drm/i915/bdw: implement semaphore signal

Semaphore signalling works similarly to previous GENs with the exception
that the per ring mailboxes no longer exist. Instead you must define
your own space, somewhere in the GTT.

The comments in the code define the layout I've opted for, which should
be fairly future proof. Ie. I tried to define offsets in abstract terms
(NUM_RINGS, seqno size, etc).

NOTE: If one wanted to move this to the HWSP they could. I've decided
one 4k object would be easier to deal with, and provide potential wins
with cache locality, but that's all speculative.

v2: Update the macro to not need the other ring's ring->id (Chris)
Update the comment to use the correct formula (Chris)

v3: Move the macros the ringbuffer.h to prevent churn in next patch
(Ville)

v4: Fixed compilation rebase conflict
commit 1ec9e26ddab06459e89a890431b2de064c5d1056
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Feb 14 14:01:11 2014 +0100

    drm/i915: Consolidate binding parameters into flags

v5: VCS2 rebase
Replace hweight_long with hweight32

v6 (Rodrigo): * Add missed VC2 gen8 ring signal init
   	      * fixing conflicst on rebase
    	      * minor fixes on address table
	      * remove WARN_ON

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
[danvet: s/BUG_ON/WARN_ON/]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 204de4032a4a41ad84beda2cc52a497c6262dd88..7ef67b656e2f093cf822cd08843e2a7f2a608491 100644 (file)
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1375,6 +1375,7 @@ struct drm_i915_private {
 
        struct pci_dev *bridge_dev;
        struct intel_engine_cs ring[I915_NUM_RINGS];
+       struct drm_i915_gem_object *semaphore_obj;
        uint32_t last_seqno, next_seqno;
 
        drm_dma_handle_t *status_page_dmah;

diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 66151ff1535d05c405ffb8563c9bcb581be9b12a..218ca7b7bb7c759c96e51f08678afe1ba86dc3a4 100644 (file)
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -240,7 +240,7 @@
 #define   MI_DISPLAY_FLIP_IVB_SPRITE_B (3 << 19)
 #define   MI_DISPLAY_FLIP_IVB_PLANE_C  (4 << 19)
 #define   MI_DISPLAY_FLIP_IVB_SPRITE_C (5 << 19)
-#define MI_SEMAPHORE_MBOX      MI_INSTR(0x16, 1) /* gen6+ */
+#define MI_SEMAPHORE_MBOX      MI_INSTR(0x16, 1) /* gen6, gen7 */
 #define   MI_SEMAPHORE_GLOBAL_GTT    (1<<22)
 #define   MI_SEMAPHORE_UPDATE      (1<<21)
 #define   MI_SEMAPHORE_COMPARE     (1<<20)
@@ -266,6 +266,8 @@
 #define   MI_RESTORE_EXT_STATE_EN      (1<<2)
 #define   MI_FORCE_RESTORE             (1<<1)
 #define   MI_RESTORE_INHIBIT           (1<<0)
+#define MI_SEMAPHORE_SIGNAL    MI_INSTR(0x1b, 0) /* GEN8+ */
+#define   MI_SEMAPHORE_TARGET(engine)  ((engine)<<15)
 #define MI_STORE_DWORD_IMM     MI_INSTR(0x20, 1)
 #define   MI_MEM_VIRTUAL       (1 << 22) /* 965+ only */
 #define MI_STORE_DWORD_INDEX   MI_INSTR(0x21, 1)
@@ -360,6 +362,7 @@
 #define   PIPE_CONTROL_TEXTURE_CACHE_INVALIDATE                (1<<10) /* GM45+ only */
 #define   PIPE_CONTROL_INDIRECT_STATE_DISABLE          (1<<9)
 #define   PIPE_CONTROL_NOTIFY                          (1<<8)
+#define   PIPE_CONTROL_FLUSH_ENABLE                    (1<<7) /* gen7+ */
 #define   PIPE_CONTROL_VF_CACHE_INVALIDATE             (1<<4)
 #define   PIPE_CONTROL_CONST_CACHE_INVALIDATE          (1<<3)
 #define   PIPE_CONTROL_STATE_CACHE_INVALIDATE          (1<<2)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index e9c8814bed36ee2a0dfd12a75f0849aaeb1c8f17..e1aac25fc84c756371efc0e16b869f57a602e540 100644 (file)
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -660,6 +660,13 @@ static int init_render_ring(struct intel_engine_cs *ring)
 static void render_ring_cleanup(struct intel_engine_cs *ring)
 {
        struct drm_device *dev = ring->dev;
+       struct drm_i915_private *dev_priv = dev->dev_private;
+
+       if (dev_priv->semaphore_obj) {
+               i915_gem_object_ggtt_unpin(dev_priv->semaphore_obj);
+               drm_gem_object_unreference(&dev_priv->semaphore_obj->base);
+               dev_priv->semaphore_obj = NULL;
+       }
 
