Reviewed and revised bo reference counting.

Change-Id: I9d827678692207101565e235b44b62ffee756dc7

8 files changed, 141 insertions, 80 deletions

diff --git a/README b/README
index e892d8f..4329914 100644
--- a/README
+++ b/README
@@ -25,3 +25,26 @@ You can also copy src/drmmode_template into src/drmmode_<yourdrivername> and bui
 $ ./configure --with-drmmode=<yourdrivername>
 
 
+Summary of bo reference counting
+--------------------------------
+The Screen takes a ref on the scanout bo in InitScreen and drops it in ScreenClose. When the scanout bo changes
+(due to a flip or a modeset) the ref is moved from the old bo to the new one by set_scanout_bo.
+
+Pixmaps take a ref on their bo(s) when created in ARMSOCCreatePixmap2 and drop it in ARMSOCDestroyPixmap.
+If ARMSOCModifyPixmapHeader points a pixmap at anything other than the scanout bo then the ref to
+the existing bo (if any) is dropped.
+If ARMSOCModifyPixmapHeader points a pixmap at the scanout bo the ref is moved from the old bo to the new
+If ARMSOCModifyPixmapHeader changes the size of the pixmap's bo the ref is dropped, a new bo created and a
+ref taken on that.
+
+resize_scanout_bo creates and takes a ref on the new bo and drops its ref when the new bo becomes the scanout bo and
+the Screen has taken a ref.
+
+The swap chain takes a ref on the src and dst bos when a swap is scheduled in ARMSOCDRI2ScheduleSwap and drops them
+in ARMSOCDRI2SwapComplete after the src becomes pARMSOC->scanout and the screen takes a ref. 
+
+The cursor takes a ref on its bo in drmmode_cursor_init and drops it in drmmode_cursor_fini
+
+
+
+
diff --git a/src/armsoc_dri2.c b/src/armsoc_dri2.c
index a4dfe7d..c0870b1 100644
--- a/src/armsoc_dri2.c
+++ b/src/armsoc_dri2.c
@@ -286,7 +286,7 @@ ARMSOCDRI2DestroyBuffer(DrawablePtr pDraw, DRI2BufferPtr buffer)
 	if (--buf->refcnt > 0)
 		return;
 
-	DEBUG_MSG("pDraw=%p, buffer=%p", pDraw, buffer);
+	DEBUG_MSG("pDraw=%p, DRIbuffer=%p", pDraw, buffer);
 
 	if (buffer->attachment == DRI2BufferBackLeft) {
 		assert(pARMSOC->driNumBufs > 1);
@@ -307,6 +307,7 @@ static void
 ARMSOCDRI2ReferenceBuffer(DRI2BufferPtr buffer)
 {
 	struct ARMSOCDRI2BufferRec *buf = ARMSOCBUF(buffer);
+
 	buf->refcnt++;
 }
 
@@ -387,9 +388,9 @@ ARMSOCDRI2GetMSC(DrawablePtr pDraw, CARD64 *ust, CARD64 *msc)
 
 #if DRI2INFOREC_VERSION >= 6
 /**
- * Validates that the swap limit range is within the range of supported
- * asynchronous buffer flips. This is bounded by the DRI2MaxBuffers option plus
- * one additional flip in case of early display usage.
+ * Called by DRI2 to validate that any new swap limit being set by
+ * DRI2 is in range. In our case the range is 1 to the DRI2MaxBuffers
+ * option, plus one in the case of early display usage.
  */
 static Bool
 ARMSOCDRI2SwapLimitValidate(DrawablePtr pDraw, int swap_limit) {
@@ -423,12 +424,12 @@ struct ARMSOCDRISwapCmd {
 	DRI2BufferPtr pDstBuffer;
 	DRI2BufferPtr pSrcBuffer;
 	DRI2SwapEventPtr func;
-	int swapCount;
+	int swapCount; /* number of crtcs with flips in flight for this swap */
 	int flags;
 	void *data;
-	struct armsoc_bo *old_src_bo;
-	struct armsoc_bo *old_dst_bo;
-	struct armsoc_bo *new_scanout;
+	struct armsoc_bo *old_src_bo;  /* Swap chain holds ref on src bo */
+	struct armsoc_bo *old_dst_bo;  /* Swap chain holds ref on dst bo */
+	struct armsoc_bo *new_scanout; /* scanout to be used after swap */
 	unsigned int swap_id;
 };
 
