intel: Replace wraparound test logic in bufmgr_fake. Again.

I'd swapped the operands, so if we weren't in lockstep with the hardware we
said the sequence was always passed.  Additionally, a race was available that
we might have failed at recovering from.  Instead, I've replaced the logic
with new stuff that should be more robust and not rely on all the parties in
userland following the same IRQ_EMIT() == 1 protocol.  Also, in a radical
departure from past efforts, include a long comment describing the failure
modes and how we're working around them.

Thanks to haihao for catching the original issue.

libdrm/intel/intel_bufmgr_fake.c

@@ -166,7 +166,7 @@ typedef struct _bufmgr_fake {
    /** Driver-supplied argument to driver callbacks */
    void *driver_priv;
    /* Pointer to kernel-updated sarea data for the last completed user irq */
-   volatile unsigned int *last_dispatch;
+   volatile int *last_dispatch;
 
    int fd;
 
@@ -264,61 +264,114 @@ _fence_emit_internal(dri_bufmgr_fake *bufmgr_fake)
       abort();
    }
 
-   /* The kernel implementation of IRQ_WAIT is broken for wraparound, and has
-    * been since it was first introduced.  It only checks for
-    * completed_seq >= seq, and thus returns success early for wrapped irq
-    * values if the CPU wins a race.
-    *
-    * We have to do it up front at emit when we discover wrap, so that another
-    * client can't race (after we drop the lock) to emit and wait and fail.
-    */
-   if (seq == 0 || seq == 1) {
-      drmCommandWriteRead(bufmgr_fake->fd, DRM_I915_FLUSH, &ie, sizeof(ie));
-   }
-
    DBG("emit 0x%08x\n", seq);
    bufmgr_fake->last_fence = seq;
    return bufmgr_fake->last_fence;
 }
 
 static void
-_fence_wait_internal(dri_bufmgr_fake *bufmgr_fake, unsigned int cookie)
+_fence_wait_internal(dri_bufmgr_fake *bufmgr_fake, int seq)
 {
    struct drm_i915_irq_wait iw;
-   unsigned int last_dispatch;
+   int hw_seq;
    int ret;
+   int kernel_lied;
 
    if (bufmgr_fake->fence_wait != NULL) {
-      bufmgr_fake->fence_wait(cookie, bufmgr_fake->fence_priv);
+      bufmgr_fake->fence_wait(seq, bufmgr_fake->fence_priv);
       return;
    }
 
    DBG("wait 0x%08x\n", iw.irq_seq);
 
