drm/i915: Keep the CRC values into a circular buffer

There are a few good properties to a circular buffer, for instance it
has a number of entries (before we were always dumping the full buffer).
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -27,6 +27,7 @@
  */
 
 #include <linux/seq_file.h>
+#include <linux/circ_buf.h>
 #include <linux/ctype.h>
 #include <linux/debugfs.h>
 #include <linux/slab.h>
@@ -1739,25 +1740,28 @@ static int i915_pipe_crc(struct seq_file *m, void *data)
 	struct drm_device *dev = node->minor->dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	enum pipe pipe = (enum pipe)node->info_ent->data;
-	const struct intel_pipe_crc *pipe_crc = &dev_priv->pipe_crc[pipe];
-	int i;
-	int start;
+	struct intel_pipe_crc *pipe_crc = &dev_priv->pipe_crc[pipe];
+	int head, tail;
 
 	if (dev_priv->pipe_crc[pipe].source == INTEL_PIPE_CRC_SOURCE_NONE) {
 		seq_puts(m, "none\n");
 		return 0;
 	}
 
-	start = atomic_read(&pipe_crc->slot) + 1;
 	seq_puts(m, " timestamp     CRC1     CRC2     CRC3     CRC4     CRC5\n");
-	for (i = 0; i < INTEL_PIPE_CRC_ENTRIES_NR; i++) {
-		const struct intel_pipe_crc_entry *entry =
-			&pipe_crc->entries[(start + i) %
-					   INTEL_PIPE_CRC_ENTRIES_NR];
+	head = atomic_read(&pipe_crc->head);
+	tail = atomic_read(&pipe_crc->tail);
+
+	while (CIRC_CNT(head, tail, INTEL_PIPE_CRC_ENTRIES_NR) >= 1) {
+		struct intel_pipe_crc_entry *entry = &pipe_crc->entries[tail];
 
 		seq_printf(m, "%12u %8x %8x %8x %8x %8x\n", entry->timestamp,
 			   entry->crc[0], entry->crc[1], entry->crc[2],
 			   entry->crc[3], entry->crc[4]);
+
+		BUILD_BUG_ON_NOT_POWER_OF_2(INTEL_PIPE_CRC_ENTRIES_NR);
+		tail = (tail + 1) & (INTEL_PIPE_CRC_ENTRIES_NR - 1);
+		atomic_set(&pipe_crc->tail, tail);
 	}
 
 	return 0;

drivers/gpu/drm/i915/i915_drv.h

@@ -1230,11 +1230,11 @@ struct intel_pipe_crc_entry {
 	uint32_t crc[5];
 };
 
-#define INTEL_PIPE_CRC_ENTRIES_NR	200
+#define INTEL_PIPE_CRC_ENTRIES_NR	128
 struct intel_pipe_crc {
 	struct intel_pipe_crc_entry entries[INTEL_PIPE_CRC_ENTRIES_NR];
 	enum intel_pipe_crc_source source;
-	atomic_t slot;
+	atomic_t head, tail;
 };
 
 typedef struct drm_i915_private {

drivers/gpu/drm/i915/i915_irq.c

@@ -30,6 +30,7 @@
 
 #include <linux/sysrq.h>
 #include <linux/slab.h>
+#include <linux/circ_buf.h>
 #include <drm/drmP.h>
 #include <drm/i915_drm.h>
 #include "i915_drv.h"
@@ -1195,20 +1196,30 @@ static void ivb_pipe_crc_update(struct drm_device *dev, enum pipe pipe)
 	struct intel_pipe_crc *pipe_crc = &dev_priv->pipe_crc[pipe];
 	struct intel_pipe_crc_entry *entry;
 	ktime_t now;
-	int ts, slot;
+	int ts, head, tail;
+
+	head = atomic_read(&pipe_crc->head);
+	tail = atomic_read(&pipe_crc->tail);
+
+	if (CIRC_SPACE(head, tail, INTEL_PIPE_CRC_ENTRIES_NR) < 1) {
+		DRM_ERROR("CRC buffer overflowing\n");
+		return;
+	}
+
+	entry = &pipe_crc->entries[head];
 
 	now = ktime_get();
 	ts = ktime_to_us(now);
 
-	slot = (atomic_read(&pipe_crc->slot) + 1) % INTEL_PIPE_CRC_ENTRIES_NR;
-	entry = &pipe_crc->entries[slot];
 	entry->timestamp = ts;
 	entry->crc[0] = I915_READ(PIPE_CRC_RES_1_IVB(pipe));
 	entry->crc[1] = I915_READ(PIPE_CRC_RES_2_IVB(pipe));
 	entry->crc[2] = I915_READ(PIPE_CRC_RES_3_IVB(pipe));
 	entry->crc[3] = I915_READ(PIPE_CRC_RES_4_IVB(pipe));
 	entry->crc[4] = I915_READ(PIPE_CRC_RES_5_IVB(pipe));
-	atomic_set(&dev_priv->pipe_crc[pipe].slot, slot);
+
+	head = (head + 1) & (INTEL_PIPE_CRC_ENTRIES_NR - 1);
+	atomic_set(&pipe_crc->head, head);
 }
 #else
 static void ivb_pipe_crc_update(struct drm_device *dev, int pipe) {}