@@ -562,33 +563,40 @@ void updateResizedBuffer(ScrnInfoPtr pScrn, void *buffer,
 	for (i = 0; i < buf->numPixmaps; i++) {
 		if (buf->pPixmaps[i] != NULL) {
 			struct ARMSOCPixmapPrivRec *priv = exaGetPixmapDriverPrivate(buf->pPixmaps[i]);
+
 			if (old_bo == priv->bo) {
 				int ret;
-				ret = armsoc_bo_get_name(resized_bo, &dri2buf->name);
-				assert(!ret);
+
+				/* Update the buffer name if this pixmap is current */
+				if (i == buf->currentPixmap) {
+					ret = armsoc_bo_get_name(resized_bo, &dri2buf->name);
+					assert(!ret);
+				}
+
+				/* pixmap takes ref on resized bo */
+				armsoc_bo_reference(resized_bo);
+				/* replace the old_bo with the resized_bo */
 				priv->bo = resized_bo;
+				/* pixmap drops ref on old bo */
+				armsoc_bo_unreference(old_bo);
 			}
 		}
 	}
 }
 
-
 void
 ARMSOCDRI2ResizeSwapChain(ScrnInfoPtr pScrn, struct armsoc_bo *old_bo,
-		struct armsoc_bo *resized_bo) {
-	PixmapPtr rootPixmap;
-	struct ARMSOCPixmapPrivRec *rootPriv;
+		struct armsoc_bo *resized_bo)
+{
 	struct ARMSOCRec *pARMSOC = ARMSOCPTR(pScrn);
 	struct ARMSOCDRISwapCmd *cmd = NULL;
 	int i;
-	int back = pARMSOC->swap_chain_count - 1;
-
-	rootPixmap = pScrn->pScreen->GetScreenPixmap(pScrn->pScreen);
-	rootPriv = exaGetPixmapDriverPrivate(rootPixmap);
-	/* We need to access the front to back (count-1) % size. */
+	int back = pARMSOC->swap_chain_count - 1; /* The last swap scheduled */
 
+	/* Update the bos for each scheduled swap in the swap chain */
 	for (i = 0; i < pARMSOC->swap_chain_size && back >= 0; i++) {
 		unsigned int idx = back % pARMSOC->swap_chain_size;
+
 		cmd = pARMSOC->swap_chain[idx];
 		back--;
 		if (!cmd)
@@ -596,12 +604,6 @@ ARMSOCDRI2ResizeSwapChain(ScrnInfoPtr pScrn, struct armsoc_bo *old_bo,
 		updateResizedBuffer(pScrn, cmd->pSrcBuffer, old_bo, resized_bo);
 		updateResizedBuffer(pScrn, cmd->pDstBuffer, old_bo, resized_bo);
 	}
-
-	/* The current front buffer might be several frames ahead of scanout. So we
-	 * need to check if the current front buffer owned the previous scanout,
-	 * i.e. the one we are deleting. If so we will transfer the ownership. */
-	if (0 == armsoc_bo_unreference(old_bo) && rootPriv->bo == old_bo)
-		rootPriv->bo = resized_bo;
 }
 
 
@@ -615,11 +617,12 @@ ARMSOCDRI2SwapComplete(struct ARMSOCDRISwapCmd *cmd)
 	unsigned int idx;
 	int status;
 
