I think I answered this. It is controlled by the ISP, indirectly anyway. Technically speaking, theres a way to do it without the ISP. Sometimes its easier then other times. For instance, with the DCM-202, you can Telnet into the modem to update it. You can with the 6120 as well, but its much more diffacult and the firmware is not freely available.
So what you have to do is contact your ISP, usually with DSLReport, and ask one of offically recognized moderators to update it for you. These are usually listed on a sticky on your ISP's board. Many times they will go through many firmwares before they find one that they think works the best and then they will test it for a couple months before pushing it to modems, but that can take months. If your having a problem that may be firmware related (IE channel bonding) sometimes getting your ISP to update to a "unofficial firmware" can help. The thing is, hackers/crackers have given firmware updating a bad name, and so its not uncommon for your ISP to give you a raised eyebrow if you ask. But in the end the modem is your property, and the firmware is released by the manufacture, and you have no access to it when it is put on your modem, so their is no risk of gaining proprietary information, so they have no valid reason to deny you. Thats another reason to go through DSLReports so you know your talking to someone who knows what the heck their talking about. LOL