Function validate_against_parent_gas_limit 

pub fn validate_against_parent_gas_limit<H, ChainSpec>(
    header: &SealedHeader<H>,
    parent: &SealedHeader<H>,
    chain_spec: &ChainSpec,
) -> Result<(), ConsensusError>
where
    H: BlockHeader,
    ChainSpec: EthChainSpec + EthereumHardforks,

Available on crate feature consensus only.

Validates gas limit against parent gas limit.

The maximum allowable difference between self and parent gas limits is determined by the parent’s gas limit divided by the GAS_LIMIT_BOUND_DIVISOR.