        if (ring->scratch.obj == NULL)
                return;
@@ -673,6 +680,80 @@ static void render_ring_cleanup(struct intel_engine_cs *ring)
        ring->scratch.obj = NULL;
 }
 
+static int gen8_rcs_signal(struct intel_engine_cs *signaller,
+                          unsigned int num_dwords)
+{
+#define MBOX_UPDATE_DWORDS 8
+       struct drm_device *dev = signaller->dev;
+       struct drm_i915_private *dev_priv = dev->dev_private;
+       struct intel_engine_cs *waiter;
+       int i, ret, num_rings;
+
+       num_rings = hweight32(INTEL_INFO(dev)->ring_mask);
+       num_dwords += (num_rings-1) * MBOX_UPDATE_DWORDS;
+#undef MBOX_UPDATE_DWORDS
+
+       ret = intel_ring_begin(signaller, num_dwords);
+       if (ret)
+               return ret;
+
+       for_each_ring(waiter, dev_priv, i) {
+               u64 gtt_offset = signaller->semaphore.signal_ggtt[i];
+               if (gtt_offset == MI_SEMAPHORE_SYNC_INVALID)
+                       continue;
+
+               intel_ring_emit(signaller, GFX_OP_PIPE_CONTROL(6));
+               intel_ring_emit(signaller, PIPE_CONTROL_GLOBAL_GTT_IVB |
+                                          PIPE_CONTROL_QW_WRITE |
+                                          PIPE_CONTROL_FLUSH_ENABLE);
+               intel_ring_emit(signaller, lower_32_bits(gtt_offset));
+               intel_ring_emit(signaller, upper_32_bits(gtt_offset));
+               intel_ring_emit(signaller, signaller->outstanding_lazy_seqno);
+               intel_ring_emit(signaller, 0);
+               intel_ring_emit(signaller, MI_SEMAPHORE_SIGNAL |
+                                          MI_SEMAPHORE_TARGET(waiter->id));
+               intel_ring_emit(signaller, 0);
+       }
+
+       return 0;
+}
+
+static int gen8_xcs_signal(struct intel_engine_cs *signaller,
+                          unsigned int num_dwords)
+{
+#define MBOX_UPDATE_DWORDS 6
+       struct drm_device *dev = signaller->dev;
+       struct drm_i915_private *dev_priv = dev->dev_private;
+       struct intel_engine_cs *waiter;
+       int i, ret, num_rings;
+
+       num_rings = hweight32(INTEL_INFO(dev)->ring_mask);
+       num_dwords += (num_rings-1) * MBOX_UPDATE_DWORDS;
+#undef MBOX_UPDATE_DWORDS
+
+       ret = intel_ring_begin(signaller, num_dwords);
+       if (ret)
+               return ret;
+
+       for_each_ring(waiter, dev_priv, i) {
+               u64 gtt_offset = signaller->semaphore.signal_ggtt[i];
+               if (gtt_offset == MI_SEMAPHORE_SYNC_INVALID)
+                       continue;
+
+               intel_ring_emit(signaller, (MI_FLUSH_DW + 1) |
+                                          MI_FLUSH_DW_OP_STOREDW);
+               intel_ring_emit(signaller, lower_32_bits(gtt_offset) |
+                                          MI_FLUSH_DW_USE_GTT);
+               intel_ring_emit(signaller, upper_32_bits(gtt_offset));
+               intel_ring_emit(signaller, signaller->outstanding_lazy_seqno);
+               intel_ring_emit(signaller, MI_SEMAPHORE_SIGNAL |
+                                          MI_SEMAPHORE_TARGET(waiter->id));
+               intel_ring_emit(signaller, 0);
+       }
+
+       return 0;
+}
+
 static int gen6_signal(struct intel_engine_cs *signaller,
                       unsigned int num_dwords)
 {
@@ -1942,12 +2023,30 @@ int intel_init_render_ring_buffer(struct drm_device *dev)
 {
        struct drm_i915_private *dev_priv = dev->dev_private;
        struct intel_engine_cs *ring = &dev_priv->ring[RCS];
+       struct drm_i915_gem_object *obj;
+       int ret;
 
