drm/vc4: hvs: Defer dlist slots deallocation

During normal operations, the cursor position update is done through an
asynchronous plane update, which on the vc4 driver basically just
modifies the right dlist word to move the plane to the new coordinates.

However, when we have the overscan margins setup, we fall back to a
regular commit when we are next to the edges. And since that commit
happens to be on a cursor plane, it's considered a legacy cursor update
by KMS.

The main difference it makes is that it won't wait for its completion
(ie, next vblank) before returning. This means if we have multiple
commits happening in rapid succession, we can have several of them
happening before the next vblank.

In parallel, our dlist allocation is tied to a CRTC state, and each time
we do a commit we end up with a new CRTC state, with the previous one
being freed. This means that we free our previous dlist entry (but don't
clear it though) every time a new one is being committed.

Now, if we were to have two commits happening before the next vblank, we
could end up freeing reusing the same dlist entries before the next
vblank.

Indeed, we would start from an initial state taking, for example, the
dlist entries 10 to 20, then start a commit taking the entries 20 to 30
and setting the dlist pointer to 20, and freeing the dlist entries 10 to
20. However, since we haven't reach vblank yet, the HVS is still using
the entries 10 to 20.

If we were to make a new commit now, chances are the allocator are going
to give the 10 to 20 entries back, and we would change their content to
match the new state. If vblank hasn't happened yet, we just corrupted
the active dlist entries.

A first attempt to solve this was made by creating an intermediate dlist
buffer to store the current (ie, as of the last commit) dlist content,
that we would update each time the HVS is done with a frame. However, if
the interrupt handler missed the vblank window, we would end up copying
our intermediate dlist to the hardware one during the composition,
essentially creating the same issue.

Since making sure that our interrupt handler runs within a fixed,
constrained, time window would require to make Linux a real-time kernel,
this seems a bit out of scope.

Instead, we can work around our original issue by keeping the dlist
slots allocation longer. That way, we won't reuse a dlist slot while
it's still in flight. In order to achieve this, instead of freeing the
dlist slot when its associated CRTC state is destroyed, we'll queue it
in a list.

A naive implementation would free the buffers in that queue when we get
our end of frame interrupt. However, there's still a race since, just
like in the shadow dlist case, we don't control when the handler for
that interrupt is going to run. Thus, we can end up with a commit adding
an old dlist allocation to our queue during the window between our
actual interrupt and when our handler will run. And since that buffer is
still being used for the composition of the current frame, we can't free
it right away, exposing us to the original bug.

Fortunately for us, the hardware provides a frame counter that is
increased each time the first line of a frame is being generated.
Associating the frame counter the image is supposed to go away to the
allocation, and then only deallocate buffers that have a counter below
or equal to the one we see when the deallocation code should prevent the
above race from occuring.

Signed-off-by: Maxime Ripard <[email protected]>

Author: mripard

drivers/gpu/drm/vc4/vc4_crtc.c

@@ -1096,14 +1096,8 @@ void vc4_crtc_destroy_state(struct drm_crtc *crtc,
 	struct vc4_dev *vc4 = to_vc4_dev(crtc->dev);
 	struct vc4_crtc_state *vc4_state = to_vc4_crtc_state(state);
 
-	if (drm_mm_node_allocated(&vc4_state->mm)) {
-		unsigned long flags;
-
-		spin_lock_irqsave(&vc4->hvs->mm_lock, flags);
-		drm_mm_remove_node(&vc4_state->mm);
-		spin_unlock_irqrestore(&vc4->hvs->mm_lock, flags);
-
-	}
+	vc4_hvs_mark_dlist_entry_stale(vc4->hvs, vc4_state->mm);
+	vc4_state->mm = NULL;
 
 	drm_atomic_helper_crtc_destroy_state(crtc, state);
 }

drivers/gpu/drm/vc4/vc4_drv.h

@@ -346,6 +346,9 @@ struct vc4_hvs {
 	struct drm_mm lbm_mm;
 	spinlock_t mm_lock;
 
+	struct list_head stale_dlist_entries;
+	struct work_struct free_dlist_work;
+
 	struct drm_mm_node mitchell_netravali_filter;
 
 	struct debugfs_regset32 regset;
@@ -648,10 +651,16 @@ struct drm_connector *vc4_get_crtc_connector(struct drm_crtc *crtc,
 struct drm_encoder *vc4_get_crtc_encoder(struct drm_crtc *crtc,
 					 struct drm_crtc_state *state);
 