-	if (--cmd->swapCount > 0)
+	if (--cmd->swapCount > 0) /* wait for all crtcs to flip */
 		return;
 
 	if ((cmd->flags & ARMSOC_SWAP_FAIL) == 0) {
-		DEBUG_MSG("%s complete: %d -> %d", swap_names[cmd->type],
+		DEBUG_MSG("swap %d %s complete: %d -> %d",
+			cmd->swap_id, swap_names[cmd->type],
 			cmd->pSrcBuffer->attachment,
 			cmd->pDstBuffer->attachment);
 		status = dixLookupDrawable(&pDraw, cmd->draw_id, serverClient,
@@ -635,7 +638,11 @@ ARMSOCDRI2SwapComplete(struct ARMSOCDRISwapCmd *cmd)
 				assert(cmd->type == DRI2_FLIP_COMPLETE);
 				set_scanout_bo(pScrn, cmd->new_scanout);
 			}
+		} else {
+			ERROR_MSG("dixLookupDrawable fail on swap complete");
 		}
+	} else {
+		ERROR_MSG("swap %d ARMSOC_SWAP_FAIL on swap complete", cmd->swap_id);
 	}
 
 	/* drop extra refcnt we obtained prior to swap:
@@ -643,14 +650,15 @@ ARMSOCDRI2SwapComplete(struct ARMSOCDRISwapCmd *cmd)
 	ARMSOCDRI2DestroyBuffer(pDraw, cmd->pSrcBuffer);
 	ARMSOCDRI2DestroyBuffer(pDraw, cmd->pDstBuffer);
 
-	/* drop extra reference of the actual buffer objects used */
+	/* swap chain drops ref on original src bo */
 	armsoc_bo_unreference(cmd->old_src_bo);
+	/* swap chain drops ref on original dst bo */
 	armsoc_bo_unreference(cmd->old_dst_bo);
 
 	if (cmd->type == DRI2_FLIP_COMPLETE)
 		pARMSOC->pending_flips--;
 
-	/* Free the swap operation and progress the swap chain. */
+	/* Free the swap cmd and remove it from the swap chain. */
 	idx = cmd->swap_id % pARMSOC->swap_chain_size;
 	assert(pARMSOC->swap_chain[idx] == cmd);
 	free(cmd);
@@ -709,10 +717,8 @@ ARMSOCDRI2ScheduleSwap(ClientPtr client, DrawablePtr pDraw,
 	DamageRegionAppend(&pDstPixmap->drawable, &region);
 	DamageRegionProcessPending(&pDstPixmap->drawable);
 
-	DEBUG_MSG("%d -> %d", pSrcBuffer->attachment, pDstBuffer->attachment);
-
-	/* obtain extra ref on buffers to avoid them going away while we await
-	 * the page flip event:
+	/* obtain extra ref on DRI buffers to avoid them going
+	 * away while we await the page flip event.
 	 */
 	ARMSOCDRI2ReferenceBuffer(pSrcBuffer);
 	ARMSOCDRI2ReferenceBuffer(pDstBuffer);
@@ -723,13 +729,16 @@ ARMSOCDRI2ScheduleSwap(ClientPtr client, DrawablePtr pDraw,
 	src_fb_id = armsoc_bo_get_fb(src_bo);
 	dst_fb_id = armsoc_bo_get_fb(dst_bo);
 
-	/* Store and reference actual buffer-objects used as they could
-	 * be exchanged under-the-hood if doing a page flip. */
+	/* Store and reference actual buffer-objects used in case
+	 * the pixmaps disappear.
+	 */
 	cmd->old_src_bo = src_bo;
 	cmd->old_dst_bo = dst_bo;
 
-	armsoc_bo_reference(src_bo);
-	armsoc_bo_reference(dst_bo);
+	/* Swap chain takes a ref on original src bo */
+	armsoc_bo_reference(cmd->old_src_bo);
+	/* Swap chain takes a ref on original dst bo */
+	armsoc_bo_reference(cmd->old_dst_bo);
 
 	/* Add swap operation to the swap chain */
 	cmd->swap_id = pARMSOC->swap_chain_count++;
@@ -737,6 +746,9 @@ ARMSOCDRI2ScheduleSwap(ClientPtr client, DrawablePtr pDraw,
 	assert(NULL == pARMSOC->swap_chain[idx]);
 	pARMSOC->swap_chain[idx] = cmd;
 
