AMD GCN: Implement circular buffering.

The GCN port outputs stdout and stderr via a shared-memory interface.
Previously the buffer was limited to 1000 write operations, which was enough
for testing purposes, but easy to exhaust.

This patch implements a new circular buffering system allowing a greater
amount of output.  The interface must allow hundreds of hardware threads to
output simultaneously.  The new limit is UINT32_MAX write operations.

Unfortunately, there's no way to tell if the host side has also been updated.
This code will misbehave unless the gcn-run from GCC is also updated (although
it's fine the other way around), but that patch has already been committed.

OK?

Andrew Stubbs
Mentor Graphics / CodeSourcery

newlib/libc/sys/amdgcn/write.c

@@ -26,10 +26,14 @@
  
    The next_output counter must be atomically incremented for each
    print output.  Only when the print data is fully written can the
-   "written" flag be set.  */
+   "written" flag be set.
+
+   The buffer is circular; the host increments the consumed counter
+   and clears the written flag as it goes, opening up slots for reuse.
+   The counters always use absolute numbers.  */
 struct output {
   int return_value;
-  int next_output;
+  unsigned int next_output;
   struct printf_data {
     int written;
     char msg[128];
@@ -39,7 +43,8 @@ struct output {
       double dvalue;
       char text[128];
     };
-  } queue[1000];
+  } queue[1024];
+  unsigned int consumed;
 };
 
 _READ_WRITE_RETURN_TYPE write (int fd, const void *buf, size_t count)
@@ -55,33 +60,49 @@ _READ_WRITE_RETURN_TYPE write (int fd, const void *buf, size_t count)
   struct output *data = (struct output *)kernargs[2];
 
   /* Each output slot allows 256 bytes, so reserve as many as we need. */
-  int slot_count = ((count+1)/256)+1;
+  unsigned int slot_count = ((count+1)/256)+1;
-  int index = __atomic_fetch_add (&data->next_output, slot_count,
+  unsigned int index = __atomic_fetch_add (&data->next_output, slot_count,
-				  __ATOMIC_ACQUIRE);
+					   __ATOMIC_ACQUIRE);
+
+  if ((unsigned int)(index + slot_count) < data->consumed)
+    {
+      /* Overflow.  */
+      errno = EFBIG;
+      return 0;
+    }
+
   for (int c = count;
-       c >= 0 && index < 1000;
+       c >= 0;
        buf += 256, c -= 256, index++)
     {
+      unsigned int slot = index % 1024;
+
+      /* Spinlock while the host catches up.  */
+      if (index >= 1024)
+	while (__atomic_load_n (&data->consumed, __ATOMIC_ACQUIRE)
+	       <= (index - 1024))
+	  asm ("s_sleep 64");
+
       if (c < 128)
 	{
-	  memcpy (data->queue[index].msg, buf, c);
+	  memcpy (data->queue[slot].msg, buf, c);
-	  data->queue[index].msg[c] = '\0';
+	  data->queue[slot].msg[c] = '\0';
-	  data->queue[index].text[0] = '\0';
+	  data->queue[slot].text[0] = '\0';
 	}
       else if (c < 256)
 	{
-	  memcpy (data->queue[index].msg, buf, 128);
+	  memcpy (data->queue[slot].msg, buf, 128);
-	  memcpy (data->queue[index].text, buf+128, c-128);
+	  memcpy (data->queue[slot].text, buf+128, c-128);
-	  data->queue[index].text[c-128] = '\0';
+	  data->queue[slot].text[c-128] = '\0';
 	}
       else
 	{
-	  memcpy (data->queue[index].msg, buf, 128);
+	  memcpy (data->queue[slot].msg, buf, 128);
-	  memcpy (data->queue[index].text, buf+128, 128);
+	  memcpy (data->queue[slot].text, buf+128, 128);
 	}
 
-      data->queue[index].type = 3; /* Raw.  */
+      data->queue[slot].type = 3; /* Raw.  */
-      __atomic_store_n (&data->queue[index].written, 1, __ATOMIC_RELEASE);
+      __atomic_store_n (&data->queue[slot].written, 1, __ATOMIC_RELEASE);
     }
 
   return count;