+struct vc4_hvs_dlist_allocation {
+	struct list_head node;
+	struct drm_mm_node mm_node;
+	unsigned int channel;
+	u8 target_frame_count;
+};
+
 struct vc4_crtc_state {
 	struct drm_crtc_state base;
-	/* Dlist area for this CRTC configuration. */
-	struct drm_mm_node mm;
+	struct vc4_hvs_dlist_allocation *mm;
 	bool txp_armed;
 	unsigned int assigned_channel;
 
@@ -1077,6 +1086,8 @@ struct vc4_hvs *__vc4_hvs_alloc(struct vc4_dev *vc4, struct platform_device *pde
 void vc4_hvs_stop_channel(struct vc4_hvs *hvs, unsigned int output);
 int vc4_hvs_get_fifo_from_output(struct vc4_hvs *hvs, unsigned int output);
 u8 vc4_hvs_get_fifo_frame_count(struct vc4_hvs *hvs, unsigned int fifo);
+void vc4_hvs_mark_dlist_entry_stale(struct vc4_hvs *hvs,
+				    struct vc4_hvs_dlist_allocation *alloc);
 int vc4_hvs_atomic_check(struct drm_crtc *crtc, struct drm_atomic_state *state);
 void vc4_hvs_atomic_begin(struct drm_crtc *crtc, struct drm_atomic_state *state);
 void vc4_hvs_atomic_enable(struct drm_crtc *crtc, struct drm_atomic_state *state);

drivers/gpu/drm/vc4/vc4_hvs.c

@@ -412,6 +412,152 @@ static void vc5_hvs_update_gamma_lut(struct vc4_hvs *hvs,
 	vc5_hvs_lut_load(hvs, vc4_crtc);
 }
 
