If, and only if, the recoverable amount of an asset is less than its carrying amount, the carrying amount of the asset shall be reduced to its recoverable amount. That reduction is an impairment loss.
An impairment loss shall be recognized immediately in profit or loss, unless the asset is carried at revalued amount in accordance with another Standard (for example, in accordance with the revaluation model in IAS 16 Property, Plant and Equipment). Any impairment loss of a revalued asset shall be treated as a revaluation decrease in accordance with that other Standard. That means as per IAS-16, which tells about revaluation model, states that:
If a revaluation results in an increase in value, it should be credited to other comprehensive income and accumulated in equity under the heading “revaluation surplus” unless it represents the reversal of a revaluation decrease of the same asset previously recognized as an expense, in which case it should be recognized in profit or loss. [IAS 16.39]
A decrease arising as a result of a revaluation should be recognized as an expense to the extent that it exceeds any amount previously credited to the revaluation surplus relating to the same asset. [IAS 16.40]
So, as a conclusion it states that if any revaluation reserve is created in earlier years then the loss should be settled first to the extend, and then can be charged through Profit and loss account. And the same treatment in AS-28.
As per IFRS, no disclosure is required. But, as per AS-28, Amount of impairment loss recognised in P/L, transferred in Revaluation Loss is required to be disclosed.