prb_first_valid_seq
u64 valid_seq = max_t(u64, seq, prb_first_valid_seq(prb));
seq = prb_first_valid_seq(prb);
atomic64_set(&user->seq, prb_first_valid_seq(prb));
atomic64_set(&user->seq, prb_first_valid_seq(prb));
u64 prb_first_valid_seq(struct printk_ringbuffer *rb);