        ring->name = "render ring";
        ring->id = RCS;
        ring->mmio_base = RENDER_RING_BASE;
 
        if (INTEL_INFO(dev)->gen >= 8) {
+               if (i915_semaphore_is_enabled(dev)) {
+                       obj = i915_gem_alloc_object(dev, 4096);
+                       if (obj == NULL) {
+                               DRM_ERROR("Failed to allocate semaphore bo. Disabling semaphores\n");
+                               i915.semaphores = 0;
+                       } else {
+                               i915_gem_object_set_cache_level(obj, I915_CACHE_LLC);
+                               ret = i915_gem_obj_ggtt_pin(obj, 0, PIN_NONBLOCK);
+                               if (ret != 0) {
+                                       drm_gem_object_unreference(&obj->base);
+                                       DRM_ERROR("Failed to pin semaphore bo. Disabling semaphores\n");
+                                       i915.semaphores = 0;
+                               } else
+                                       dev_priv->semaphore_obj = obj;
+                       }
+               }
                ring->add_request = gen6_add_request;
                ring->flush = gen8_render_ring_flush;
                ring->irq_get = gen8_ring_get_irq;
@@ -1956,18 +2055,10 @@ int intel_init_render_ring_buffer(struct drm_device *dev)
                ring->get_seqno = gen6_ring_get_seqno;
                ring->set_seqno = ring_set_seqno;
                if (i915_semaphore_is_enabled(dev)) {
+                       WARN_ON(!dev_priv->semaphore_obj);
                        ring->semaphore.sync_to = gen6_ring_sync;
-                       ring->semaphore.signal = gen6_signal;
-                       ring->semaphore.mbox.wait[RCS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[VCS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[BCS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[VECS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[VCS2] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.signal[RCS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[VCS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[BCS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[VECS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[VCS2] = GEN6_NOSYNC;
+                       ring->semaphore.signal = gen8_rcs_signal;
+                       GEN8_RING_SEMAPHORE_INIT;
                }
        } else if (INTEL_INFO(dev)->gen >= 6) {
                ring->add_request = gen6_add_request;
@@ -2045,9 +2136,6 @@ int intel_init_render_ring_buffer(struct drm_device *dev)
 