-   /* The kernel implementation of IRQ_WAIT is broken for wraparound, and has
+   iw.irq_seq = seq;
-    * been since it was first introduced.  It only checks for
+
-    * completed_seq >= seq, and thus never returns for pre-wrapped irq values
+   /* The kernel IRQ_WAIT implementation is all sorts of broken.
-    * if the GPU wins the race.
+    * 1) It returns 1 to 0x7fffffff instead of using the full 32-bit unsigned
+    *    range.
+    * 2) It returns 0 if hw_seq >= seq, not seq - hw_seq < 0 on the 32-bit
+    *    signed range.
+    * 3) It waits if seq < hw_seq, not seq - hw_seq > 0 on the 32-bit
+    *    signed range.
+    * 4) It returns -EBUSY in 3 seconds even if the hardware is still
+    *    successfully chewing through buffers.
     *
-    * So, check if it looks like a pre-wrapped value and just return success.
+    * Assume that in userland we treat sequence numbers as ints, which makes
+    * some of the comparisons convenient, since the sequence numbers are
+    * all postive signed integers.
+    *
+    * From this we get several cases we need to handle.  Here's a timeline.
+    * 0x2   0x7                                         0x7ffffff8   0x7ffffffd
+    *   |    |                                                   |    |
+    * -------------------------------------------------------------------
+    *
+    * A) Normal wait for hw to catch up
+    * hw_seq seq
+    *   |    |
+    * -------------------------------------------------------------------
+    * seq - hw_seq = 5.  If we call IRQ_WAIT, it will wait for hw to catch up.
+    *
+    * B) Normal wait for a sequence number that's already passed.
+    * seq    hw_seq
+    *   |    |
+    * -------------------------------------------------------------------
+    * seq - hw_seq = -5.  If we call IRQ_WAIT, it returns 0 quickly.
+    *
+    * C) Hardware has already wrapped around ahead of us
+    * hw_seq                                                         seq
+    *   |                                                             |
+    * -------------------------------------------------------------------
+    * seq - hw_seq = 0x80000000 - 5.  If we called IRQ_WAIT, it would wait
+    * for hw_seq >= seq, which may never occur.  Thus, we want to catch this
+    * in userland and return 0.
+    *
+    * D) We've wrapped around ahead of the hardware.
+    * seq                                                           hw_seq
+    *   |                                                             |
+    * -------------------------------------------------------------------
+    * seq - hw_seq = -(0x80000000 - 5).  If we called IRQ_WAIT, it would return
+    * 0 quickly because hw_seq >= seq, even though the hardware isn't caught up.
+    * Thus, we need to catch this early return in userland and bother the
+    * kernel until the hardware really does catch up.
+    *
+    * E) Hardware might wrap after we test in userland.
+    *                                                         hw_seq  seq
+    *                                                            |    |
+    * -------------------------------------------------------------------
+    * seq - hw_seq = 5.  If we call IRQ_WAIT, it will likely see seq >= hw_seq
+    * and wait.  However, suppose hw_seq wraps before we make it into the
+    * kernel.  The kernel sees hw_seq >= seq and waits for 3 seconds then
+    * returns -EBUSY.  This is case C).  We should catch this and then return
+    * successfully.
     */
-   if (*bufmgr_fake->last_dispatch - cookie > 0x4000000)
-      return;
-
-   iw.irq_seq = cookie;
-
    do {
-      last_dispatch = *bufmgr_fake->last_dispatch;
+      /* Keep a copy of last_dispatch so that if the wait -EBUSYs because the
+       * hardware didn't catch up in 3 seconds, we can see if it at least made
+       * progress and retry.
+       */
+      hw_seq = *bufmgr_fake->last_dispatch;
+
+      /* Catch case C */
+      if (seq - hw_seq > 0x40000000)
+	 return;
+
       ret = drmCommandWrite(bufmgr_fake->fd, DRM_I915_IRQ_WAIT,
 			    &iw, sizeof(iw));
-   } while (ret == -EAGAIN || ret == -EINTR ||
+      /* Catch case D */
-	    (ret == -EBUSY && last_dispatch != *bufmgr_fake->last_dispatch));
+      kernel_lied = (ret == 0) && (seq - *bufmgr_fake->last_dispatch <
+				   -0x40000000);
+
+      /* Catch case E */
+      if (ret == -EBUSY && (seq - *bufmgr_fake->last_dispatch > 0x40000000))
+	 ret = 0;
+   } while (kernel_lied || ret == -EAGAIN || ret == -EINTR ||
+	    (ret == -EBUSY && hw_seq != *bufmgr_fake->last_dispatch));
 
    if (ret != 0) {
       drmMsg("%s:%d: Error %d waiting for fence.\n", __FILE__, __LINE__, ret);
       abort();
    }
-   clear_fenced(bufmgr_fake, cookie);
+   clear_fenced(bufmgr_fake, seq);
 }
 
 static int
@@ -1312,7 +1365,7 @@ void intel_bufmgr_fake_set_last_dispatch(dri_bufmgr *bufmgr,
 {
    dri_bufmgr_fake *bufmgr_fake = (dri_bufmgr_fake *)bufmgr;
 
-   bufmgr_fake->last_dispatch = last_dispatch;
+   bufmgr_fake->last_dispatch = (volatile int *)last_dispatch;
 }
 
 dri_bufmgr *
@@ -1349,7 +1402,7 @@ intel_bufmgr_fake_init(int fd,
    bufmgr_fake->bufmgr.debug = 0;
 
    bufmgr_fake->fd = fd;
-   bufmgr_fake->last_dispatch = last_dispatch;
+   bufmgr_fake->last_dispatch = (volatile int *)last_dispatch;
 
    return &bufmgr_fake->bufmgr;
 }