(1) Innodb can be run from read only media like DVD/CD etc

(2) User can explicitly set instance in read only mode though its on read-write media

Start server in Read Only Mode:

"--innodb-read-only" boolean configuration parameter is introduced  for starting server in read only mode. Server will not start in read only mode if it was not shutdown cleanly/safely earlier. 

User must pass/set parameter which try to create file/log on read/write media. Else server will not start. Like , user will need to pass "--pid-file=" and/or "--event-scheduler=disabled"  if datadir is on read only media etc

There exits a "--read-only" variable which differs from newly added --innodb-read-only.[If datadir is without write permission then server will give error even when --read-only option is provided ]

--read-only
1) When datadir is RW and server started with --read-only :  root/super user have permission for operations. Other users will not have permission to change.
2) When datadir is on read only media and server started with --read-only : all operations blocked for users (including root).

--innodb-read-only
1) When datadir is RW and server started with --innodb-read-only : 
a) No user will be able to modify.  DCL(like create user/grant/revoke etc) commands will work for root user. (It's non innodb issue)
b) When both  --read-only and --innodb-read-only are given, then --innodb-read-only takes effect.(As I checked that root user can not do DML/DDL but can perform DCL )
2) When datadir is on read only media and server started with --read-only : all operations blocked for users (including root).