From 1222fd40b7b0b79b40977bd2ab51ecae5f730039 Mon Sep 17 00:00:00 2001
From: Geoffrey McRae <geoff@hostfission.com>
Date: Sun, 12 Apr 2020 13:14:53 +1000
Subject: [PATCH] [common] fix FrameBuffer to use atomics correctly

Might Fix #248
---
 VERSION                             |  2 +-
 c-host/src/app.c                    |  2 +-
 common/include/common/framebuffer.h | 21 +++++++----------
 common/src/framebuffer.c            | 35 ++++++++++++++++++++++++-----
 4 files changed, 39 insertions(+), 21 deletions(-)

diff --git a/VERSION b/VERSION
index 05305d61..49410ff2 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-B1-163-g577c54d20d+1
\ No newline at end of file
+B1-165-gb5f4c639fd+1
\ No newline at end of file
diff --git a/c-host/src/app.c b/c-host/src/app.c
index f2215e81..ce58c9fc 100644
--- a/c-host/src/app.c
+++ b/c-host/src/app.c
@@ -187,7 +187,7 @@ static int frameThread(void * opaque)
     fi->height  = frame.height;
     fi->stride  = frame.stride;
     fi->pitch   = frame.pitch;
-    fi->offset  = pageSize - sizeof(FrameBuffer);
+    fi->offset  = pageSize - FrameBufferStructSize;
     frameValid  = true;
 
     // put the framebuffer on the border of the next page
diff --git a/common/include/common/framebuffer.h b/common/include/common/framebuffer.h
index 20cd479f..21a52e37 100644
--- a/common/include/common/framebuffer.h
+++ b/common/include/common/framebuffer.h
@@ -23,24 +23,19 @@ Place, Suite 330, Boston, MA 02111-1307 USA
 #include <stdbool.h>
 #include <stdint.h>
 
-
-typedef struct stFrameBuffer
-{
-  uint64_t  wp;
-  uint8_t   data[0];
-}
-FrameBuffer;
-
+typedef struct stFrameBuffer FrameBuffer;
 
 typedef bool (*FrameBufferReadFn)(void * opaque, const void * src, size_t size);
 
+/**
+ * The size of the FrameBuffer struct
+ */
+extern const size_t FrameBufferStructSize;
+
 /**
  * Wait for the framebuffer to fill to the specified size
  */
-static inline void framebuffer_wait(const FrameBuffer * frame, size_t size)
-{
-  while(frame->wp != size) {}
-}
+void framebuffer_wait(const FrameBuffer * frame, size_t size);
 
 /**
  * Read data from the KVMFRFrame into the dst buffer
@@ -60,4 +55,4 @@ void framebuffer_prepare(FrameBuffer * frame);
 /**
  * Write data from the src buffer into the KVMFRFrame
  */
-bool framebuffer_write(FrameBuffer * frame, const void * src, size_t size);
\ No newline at end of file
+bool framebuffer_write(FrameBuffer * frame, const void * src, size_t size);
diff --git a/common/src/framebuffer.c b/common/src/framebuffer.c
index 4e2f7e06..6de5f8a2 100644
--- a/common/src/framebuffer.c
+++ b/common/src/framebuffer.c
@@ -21,19 +21,38 @@ Place, Suite 330, Boston, MA 02111-1307 USA
 #include "common/debug.h"
 
 #include <string.h>
+#include <stdatomic.h>
+
 #define FB_CHUNK_SIZE 1024
 
+struct stFrameBuffer
+{
+  atomic_uint_least32_t wp;
+  uint8_t               data[0];
+};
+
+const size_t FrameBufferStructSize = sizeof(FrameBuffer);
+
+void framebuffer_wait(const FrameBuffer * frame, size_t size)
+{
+  while(atomic_load_explicit(&frame->wp, memory_order_relaxed) != size) {}
+}
+
 bool framebuffer_read(const FrameBuffer * frame, void * dst, size_t size)
 {
   uint8_t *d  = (uint8_t*)dst;
   uint64_t rp = 0;
   while(rp < size)
   {
+    uint_least32_t wp;
+
     /* spinlock */
-    while(rp == frame->wp) { }
+    do
+      wp = atomic_load_explicit(&frame->wp, memory_order_relaxed);
+    while(rp == wp);
 
     /* copy what we can */
-    uint64_t avail = frame->wp - rp;
+    uint64_t avail = wp - rp;
     avail = avail > size ? size : avail;
 
     memcpy(d, frame->data + rp, avail);
@@ -50,11 +69,15 @@ bool framebuffer_read_fn(const FrameBuffer * frame, FrameBufferReadFn fn, size_t
   uint64_t rp = 0;
   while(rp < size)
   {
+    uint_least32_t wp;
+
     /* spinlock */
-    while(rp == frame->wp) { }
+    do
+      wp = atomic_load_explicit(&frame->wp, memory_order_relaxed);
+    while(rp == wp);
 
     /* copy what we can */
-    uint64_t avail = frame->wp - rp;
+    uint64_t avail = wp - rp;
     avail = avail > size ? size : avail;
 
     if (!fn(opaque, frame->data + rp, avail))
@@ -82,8 +105,8 @@ bool framebuffer_write(FrameBuffer * frame, const void * src, size_t size)
   {
     size_t copy = size < FB_CHUNK_SIZE ? FB_CHUNK_SIZE : size;
     memcpy(frame->data + frame->wp, src, copy);
-    __sync_fetch_and_add(&frame->wp, copy);
+    atomic_fetch_add(&frame->wp, copy);
     size -= copy;
   }
   return true;
-}
\ No newline at end of file
+}