        /* Workaround batchbuffer to combat CS tlb bug. */
        if (HAS_BROKEN_CS_TLB(dev)) {
-               struct drm_i915_gem_object *obj;
-               int ret;
-
                obj = i915_gem_alloc_object(dev, I830_BATCH_LIMIT);
                if (obj == NULL) {
                        DRM_ERROR("Failed to allocate batch bo\n");
@@ -2180,25 +2268,8 @@ int intel_init_bsd_ring_buffer(struct drm_device *dev)
                                gen8_ring_dispatch_execbuffer;
                        if (i915_semaphore_is_enabled(dev)) {
                                ring->semaphore.sync_to = gen6_ring_sync;
-                               ring->semaphore.signal = gen6_signal;
-                               /*
-                                * The current semaphore is only applied on
-                                * pre-gen8 platform.  And there is no VCS2 ring
-                                * on the pre-gen8 platform. So the semaphore
-                                * between VCS and VCS2 is initialized as
-                                * INVALID.  Gen8 will initialize the sema
-                                * between VCS2 and VCS later.
-                                */
-                               ring->semaphore.mbox.wait[RCS] = MI_SEMAPHORE_SYNC_INVALID;
-                               ring->semaphore.mbox.wait[VCS] = MI_SEMAPHORE_SYNC_INVALID;
-                               ring->semaphore.mbox.wait[BCS] = MI_SEMAPHORE_SYNC_INVALID;
-                               ring->semaphore.mbox.wait[VECS] = MI_SEMAPHORE_SYNC_INVALID;
-                               ring->semaphore.mbox.wait[VCS2] = MI_SEMAPHORE_SYNC_INVALID;
-                               ring->semaphore.mbox.signal[RCS] = GEN6_NOSYNC;
-                               ring->semaphore.mbox.signal[VCS] = GEN6_NOSYNC;
-                               ring->semaphore.mbox.signal[BCS] = GEN6_NOSYNC;
-                               ring->semaphore.mbox.signal[VECS] = GEN6_NOSYNC;
-                               ring->semaphore.mbox.signal[VCS2] = GEN6_NOSYNC;
+                               ring->semaphore.signal = gen8_xcs_signal;
+                               GEN8_RING_SEMAPHORE_INIT;
                        }
                } else {
                        ring->irq_enable_mask = GT_BSD_USER_INTERRUPT;
@@ -2273,24 +2344,10 @@ int intel_init_bsd2_ring_buffer(struct drm_device *dev)
        ring->dispatch_execbuffer =
                        gen8_ring_dispatch_execbuffer;
        ring->semaphore.sync_to = gen6_ring_sync;
-       ring->semaphore.signal = gen6_signal;
-       /*
-        * The current semaphore is only applied on the pre-gen8. And there
-        * is no bsd2 ring on the pre-gen8. So now the semaphore_register
-        * between VCS2 and other ring is initialized as invalid.
-        * Gen8 will initialize the sema between VCS2 and other ring later.
-        */
-       ring->semaphore.mbox.wait[RCS] = MI_SEMAPHORE_SYNC_INVALID;
-       ring->semaphore.mbox.wait[VCS] = MI_SEMAPHORE_SYNC_INVALID;
-       ring->semaphore.mbox.wait[BCS] = MI_SEMAPHORE_SYNC_INVALID;
-       ring->semaphore.mbox.wait[VECS] = MI_SEMAPHORE_SYNC_INVALID;
-       ring->semaphore.mbox.wait[VCS2] = MI_SEMAPHORE_SYNC_INVALID;
-       ring->semaphore.mbox.signal[RCS] = GEN6_NOSYNC;
-       ring->semaphore.mbox.signal[VCS] = GEN6_NOSYNC;
-       ring->semaphore.mbox.signal[BCS] = GEN6_NOSYNC;
-       ring->semaphore.mbox.signal[VECS] = GEN6_NOSYNC;
-       ring->semaphore.mbox.signal[VCS2] = GEN6_NOSYNC;
-
+       if (i915_semaphore_is_enabled(dev)) {
+               ring->semaphore.signal = gen8_xcs_signal;
+               GEN8_RING_SEMAPHORE_INIT;
+       }
        ring->init = init_ring_common;
 