+	DEBUG_MSG("SWAP %d SCHEDULED : %d -> %d ", cmd->swap_id,
+				pSrcBuffer->attachment, pDstBuffer->attachment);
+
 	new_canflip = canflip(pDraw);
 	if ((src->previous_canflip != new_canflip) ||
 	    (dst->previous_canflip != new_canflip)) {
@@ -775,7 +787,7 @@ ARMSOCDRI2ScheduleSwap(ClientPtr client, DrawablePtr pDraw,
 			(armsoc_bo_height(src_bo) == armsoc_bo_height(dst_bo));
 
 	if (do_flip) {
-		DEBUG_MSG("can flip:  %d -> %d", src_fb_id, dst_fb_id);
+		DEBUG_MSG("FLIPPING:  FB%d -> FB%d", src_fb_id, dst_fb_id);
 		cmd->type = DRI2_FLIP_COMPLETE;
 		/* TODO: MIDEGL-1461: Handle rollback if multiple CRTC flip is
 		 * only partially successful
@@ -800,7 +812,6 @@ ARMSOCDRI2ScheduleSwap(ClientPtr client, DrawablePtr pDraw,
 				cmd->swapCount = 0;
 
 			cmd->new_scanout = boFromBuffer(pDstBuffer);
-
 			if (cmd->swapCount == 0)
 				ARMSOCDRI2SwapComplete(cmd);
 
@@ -814,9 +825,9 @@ ARMSOCDRI2ScheduleSwap(ClientPtr client, DrawablePtr pDraw,
 			else
 				cmd->swapCount = 0;
 
-			/* Here we have successfully scheduled a flip. We now
-			 * need to exchange buffers between src and dst pixmaps
-			 * and obtain the next buffer.
+			/* Flip successfully scheduled.
+			 * Now exchange bos between src and dst pixmaps
+			 * and select the next bo for the back buffer.
 			 */
 			if (ret) {
 				assert(cmd->type == DRI2_FLIP_COMPLETE);
@@ -826,10 +837,11 @@ ARMSOCDRI2ScheduleSwap(ClientPtr client, DrawablePtr pDraw,
 					nextBuffer(pDraw, ARMSOCBUF(pSrcBuffer));
 			}
 
-			/* We need to store the new scanout now as the
-			 * destination buffer might be switched before the cb. */
+			/* Store the new scanout bo now as the destination
+			 * buffer bo might be exchanged if another swap is
+			 * scheduled before this swap completes
+			 */
 			cmd->new_scanout = boFromBuffer(pDstBuffer);
-
 			if (cmd->swapCount == 0)
 				ARMSOCDRI2SwapComplete(cmd);
 		}
@@ -842,6 +854,8 @@ ARMSOCDRI2ScheduleSwap(ClientPtr client, DrawablePtr pDraw,
 				.y2 = pDraw->height,
 		};
 		RegionRec region;
+
+		DEBUG_MSG("BLITTING");
 		RegionInit(&region, &box, 0);
 		ARMSOCDRI2CopyRegion(pDraw, &region, pDstBuffer, pSrcBuffer);
 		cmd->type = DRI2_BLIT_COMPLETE;
@@ -934,7 +948,7 @@ ARMSOCDRI2ScreenInit(ScreenPtr pScreen)
 #endif
 	}
 	pARMSOC->swap_chain = calloc(pARMSOC->swap_chain_size,
-		sizeof *pARMSOC->swap_chain);
+		sizeof(*pARMSOC->swap_chain));
 
 	INFO_MSG("Setting swap chain size: %d ", pARMSOC->swap_chain_size);
 
diff --git a/src/armsoc_driver.c b/src/armsoc_driver.c
index ddd650b..6e21182 100644
--- a/src/armsoc_driver.c
+++ b/src/armsoc_driver.c
@@ -663,7 +663,7 @@ ARMSOCProbe(DriverPtr drv, int flags)
 			 * driverPrivate field.
 			 */
 			pScrn->driverPrivate =
-					calloc(1, sizeof *pARMSOC);
+					calloc(1, sizeof(*pARMSOC));
 			if (!pScrn->driverPrivate)
 				return FALSE;
 