+static void vc4_hvs_irq_enable_eof(const struct vc4_hvs *hvs,
+				   unsigned int channel)
+{
+	struct vc4_dev *vc4 = hvs->vc4;
+	u32 irq_mask = vc4->is_vc5 ?
+		SCALER5_DISPCTRL_DSPEIEOF(channel) :
+		SCALER_DISPCTRL_DSPEIEOF(channel);
+
+	HVS_WRITE(SCALER_DISPCTRL,
+		  HVS_READ(SCALER_DISPCTRL) | irq_mask);
+}
+
+static void vc4_hvs_irq_clear_eof(const struct vc4_hvs *hvs,
+				  unsigned int channel)
+{
+	struct vc4_dev *vc4 = hvs->vc4;
+	u32 irq_mask = vc4->is_vc5 ?
+		SCALER5_DISPCTRL_DSPEIEOF(channel) :
+		SCALER_DISPCTRL_DSPEIEOF(channel);
+
+	HVS_WRITE(SCALER_DISPCTRL,
+		  HVS_READ(SCALER_DISPCTRL) & ~irq_mask);
+}
+
+static struct vc4_hvs_dlist_allocation *
+vc4_hvs_alloc_dlist_entry(struct vc4_hvs *hvs,
+			  unsigned int channel,
+			  size_t dlist_count)
+{
+	struct vc4_hvs_dlist_allocation *alloc;
+	unsigned long flags;
+	int ret;
+
+	if (channel == VC4_HVS_CHANNEL_DISABLED)
+		return NULL;
+
+	alloc = kzalloc(sizeof(*alloc), GFP_KERNEL);
+	if (!alloc)
+		return ERR_PTR(-ENOMEM);
+
+	spin_lock_irqsave(&hvs->mm_lock, flags);
+	ret = drm_mm_insert_node(&hvs->dlist_mm, &alloc->mm_node,
+				 dlist_count);
+	spin_unlock_irqrestore(&hvs->mm_lock, flags);
+	if (ret)
+		return ERR_PTR(ret);
+
+	alloc->channel = channel;
+
+	return alloc;
+}
+
+void vc4_hvs_mark_dlist_entry_stale(struct vc4_hvs *hvs,
+				    struct vc4_hvs_dlist_allocation *alloc)
+{
+	unsigned long flags;
+	u8 frcnt;
+
+	if (!alloc)
+		return;
+
+	if (!drm_mm_node_allocated(&alloc->mm_node))
+		return;
+
+	frcnt = vc4_hvs_get_fifo_frame_count(hvs, alloc->channel);
+	alloc->target_frame_count = (frcnt + 1) & ((1 << 6) - 1);
+
+	spin_lock_irqsave(&hvs->mm_lock, flags);
+
+	list_add_tail(&alloc->node, &hvs->stale_dlist_entries);
+
+	HVS_WRITE(SCALER_DISPSTAT, SCALER_DISPSTAT_EOF(alloc->channel));
+	vc4_hvs_irq_enable_eof(hvs, alloc->channel);
+
+	spin_unlock_irqrestore(&hvs->mm_lock, flags);
+}
+
+static void vc4_hvs_schedule_dlist_sweep(struct vc4_hvs *hvs,
+					 unsigned int channel)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&hvs->mm_lock, flags);
+
+	if (!list_empty(&hvs->stale_dlist_entries))
+		queue_work(system_unbound_wq, &hvs->free_dlist_work);
+
+	vc4_hvs_irq_clear_eof(hvs, channel);
+
+	spin_unlock_irqrestore(&hvs->mm_lock, flags);
+}
+
+/*
+ * Frame counts are essentially sequence numbers over 6 bits, and we
+ * thus can use sequence number arithmetic and follow the RFC1982 to
+ * implement proper comparison between them.
+ */
+static bool vc4_hvs_frcnt_lte(u8 cnt1, u8 cnt2)
+{
+	return (s8)((cnt1 << 2) - (cnt2 << 2)) <= 0;
+}
+
+/*
+ * Some atomic commits (legacy cursor updates, mostly) will not wait for
+ * the next vblank and will just return once the commit has been pushed
+ * to the hardware.
+ *
+ * On the hardware side, our HVS stores the planes parameters in its
+ * context RAM, and will use part of the RAM to store data during the
+ * frame rendering.
+ *
+ * This interacts badly if we get multiple commits before the next
+ * vblank since we could end up overwriting the DLIST entries used by
+ * previous commits if our dlist allocation reuses that entry. In such a
+ * case, we would overwrite the data currently being used by the
+ * hardware, resulting in a corrupted frame.
+ *
+ * In order to work around this, we'll queue the dlist entries in a list
+ * once the associated CRTC state is destroyed. The HVS only allows us
+ * to know which entry is being active, but not which one are no longer
+ * being used, so in order to avoid freeing entries that are still used
+ * by the hardware we add a guesstimate of the frame count where our
+ * entry will no longer be used, and thus will only free those entries
+ * when we will have reached that frame count.
+ */
+static void vc4_hvs_dlist_free_work(struct work_struct *work)
+{
+	struct vc4_hvs *hvs = container_of(work, struct vc4_hvs, free_dlist_work);
+	struct vc4_hvs_dlist_allocation *cur, *next;
+	unsigned long flags;
+
+	spin_lock_irqsave(&hvs->mm_lock, flags);
+	list_for_each_entry_safe(cur, next, &hvs->stale_dlist_entries, node) {
+		u8 frcnt;
+
+		frcnt = vc4_hvs_get_fifo_frame_count(hvs, cur->channel);
+		if (!vc4_hvs_frcnt_lte(cur->target_frame_count, frcnt))
+			continue;
+
+		list_del(&cur->node);
+		drm_mm_remove_node(&cur->mm_node);
+		kfree(cur);
+	}
+	spin_unlock_irqrestore(&hvs->mm_lock, flags);
+}
+
 u8 vc4_hvs_get_fifo_frame_count(struct vc4_hvs *hvs, unsigned int fifo)
 {
 	struct drm_device *drm = &hvs->vc4->base;
@@ -643,13 +789,12 @@ int vc4_hvs_atomic_check(struct drm_crtc *crtc, struct drm_atomic_state *state)
 {
 	struct drm_crtc_state *crtc_state = drm_atomic_get_new_crtc_state(state, crtc);
 	struct vc4_crtc_state *vc4_state = to_vc4_crtc_state(crtc_state);
+	struct vc4_hvs_dlist_allocation *alloc;
 	struct drm_device *dev = crtc->dev;
 	struct vc4_dev *vc4 = to_vc4_dev(dev);
 	struct drm_plane *plane;
-	unsigned long flags;
 	const struct drm_plane_state *plane_state;
 	u32 dlist_count = 0;
-	int ret;
 