        return intel_init_ring_buffer(dev, ring);
@@ -2318,17 +2375,8 @@ int intel_init_blt_ring_buffer(struct drm_device *dev)
                ring->dispatch_execbuffer = gen8_ring_dispatch_execbuffer;
                if (i915_semaphore_is_enabled(dev)) {
                        ring->semaphore.sync_to = gen6_ring_sync;
-                       ring->semaphore.signal = gen6_signal;
-                       ring->semaphore.mbox.wait[RCS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[VCS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[BCS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[VECS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[VCS2] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.signal[RCS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[VCS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[BCS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[VECS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[VCS2] = GEN6_NOSYNC;
+                       ring->semaphore.signal = gen8_xcs_signal;
+                       GEN8_RING_SEMAPHORE_INIT;
                }
        } else {
                ring->irq_enable_mask = GT_BLT_USER_INTERRUPT;
@@ -2385,17 +2433,8 @@ int intel_init_vebox_ring_buffer(struct drm_device *dev)
                ring->dispatch_execbuffer = gen8_ring_dispatch_execbuffer;
                if (i915_semaphore_is_enabled(dev)) {
                        ring->semaphore.sync_to = gen6_ring_sync;
-                       ring->semaphore.signal = gen6_signal;
-                       ring->semaphore.mbox.wait[RCS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[VCS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[BCS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[VECS] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.wait[VCS2] = MI_SEMAPHORE_SYNC_INVALID;
-                       ring->semaphore.mbox.signal[RCS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[VCS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[BCS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[VECS] = GEN6_NOSYNC;
-                       ring->semaphore.mbox.signal[VCS2] = GEN6_NOSYNC;
+                       ring->semaphore.signal = gen8_xcs_signal;
+                       GEN8_RING_SEMAPHORE_INIT;
                }
        } else {
                ring->irq_enable_mask = PM_VEBOX_USER_INTERRUPT;

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 2e8b51645bcbeb84b7f3f39515f753d5b6ef972b..17ef9cdf1b8ddc7a81ad18269b8582a71def8bea 100644 (file)
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -40,6 +40,32 @@ struct  intel_hw_status_page {
 #define I915_READ_MODE(ring) I915_READ(RING_MI_MODE((ring)->mmio_base))
 #define I915_WRITE_MODE(ring, val) I915_WRITE(RING_MI_MODE((ring)->mmio_base), val)
 
+/* seqno size is actually only a uint32, but since we plan to use MI_FLUSH_DW to
+ * do the writes, and that must have qw aligned offsets, simply pretend it's 8b.
+ */
+#define i915_semaphore_seqno_size sizeof(uint64_t)
+#define GEN8_SIGNAL_OFFSET(__ring, to)                      \
+       (i915_gem_obj_ggtt_offset(dev_priv->semaphore_obj) + \
+       ((__ring)->id * I915_NUM_RINGS * i915_semaphore_seqno_size) +   \
+       (i915_semaphore_seqno_size * (to)))
+
+#define GEN8_WAIT_OFFSET(__ring, from)                      \
+       (i915_gem_obj_ggtt_offset(dev_priv->semaphore_obj) + \
+       ((from) * I915_NUM_RINGS * i915_semaphore_seqno_size) + \
+       (i915_semaphore_seqno_size * (__ring)->id))
+
+#define GEN8_RING_SEMAPHORE_INIT do { \
+       if (!dev_priv->semaphore_obj) { \
+               break; \
+       } \
+       ring->semaphore.signal_ggtt[RCS] = GEN8_SIGNAL_OFFSET(ring, RCS); \
+       ring->semaphore.signal_ggtt[VCS] = GEN8_SIGNAL_OFFSET(ring, VCS); \
+       ring->semaphore.signal_ggtt[BCS] = GEN8_SIGNAL_OFFSET(ring, BCS); \
+       ring->semaphore.signal_ggtt[VECS] = GEN8_SIGNAL_OFFSET(ring, VECS); \
+       ring->semaphore.signal_ggtt[VCS2] = GEN8_SIGNAL_OFFSET(ring, VCS2); \
+       ring->semaphore.signal_ggtt[ring->id] = MI_SEMAPHORE_SYNC_INVALID; \
+       } while(0)
+
 enum intel_ring_hangcheck_action {
        HANGCHECK_IDLE = 0,
        HANGCHECK_WAIT,
@@ -127,15 +153,55 @@ struct  intel_engine_cs {
 #define I915_DISPATCH_PINNED 0x2
        void            (*cleanup)(struct intel_engine_cs *ring);
 