@@ -973,6 +973,7 @@ ARMSOCScreenInit(SCREEN_INIT_ARGS_DECL)
 			pScrn->virtualX, pScrn->virtualY,
 			depth, pScrn->bitsPerPixel);
 	assert(!pARMSOC->scanout);
+	/* Screen creates and takes a ref on the scanout bo */
 	pARMSOC->scanout = armsoc_bo_new_with_dim(pARMSOC->dev, pScrn->virtualX,
 			pScrn->virtualY, depth, pScrn->bitsPerPixel,
 			ARMSOC_BO_SCANOUT);
@@ -1163,7 +1164,7 @@ fail3:
 	miClearVisualTypes();
 
 fail2:
-	/* release the scanout buffer */
+	/* Screen drops its ref on scanout bo on failure exit */
 	armsoc_bo_unreference(pARMSOC->scanout);
 	pARMSOC->scanout = NULL;
 	pScrn->displayWidth = 0;
@@ -1228,7 +1229,9 @@ ARMSOCCloseScreen(CLOSE_SCREEN_ARGS_DECL)
 		if (pARMSOC->pARMSOCEXA->CloseScreen)
 			pARMSOC->pARMSOCEXA->CloseScreen(CLOSE_SCREEN_ARGS);
 
-	/* scanout buffer is released when root pixmap is destroyed */
+	assert(pARMSOC->scanout);
+	/* Screen drops its ref on the scanout buffer */
+	armsoc_bo_unreference(pARMSOC->scanout);
 	pARMSOC->scanout = NULL;
 
 	pScrn->displayWidth = 0;
@@ -1243,8 +1246,6 @@ ARMSOCCloseScreen(CLOSE_SCREEN_ARGS_DECL)
 	return ret;
 }
 
-
-
 /**
  * Adjust the screen pixmap for the current location of the front buffer.
  * This is done at EnterVT when buffers are bound as long as the resources
diff --git a/src/armsoc_driver.h b/src/armsoc_driver.h
index a22fe5d..4621dcc 100644
--- a/src/armsoc_driver.h
+++ b/src/armsoc_driver.h
@@ -154,12 +154,13 @@ struct ARMSOCRec {
 	/* The Swap Chain stores the pending swap operations */
 	struct ARMSOCDRISwapCmd            **swap_chain;
 
-	/* This is the swap chain's count used to track last swap cmd. */
+	/* Count of swaps scheduled since startup.
+	 * Used as swap_id of the next swap cmd */
 	unsigned int                       swap_chain_count;
 
-	/* Defines swap chain size. */
+	/* Size of the swap chain. Set to 1 if DRI2SwapLimit unsupported,
+	 * driNumBufs if early display enabled, otherwise driNumBufs-1 */
 	unsigned int                       swap_chain_size;
-
 };
 
 /*
diff --git a/src/armsoc_dumb.c b/src/armsoc_dumb.c
index 74f512a..8f29b7a 100644
--- a/src/armsoc_dumb.c
+++ b/src/armsoc_dumb.c
@@ -70,7 +70,7 @@ struct armsoc_device *armsoc_device_new(int fd,
 			int (*create_custom_gem)(int fd,
 				struct armsoc_create_gem *create_gem))
 {
-	struct armsoc_device *new_dev = calloc(1, sizeof *new_dev);
+	struct armsoc_device *new_dev = calloc(1, sizeof(*new_dev));
 	if (!new_dev)
 		return NULL;
 
@@ -201,18 +201,14 @@ static void armsoc_bo_del(struct armsoc_bo *bo)
 	free(bo);
 }
 
-int armsoc_bo_unreference(struct armsoc_bo *bo)
+void armsoc_bo_unreference(struct armsoc_bo *bo)
 {
-	int refcnt;
 	if (!bo)
-		return 0;
+		return;
 
 	assert(bo->refcnt > 0);
-	refcnt = bo->refcnt;
 	if (--bo->refcnt == 0)
 		armsoc_bo_del(bo);
-
-	return --refcnt;
 }
 
 void armsoc_bo_reference(struct armsoc_bo *bo)
@@ -361,9 +357,10 @@ int armsoc_bo_add_fb(struct armsoc_bo *bo)
 		bo->bpp, bo->pitch, bo->handle, &bo->fb_id);
 
