There's no penalty for redirecting an entire root-level domain to a subdomain, as long as your sitemap and internal linking strategy all point to the subdomain.
From Google's perspective, the problem with 301s is that they take longer to load (since you've got to request two separate URLs) and they make your URL structure less stable. Neither of those is a problem if you make it clear to Google that your content belongs on a subdomain.
From Google's perspective, the problem with 301s is that they take longer to load (since you've got to request two separate URLs) and they make your URL structure less stable. Neither of those is a problem if you make it clear to Google that your content belongs on a subdomain.