I didn't had any issues with 5.4.x.  Currently using 5.4.4.2, but I think there might be later GA version available now as well. If you use update guide as in release notes, you can't go wrong.

Hi Hrvoje,

Thank you for your reply.  Are you using replication?  I agree, release notes are mandatory to read.

Steven

I'm using DD replication integrated with backup application via DD Boost (CCR in NW world).

Make sure you have enough space in ddvar. because the DD120 has linited storage, the upgrade may fill ddvar to the point where the system reports ddvar is full and stops the upgrade.

General Considerations :

1. Space utilization, system alerts, cleaning, and
   replication statistics are reset after a controller upgrade. In many cases,
   this is the desired behavior after upgrading the chassis. If this information
   must be retained, contact EMC support prior to starting the upgrade process.
2. Review the Release Notes of the DD OS you are
   upgrading to as part of the controller upgrade process.
3. For releases prior to DD OS 5.4, the source and
   destination controller must use the same DD OS major version but may have
   different minor versions. For DD OS 5.4 and later, the same release must be
   installed on the source and destination controller before you start to remove
   the source controller.
4. The source file system usage must be 95% or less
   (free space 5% or more) to support a DD OS upgrade.
5. If your upgrade plan includes adding software
   features such as Extended Retention or Replication, make the changes before you
   start the controller upgrade or after the controller upgrade is complete. Do
   not change software features while a controller upgrade is in progress.
6. EMC does not support migration of PCI and I/O
   option cards from a source system to a destination system in the controller
   upgrade procedure. New PCI and I/O option cards must be purchased for the new
   system.
7. EMC Data Domain does not support migration of
   any optional software license or capacity licenses. Shelves can be migrated,
   however, Shelf Capacity licenses are required. If the controller upgrade will
   result in a mixture of different shelves with different types of drives, refer
   to the ES20 and ES30 shelf mixing guidelines on the support portal prior to the
   upgrade.
8. Data Domain systems are heavy. To avoid
   personnel injury or equipment damage, plan to use two people or a mechanical
   lift when moving a chassis or shelf.
9. The system controller upgrade process preserves
   the configuration settings for CIFS, NFS, and DD Boost communication.
10. For WWN migration on systems configured for
    Fibre Channel, the same DD OS version, which must be version 5.3 or later, is
    required on the source and destination DDRs.
11. The upgrade process will preserve the VTL and
    DDBoost over Fibre Channel configuration from the original system. However, the
    Fibre Channel ports can differ between the source and destination systems,
    which requires the migration of the Fibre Channel configuration to the
    appropriate ports on the destination system. The FC migration procedure is
    covered later in this document.
12. After WWN migration and if the source system
    will be redeployed, the Fibre Channel configuration on the source system must
    be removed or changed to avoid conflicts with the destination system.
13. The WWN configuration for DD OS version 5.3 or
    later is persistent across system reboot, DD OS upgrade, and controller
    upgrade. However, the WWN configuration is reset to the default values after a
    DD OS reinstall.
14. The network configurations of the system
    controller being replaced may not be applicable to the new upgraded system
    controller since the network slots/names will change. All network configuration
    settings of the replaced system controller must be recorded carefully to
    recreate the network configuration on the new system controller.
15. If the file system is encrypted, make sure to
    acquire the required passwords or passphrase before beginning the upgrade.
16. If VTL is used on the source system and if the
    source system is redeployed, there is a possibility of VTL drive serial number
    collisions if the same backup software accesses the target system and the
    redeployed system.
17. Record the replication configuration.
18. If DD Boost is supported on the source system
    and you are planning to upgrade DD
19. OS, determine if the installed DD Boost plug-in
    software is compatible with the target DD OS version. You might need to upgrade
    the DD Boost plug-in software when you upgrade DD OS.

Latest GA Release Available for DD is 5.5.0.9                    

One critical thing that should always be performed is a full and verified clean reboot before you upgrade (wait 20 minutes after filesystem starts and verify "alerts show current" and "system show nvram" -- to ensure batteries are charged).

It removes so many concerns, if it doesn't come back it's a reboot problem rather than an upgrade failure (which is pretty rare anyway)

5.2 to 5.4 is a straight upgrade path.

Many of the steps above are for a controller upgrade (headswap upgrade) not for a simple DDOS code upgrade that I believe you want to achieve (your model doesn't support doing this anyway, since it does not support external shelves and thus a physical controller upgrade).

You don't need 5.5 - your suggested GA target release of 5.4.4.3 (IMHO) is solid for a DD120 and would be my choice.

Regards, Jonathan