My knowledge on GAAP and IFRS might be outdated, but I do not think they made significant changes to the rules… Let me know if I made any mistakes.
Impairment happens when the carrying value of the asset is NOT RECOVERABLE. Thus, you write the asset down and incur a loss.GAAP and IFRS each has their own rules in determining asset impairment (GAAP = two-step, IFRS = one-step).
Revaluation is just a method to value assets, and this method is only acceptable under IFRS. When asset is valued under revaluation, the carrying amount is simply the fair value - any depreciation.
So, if you value a long-term asset using revaluation method, and the current carrying amount is below the fair value - any depreciation, you have an impairment loss.
As far as reversal goes,
GAAP - for asset held under use, NO REVERSALS are allowed
GAAP - for asset held for sale, REVERSALS ALLOWED (a test of impairment happens when asset is categorized as held for sale and deprecations on the asset no longer applies)
IFRS - impairment loss may be reversed to its original value; ANY EXCESS is recorded as revaluation surplus which is a component of the OCI