm_utils: added documentation of current windows spill solution

components/freertos/xt_asm_utils.h

@@ -1,3 +1,28 @@
+/* Copyright 2015-2018 Espressif Systems (Shanghai) PTE LTD
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/* File adapted to use on IDF FreeRTOS component, extracted 
+ * originally from zephyr RTOS code base
+ */
+
+/*
+ * Copyright (c) 2017, Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
 #ifndef __XT_ASM_UTILS_H
 #define __XT_ASM_UTILS_H
 
@@ -59,5 +84,4 @@
 #endif
 .endm
 
-
 #endif

components/freertos/xtensa_context.S

@@ -169,25 +169,25 @@ _xt_context_save:
     s32i    a9,  sp, XT_STK_OVLY        /* save overlay state */
     #endif
 
-	rsr     a2, PS
-	movi    a3, PS_INTLEVEL_MASK
+	rsr     a2, PS                      /* We need to enable window execptions to */ 
+	movi    a3, PS_INTLEVEL_MASK        /* perform spill registers*/
 	and     a2, a2, a3
 	bnez    a2, _not_l1
 	rsr     a2, PS
-	movi    a3, PS_INTLEVEL(1)
-	or      a2, a2, a3
-	wsr     a2, PS
+	movi    a3, PS_INTLEVEL(1)          /* For some curious reason the level 1 interrupts */
+	or      a2, a2, a3                  /* dont set the intlevel correctly on PS, we need to */     
+	wsr     a2, PS                      /* do this manually */
 
 _not_l1:
-	rsr     a2, PS
+	rsr     a2, PS                      /* finally umask the window exceptions */
 	movi    a3, ~(PS_EXCM_MASK)
 	and     a2, a2, a3
 	wsr     a2, PS
     rsync
 
-    addi    sp,  sp, XT_STK_FRMSZ      /* restore the interruptee's SP */
-    SPILL_ALL_WINDOWS
-    addi    sp,  sp, -XT_STK_FRMSZ
+    addi    sp,  sp, XT_STK_FRMSZ      /* go back to spill register region */
+    SPILL_ALL_WINDOWS                  /* place the live register windows there */ 
+    addi    sp,  sp, -XT_STK_FRMSZ     /* return the current stack pointer and proceed with conext save*/
     #endif
 
     l32i    a12, sp, XT_STK_TMP0        /* temp. save stuff in stack frame */