- /*
- * 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;
- }
-