1 files changed, 43 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 685b0ebe5fb2..0ac2608ba150 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -224,6 +224,49 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
                 diff = (flags & DRM_CALLED_FROM_VBLIRQ) != 0;
         }
 
+        /*
+         * FIMXE: Need to replace this hack with proper seqlocks.
+         *
+         * Restrict the bump of the software vblank counter to a safe maximum
+         * value of +1 whenever there is the possibility that concurrent readers
+         * of vblank timestamps could be active at the moment, as the current
+         * implementation of the timestamp caching and updating is not safe
+         * against concurrent readers for calls to store_vblank() with a bump
+         * of anything but +1. A bump != 1 would very likely return corrupted
+         * timestamps to userspace, because the same slot in the cache could
+         * be concurrently written by store_vblank() and read by one of those
+         * readers without the read-retry logic detecting the collision.
+         *
+         * Concurrent readers can exist when we are called from the
+         * drm_vblank_off() or drm_vblank_on() functions and other non-vblank-
+         * irq callers. However, all those calls to us are happening with the
+         * vbl_lock locked to prevent drm_vblank_get(), so the vblank refcount
+         * can't increase while we are executing. Therefore a zero refcount at
+         * this point is safe for arbitrary counter bumps if we are called
+         * outside vblank irq, a non-zero count is not 100% safe. Unfortunately
+         * we must also accept a refcount of 1, as whenever we are called from
+         * drm_vblank_get() -> drm_vblank_enable() the refcount will be 1 and
+         * we must let that one pass through in order to not lose vblank counts
+         * during vblank irq off - which would completely defeat the whole
+         * point of this routine.
+         *
+         * Whenever we are called from vblank irq, we have to assume concurrent
+         * readers exist or can show up any time during our execution, even if
+         * the refcount is currently zero, as vblank irqs are usually only
+         * enabled due to the presence of readers, and because when we are called
+         * from vblank irq we can't hold the vbl_lock to protect us from sudden
+         * bumps in vblank refcount. Therefore also restrict bumps to +1 when
+         * called from vblank irq.
+         */
+        if ((diff > 1) && (atomic_read(&vblank->refcount) > 1 ||
+            (flags & DRM_CALLED_FROM_VBLIRQ))) {
+                DRM_DEBUG_VBL("clamping vblank bump to 1 on crtc %u: diffr=%u "
+                              "refcount %u, vblirq %u\n", pipe, diff,
+                              atomic_read(&vblank->refcount),
+                              (flags & DRM_CALLED_FROM_VBLIRQ) != 0);
+                diff = 1;
+        }
+
         DRM_DEBUG_VBL("updating vblank count on crtc %u:"
                       " current=%u, diff=%u, hw=%u hw_last=%u\n",
                       pipe, vblank->count, diff, cur_vblank, vblank->last);