+       /* GEN8 signal/wait table - never trust comments!
+        *        signal to     signal to    signal to   signal to      signal to
+        *          RCS            VCS          BCS        VECS          VCS2
+        *      --------------------------------------------------------------------
+        *  RCS | NOP (0x00) | VCS (0x08) | BCS (0x10) | VECS (0x18) | VCS2 (0x20) |
+        *      |-------------------------------------------------------------------
+        *  VCS | RCS (0x28) | NOP (0x30) | BCS (0x38) | VECS (0x40) | VCS2 (0x48) |
+        *      |-------------------------------------------------------------------
+        *  BCS | RCS (0x50) | VCS (0x58) | NOP (0x60) | VECS (0x68) | VCS2 (0x70) |
+        *      |-------------------------------------------------------------------
+        * VECS | RCS (0x78) | VCS (0x80) | BCS (0x88) |  NOP (0x90) | VCS2 (0x98) |
+        *      |-------------------------------------------------------------------
+        * VCS2 | RCS (0xa0) | VCS (0xa8) | BCS (0xb0) | VECS (0xb8) | NOP  (0xc0) |
+        *      |-------------------------------------------------------------------
+        *
+        * Generalization:
+        *  f(x, y) := (x->id * NUM_RINGS * seqno_size) + (seqno_size * y->id)
+        *  ie. transpose of g(x, y)
+        *
+        *       sync from      sync from    sync from    sync from     sync from
+        *          RCS            VCS          BCS        VECS          VCS2
+        *      --------------------------------------------------------------------
+        *  RCS | NOP (0x00) | VCS (0x28) | BCS (0x50) | VECS (0x78) | VCS2 (0xa0) |
+        *      |-------------------------------------------------------------------
+        *  VCS | RCS (0x08) | NOP (0x30) | BCS (0x58) | VECS (0x80) | VCS2 (0xa8) |
+        *      |-------------------------------------------------------------------
+        *  BCS | RCS (0x10) | VCS (0x38) | NOP (0x60) | VECS (0x88) | VCS2 (0xb0) |
+        *      |-------------------------------------------------------------------
+        * VECS | RCS (0x18) | VCS (0x40) | BCS (0x68) |  NOP (0x90) | VCS2 (0xb8) |
+        *      |-------------------------------------------------------------------
+        * VCS2 | RCS (0x20) | VCS (0x48) | BCS (0x70) | VECS (0x98) |  NOP (0xc0) |
+        *      |-------------------------------------------------------------------
+        *
+        * Generalization:
+        *  g(x, y) := (y->id * NUM_RINGS * seqno_size) + (seqno_size * x->id)
+        *  ie. transpose of f(x, y)
+        */
        struct {
                u32     sync_seqno[I915_NUM_RINGS-1];
 
-               struct {
-                       /* our mbox written by others */
-                       u32             wait[I915_NUM_RINGS];
-                       /* mboxes this ring signals to */
-                       u32             signal[I915_NUM_RINGS];
-               } mbox;
+               union {
+                       struct {
+                               /* our mbox written by others */
+                               u32             wait[I915_NUM_RINGS];
+                               /* mboxes this ring signals to */
+                               u32             signal[I915_NUM_RINGS];
+                       } mbox;
+                       u64             signal_ggtt[I915_NUM_RINGS];
+               };
 
                /* AKA wait() */
                int     (*sync_to)(struct intel_engine_cs *ring,