Update some comments

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@3073 42af7a65-404d-4744-a932-0658087f49c3

2 files changed, 37 insertions, 2 deletions

diff --git a/nuttx/arch/avr/src/avr32/up_fullcontextrestore.S b/nuttx/arch/avr/src/avr32/up_fullcontextrestore.S
index c40239307..c883c58c7 100755
--- a/nuttx/arch/avr/src/avr32/up_fullcontextrestore.S
+++ b/nuttx/arch/avr/src/avr32/up_fullcontextrestore.S
@@ -53,6 +53,28 @@
 /****************************************************************************

  * Name: up_fullcontextrestore

  *

+ * Descripion:

+ *   Restore the full-running contex of a thread.

+ *

+ *   NOTE: Thus function must handle one very strange case.  That is when

+ *   this function is called with up_sigdeliver().  That case is strange in

+ *   two ways:

+ *

+ *   1. It is not a context switch between threads.  Rather, up_fullcontextrestore

+ *      must behave more it more like a longjmp within the same task, using the

+ *      same stack.

+ *   2. In this case, this function is called with r12 pointing to a register

+ *      save area on the stack to be destroyed.  This is dangerous for two

+ *      reasons: (a) there is a period of time where the stack contents still

+ *      contain valid data, but are outside of range protected by the stack

+ *      pointer (hence, interrupts must be disabled), and (b) there is the

+ *      very real possibility that the new stack pointer might overlap with

+ *      the register save area and stack usage in this function might corrupt

+ *      the register save data before the state is restored.

+ *

+ * Input Parameters:

+ *   r12 = A pointer to the register save area of the thread to be restored.

+ *

  * C Prototype:

  *  void up_fullcontextrestore(uint32_t *regs);

  *

diff --git a/nuttx/arch/avr/src/avr32/up_sigdeliver.c b/nuttx/arch/avr/src/avr32/up_sigdeliver.c
index aa3940b79..b000f118a 100644
--- a/nuttx/arch/avr/src/avr32/up_sigdeliver.c
+++ b/nuttx/arch/avr/src/avr32/up_sigdeliver.c
@@ -82,7 +82,7 @@
 void up_sigdeliver(void)
 {
   _TCB  *rtcb = (_TCB*)g_readytorun.head;
-  uint32_t regs[XCPTCONTEXT_REGS];
+  uint32_t regs[XCPTCONTEXT_REGS+3];  /* Why +3? See below */
   sig_deliver_t sigdeliver;
 
   /* Save the errno.  This must be preserved throughout the signal handling
@@ -128,7 +128,20 @@ void up_sigdeliver(void)
   (void)irqsave();
   rtcb->pterrno = saved_errno;
 
-  /* Then restore the correct state for this thread of execution. */
+  /* Then restore the correct state for this thread of execution. This is an
+   * unusual case that must be handled by up_fullcontextresore. This case is
+   * unusal in two ways:
+   *
+   *   1. It is not a context switch between threads.  Rather, up_fullcontextrestore
+   *      must behave more it more like a longjmp within the same task, using
+   *      he same stack.
+   *   2. In this case, up_fullcontextrestore is called with r12 pointing to
+   *      a register save area on the stack to be destroyed.  This is
+   *      dangerous because there is the very real possibility that the new
+   *      stack pointer might overlap with the register save area and hat stack
+   *      usage in up_fullcontextrestore might corrupt the register save data
+   *      before the state is restored.
+   */
 
   up_ledoff(LED_SIGNAL);
   up_fullcontextrestore(regs);