Quite limiting, because http://mail.google.com/mail/feed/atom/inbox/
AND https://mail.google.com/mail/feed/atom/
shows all unread mail, not simply the inbox, as it should in my opintion. Then we could have /read
and /unread
but instead /read
doesn't exist and /unread
gives you all unread, not simply the unread from the inbox.
So, http://mail.google.com/mail/feed/atom/all
may be what you want as it gives you literally ALL mail. At the moment, using an atom feed alone like that, you would only be able to get ALL mail, like you asked for, but I'm not sure that's what you're asking for...I'd assume you want ALL (meaning read/unread) from the INBOX. At the moment, that's not possible, using the feed, and you are stuck with all mail from all labels where they necessarily and likely don't all live in the inbox (many have items skip the inbox purposefully).
References:
http://googlesystem.blogspot.com/2008/03/feed-for-unread-gmail-messages.html
https://developers.google.com/google-apps/gmail/gmail_inbox_feed