 	if (ret < 0 && bo->bpp == 32 && bo->depth == 32 && bo->dev->alpha_supported) {
-		/* The DRM driver may not support an alpha channel but it is possible
-		 * to continue by ignoring the alpha, so if an attempt to create
-		 * a depth 32, bpp 32 framebuffer fails we retry with depth 24, bpp 32
+		/* The DRM driver may not support an alpha channel but
+		 * it is possible to continue by ignoring the alpha, so
+		 * if an attempt to create a depth 32, bpp 32 framebuffer
+		 * fails we retry with depth 24, bpp 32
 		 */
 		xf86DrvMsg(-1, X_WARNING,
 				"depth 32 FB unsupported : falling back to depth 24\n");
diff --git a/src/armsoc_dumb.h b/src/armsoc_dumb.h
index c808f98..a299ccf 100644
--- a/src/armsoc_dumb.h
+++ b/src/armsoc_dumb.h
@@ -81,7 +81,7 @@ uint32_t armsoc_bo_bpp(struct armsoc_bo *bo);
 uint32_t armsoc_bo_pitch(struct armsoc_bo *bo);
 
 void armsoc_bo_reference(struct armsoc_bo *bo);
-int armsoc_bo_unreference(struct armsoc_bo *bo);
+void armsoc_bo_unreference(struct armsoc_bo *bo);
 
 /* When dmabuf is set on a bo, armsoc_bo_cpu_prep()
  *  waits for KDS shared access
diff --git a/src/armsoc_exa.c b/src/armsoc_exa.c
index fc897ef..a578a27 100644
--- a/src/armsoc_exa.c
+++ b/src/armsoc_exa.c
@@ -1,4 +1,3 @@
-/* -*- mode: C; c-file-style: "k&r"; tab-width 4; indent-tabs-mode: t; -*- */
 
 /*
  * Copyright © 2011 Texas Instruments, Inc
@@ -78,7 +77,7 @@ ARMSOCCreatePixmap2(ScreenPtr pScreen, int width, int height,
 		int depth, int usage_hint, int bitsPerPixel,
 		int *new_fb_pitch)
 {
-	struct ARMSOCPixmapPrivRec *priv = calloc(1, sizeof *priv);
+	struct ARMSOCPixmapPrivRec *priv = calloc(1, sizeof(*priv));
 	ScrnInfoPtr pScrn = xf86ScreenToScrn(pScreen);
 	struct ARMSOCRec *pARMSOC = ARMSOCPTR(pScrn);
 	enum armsoc_buf_type buf_type = ARMSOC_BO_NON_SCANOUT;
@@ -90,6 +89,7 @@ ARMSOCCreatePixmap2(ScreenPtr pScreen, int width, int height,
 		buf_type = ARMSOC_BO_SCANOUT;
 
 	if (width > 0 && height > 0 && depth > 0 && bitsPerPixel > 0) {
+		/* Pixmap creates and takes a ref on its bo */
 		priv->bo = armsoc_bo_new_with_dim(pARMSOC->dev,
 				width,
 				height,
@@ -103,6 +103,7 @@ ARMSOCCreatePixmap2(ScreenPtr pScreen, int width, int height,
 			WARNING_MSG(
 					"Scanout buffer allocation failed, falling back to non-scanout");
 			buf_type = ARMSOC_BO_NON_SCANOUT;
+			/* Pixmap creates and takes a ref on its bo */
 			priv->bo = armsoc_bo_new_with_dim(pARMSOC->dev,
 					width,
 					height,
@@ -141,6 +142,7 @@ ARMSOCDestroyPixmap(ScreenPtr pScreen, void *driverPriv)
 	 * backing this pixmap. */
 	if (priv->bo) {
 		assert(!armsoc_bo_has_dmabuf(priv->bo));
+		/* pixmap drops ref on its bo */
 		armsoc_bo_unreference(priv->bo);
 	}
 
@@ -172,6 +174,10 @@ ARMSOCModifyPixmapHeader(PixmapPtr pPixmap, int width, int height,
 		/* scratch-pixmap (see GetScratchPixmapHeader()) gets recycled,
 		 * so could have a previous bo!
 		 */
+
+		/* scratch pixmap should not have a dmabuf */
+		assert(!armsoc_bo_has_dmabuf(priv->bo));
+		/* Pixmap drops ref on its old bo */
 		armsoc_bo_unreference(priv->bo);
 		priv->bo = NULL;
 