 	/* The pixelvalve can only feed one encoder (and encoders are
 	 * 1:1 with connectors.)
@@ -662,12 +807,11 @@ int vc4_hvs_atomic_check(struct drm_crtc *crtc, struct drm_atomic_state *state)
 
 	dlist_count++; /* Account for SCALER_CTL0_END. */
 
-	spin_lock_irqsave(&vc4->hvs->mm_lock, flags);
-	ret = drm_mm_insert_node(&vc4->hvs->dlist_mm, &vc4_state->mm,
-				 dlist_count);
-	spin_unlock_irqrestore(&vc4->hvs->mm_lock, flags);
-	if (ret)
-		return ret;
+	alloc = vc4_hvs_alloc_dlist_entry(vc4->hvs, vc4_state->assigned_channel, dlist_count);
+	if (IS_ERR(alloc))
+		return PTR_ERR(alloc);
+
+	vc4_state->mm = alloc;
 
 	return vc4_hvs_gamma_check(crtc, state);
 }
@@ -683,8 +827,9 @@ static void vc4_hvs_install_dlist(struct drm_crtc *crtc)
 	if (!drm_dev_enter(dev, &idx))
 		return;
 
+	WARN_ON(!vc4_state->mm);
 	HVS_WRITE(SCALER_DISPLISTX(vc4_state->assigned_channel),
-		  vc4_state->mm.start);
+		  vc4_state->mm->mm_node.start);
 
 	drm_dev_exit(idx);
 }
@@ -711,8 +856,10 @@ static void vc4_hvs_update_dlist(struct drm_crtc *crtc)
 		spin_unlock_irqrestore(&dev->event_lock, flags);
 	}
 
+	WARN_ON(!vc4_state->mm);
+
 	spin_lock_irqsave(&vc4_crtc->irq_lock, flags);
-	vc4_crtc->current_dlist = vc4_state->mm.start;
+	vc4_crtc->current_dlist = vc4_state->mm->mm_node.start;
 	spin_unlock_irqrestore(&vc4_crtc->irq_lock, flags);
 }
 
@@ -769,8 +916,7 @@ void vc4_hvs_atomic_flush(struct drm_crtc *crtc,
 	struct vc4_plane_state *vc4_plane_state;
 	bool debug_dump_regs = false;
 	bool enable_bg_fill = false;
-	u32 __iomem *dlist_start = vc4->hvs->dlist + vc4_state->mm.start;
-	u32 __iomem *dlist_next = dlist_start;
+	u32 __iomem *dlist_start, *dlist_next;
 	unsigned int zpos = 0;
 	bool found = false;
 	int idx;
@@ -788,6 +934,9 @@ void vc4_hvs_atomic_flush(struct drm_crtc *crtc,
 		vc4_hvs_dump_state(hvs);
 	}
 
+	dlist_start = vc4->hvs->dlist + vc4_state->mm->mm_node.start;
+	dlist_next = dlist_start;
+
 	/* Copy all the active planes' dlist contents to the hardware dlist. */
 	do {
 		found = false;
@@ -821,7 +970,8 @@ void vc4_hvs_atomic_flush(struct drm_crtc *crtc,
 	writel(SCALER_CTL0_END, dlist_next);
 	dlist_next++;
 
-	WARN_ON_ONCE(dlist_next - dlist_start != vc4_state->mm.size);
+	WARN_ON(!vc4_state->mm);
+	WARN_ON_ONCE(dlist_next - dlist_start != vc4_state->mm->mm_node.size);
 
 	if (enable_bg_fill)
 		/* This sets a black background color fill, as is the case
@@ -960,6 +1110,11 @@ static irqreturn_t vc4_hvs_irq_handler(int irq, void *data)
 
 			irqret = IRQ_HANDLED;
 		}
+
+		if (status & SCALER_DISPSTAT_EOF(channel)) {
+			vc4_hvs_schedule_dlist_sweep(hvs, channel);
+			irqret = IRQ_HANDLED;
+		}
 	}
 
 	/* Clear every per-channel interrupt flag. */
@@ -1024,6 +1179,9 @@ struct vc4_hvs *__vc4_hvs_alloc(struct vc4_dev *vc4, struct platform_device *pde
 
 	spin_lock_init(&hvs->mm_lock);
 
+	INIT_LIST_HEAD(&hvs->stale_dlist_entries);
+	INIT_WORK(&hvs->free_dlist_work, vc4_hvs_dlist_free_work);
+
 	/* Set up the HVS display list memory manager.  We never
 	 * overwrite the setup from the bootloader (just 128b out of
 	 * our 16K), since we don't want to scramble the screen when