@@ -179,15 +185,19 @@ ARMSOCModifyPixmapHeader(PixmapPtr pPixmap, int width, int height,
 		return FALSE;
 	}
 
-	if (pPixData == armsoc_bo_map(pARMSOC->scanout)) {
+	/* Replacing the pixmap's current bo with the scanout bo */
+	if (pPixData == armsoc_bo_map(pARMSOC->scanout) && priv->bo != pARMSOC->scanout) {
 		struct armsoc_bo *old_bo = priv->bo;
+
 		priv->bo = pARMSOC->scanout;
+		/* pixmap takes a ref on its new bo */
 		armsoc_bo_reference(priv->bo);
 
 		if (old_bo) {
 			/* We are detaching the old_bo so clear it now. */
 			if (armsoc_bo_has_dmabuf(old_bo))
 				armsoc_bo_clear_dmabuf(old_bo);
+			/* pixmap drops ref on previous bo */
 			armsoc_bo_unreference(old_bo);
 		}
 	}
@@ -220,8 +230,9 @@ ARMSOCModifyPixmapHeader(PixmapPtr pPixmap, int width, int height,
 	if (armsoc_bo_width(priv->bo) != pPixmap->drawable.width ||
 	    armsoc_bo_height(priv->bo) != pPixmap->drawable.height ||
 	    armsoc_bo_bpp(priv->bo) != pPixmap->drawable.bitsPerPixel) {
-		/* re-allocate buffer! */
+		/* pixmap drops ref on its old bo */
 		armsoc_bo_unreference(priv->bo);
+		/* pixmap creates new bo and takes ref on it */
 		priv->bo = armsoc_bo_new_with_dim(pARMSOC->dev,
 				pPixmap->drawable.width,
 				pPixmap->drawable.height,
@@ -235,6 +246,7 @@ ARMSOCModifyPixmapHeader(PixmapPtr pPixmap, int width, int height,
 			WARNING_MSG(
 					"Scanout buffer allocation failed, falling back to non-scanout");
 			buf_type = ARMSOC_BO_NON_SCANOUT;
+			/* pixmap creates new bo and takes ref on it */
 			priv->bo = armsoc_bo_new_with_dim(pARMSOC->dev,
 					pPixmap->drawable.width,
 					pPixmap->drawable.height,
diff --git a/src/drmmode_display.c b/src/drmmode_display.c
index ca7a42e..bfa1905 100644
--- a/src/drmmode_display.c
+++ b/src/drmmode_display.c
@@ -1458,10 +1458,10 @@ void set_scanout_bo(ScrnInfoPtr pScrn, struct armsoc_bo *bo)
 	/* It had better have a framebuffer if we're scanning it out */
 	assert(armsoc_bo_get_fb(bo));
 
-	armsoc_bo_reference(bo);
+	armsoc_bo_reference(bo); /* Screen takes ref on new scanout bo */
 	pARMSOC->scanout = bo;
 	if (old_scanout)
-		armsoc_bo_unreference(old_scanout);
+		armsoc_bo_unreference(old_scanout); /* Screen drops ref on old scanout bo */
 }
 
 static Bool resize_scanout_bo(ScrnInfoPtr pScrn, int width, int height)
@@ -1475,12 +1475,12 @@ static Bool resize_scanout_bo(ScrnInfoPtr pScrn, int width, int height)
 
 	depth = armsoc_bo_depth(pARMSOC->scanout);
 	bpp = armsoc_bo_bpp(pARMSOC->scanout);
-	DEBUG_MSG("Resize: %dx%d %d,%d", width, height, depth, bpp );
+	DEBUG_MSG("Resize: %dx%d %d,%d", width, height, depth, bpp);
 
 	/* We don't expect the depth and bpp to change for the screen
 	 * assert this here as a check */
-	assert( depth == pScrn->bitsPerPixel );
-	assert( bpp == pScrn->bitsPerPixel );
+	assert(depth == pScrn->bitsPerPixel);
+	assert(bpp == pScrn->bitsPerPixel);
 
 	pScrn->virtualX = width;
 	pScrn->virtualY = height;
@@ -1489,7 +1489,7 @@ static Bool resize_scanout_bo(ScrnInfoPtr pScrn, int width, int height)
 		(height != armsoc_bo_height(pARMSOC->scanout))) {
 		struct armsoc_bo *new_scanout;
 
-		/* allocate new scanout buffer */
+		/* resize_scanout_bo creates and takes ref on new scanout bo */
 		new_scanout = armsoc_bo_new_with_dim(pARMSOC->dev,
 				width, height,
 				depth, bpp,
@@ -1526,10 +1526,12 @@ static Bool resize_scanout_bo(ScrnInfoPtr pScrn, int width, int height)
 			pitch = armsoc_bo_pitch(pARMSOC->scanout);
 		} else {
 			struct armsoc_bo *old_scanout = pARMSOC->scanout;
+
 			DEBUG_MSG("allocated new scanout buffer okay");
 			pitch = armsoc_bo_pitch(new_scanout);
 			/* clear new BO and add FB */
 			if (armsoc_bo_clear(new_scanout)) {
+				/* resize_scanout_bo drops ref on new scanout on failure exit */
 				armsoc_bo_unreference(new_scanout);
 				return FALSE;
 			}
@@ -1537,6 +1539,7 @@ static Bool resize_scanout_bo(ScrnInfoPtr pScrn, int width, int height)
 			if (armsoc_bo_add_fb(new_scanout)) {
 				ERROR_MSG(
 						"Failed to add framebuffer to the new scanout buffer");
+				/* resize_scanout_bo drops ref on new scanout on failure exit */
 				armsoc_bo_unreference(new_scanout);
 				return FALSE;
 			}
@@ -1551,14 +1554,14 @@ static Bool resize_scanout_bo(ScrnInfoPtr pScrn, int width, int height)
 					ERROR_MSG(
 							"Unable to attach dma_buf fd to new scanout buffer - %d (%s)\n",
 							res, strerror(res));
-					armsoc_bo_unreference(new_scanout);
+					armsoc_bo_unreference(new_scanout); /* resize_scanout_bo drops ref on new scanout on failure exit */
 					return FALSE;
 				}
 			}
 			/* use new scanout buffer */
 			set_scanout_bo(pScrn, new_scanout);
+			armsoc_bo_unreference(new_scanout); /* Screen has now taken ref on new_scanout so resize_scanout_bo drops it */
 
-			/* Resize swap chain will delete old_scanout */
 			ARMSOCDRI2ResizeSwapChain(pScrn, old_scanout, new_scanout);
 		}
 		pScrn->displayWidth = pitch / ((pScrn->bitsPerPixel + 7) / 8);
@@ -1568,10 +1571,20 @@ static Bool resize_scanout_bo(ScrnInfoPtr pScrn, int width, int height)
 	if (pScreen && pScreen->ModifyPixmapHeader) {
 		PixmapPtr rootPixmap = pScreen->GetScreenPixmap(pScreen);
 
+		/* Wrap the screen pixmap around the new scanout bo.
+		 * If we are n-buffering and the scanout bo is behind the
+		 * screen pixmap by a few flips, the bo which is being resized
+		 * may not belong to the screen pixmap. However we need to
+		 * resize the screen pixmap in order to continue flipping.
+		 * For now we let this happen and the swap chain reference
+		 * on the screen pixmap's existing bo will prevent it being
+		 * deleted here. Things may look odd until the swap chain
+		 * works through. This needs improvement.
+		 */
 		pScreen->ModifyPixmapHeader(rootPixmap,
-				pScrn->virtualX, pScrn->virtualY,
-				depth, bpp, pitch,
-				armsoc_bo_map(pARMSOC->scanout));
+			pScrn->virtualX, pScrn->virtualY,
+			depth, bpp, pitch,
+			armsoc_bo_map(pARMSOC->scanout));
 
 		/* Bump the serial number to ensure that all existing DRI2
 		 * buffers are invalidated.