[Desire HD Android Development] preacher65:
About ro.ril.hsxpa? Okay, you asked for it! :p I'll try my best. ;)

Here's the short version: Everything's fine, there's nothing to worry about. Both settings are okay. But "U" is recommended by the Revolution Team.

Long version:

There is a file called build.prop in the ROM, which has a whole bunch of settings in it. For reasons known only to HTC, on the DHD some of these settings appear twice. One of them, ro.ril.hsxpa, appears with two different values (2 and 3). I don't know why, maybe the HTC engineer was drunk. :D

In 5.1.9 the "ro.ril.hsxpa=2" setting was the one that took effect. In 5.1.10, Mike cleaned up the duplicates and "ro.ril.hsxpa=3" was the one left in the ROM. In the "stealth" update he changed it back from 3 to 2.

What does it mean? Well the first difference is that the letter that's displayed in the middle of your radio version number will change from "U" to "P". :)

The second difference is what kind of mobile internet you MIGHT get. The known settings are these, and of course it depends on what your network supports and what signal you're getting at any given time:

ro.ril.hsxpa=0 - UMTS r99

ro.ril.hsxpa=1 - HSDPA, this gives high speed downloads

ro.ril.hsxpa=2 - HSDPA/HSUPA (also called "HSPA"), this gives high speed downloads and uploads, and "U" in the radio name

ro.ril.hsxpa=3 is relatively a new setting, and current consensus seems to be that it will give you HSPA+ (or "Evolved HSPA", yes it's really called that). This gives "P" in the radio name (presumably for "plus"). However this is an educated guess, since as far as I know HTC have not released this info officially.

HSPA+ is faster than HSPA (so "P" would be faster than "U"), BUT it depends on whether your network supports it. Not every network does. (Edit: But see below for jkoljo's post, where some real world testing indicates "U" gives faster results than "P".)

(I've seen a lot of people posting speed tests in other threads, but one thing to remember before doing any tests is that mobile internet speed varies according to where you are, how strong the signal is, and how busy the network is.)

If you have HSPA+ enabled ("P" in radio name) but your network doesn't support it, you should still get normal HSPA (the same as when you had "U" in radio name). Quite a few people in the Inspire 4G forums have been using the P setting for some time with no bad effects, see wjsturgis3's recent posts in this thread. It's still a little uncertain what the exact effects are of enabling HSPA+ if your network doesn't support it, but so far it doesn't seem to do any harm.

So whether you have U or P in your radio name, your phone might work the same, as many networks do not support HSPA+ yet.

As Mike said, no need to change anything unless you have any problems. And evidence so far indicates you won't have problems - remember the Revolution Team tested this ROM for days before it was released, and they would surely have noticed and fixed any issues. :D

Phew, that must be my longest post ever. :o Anyway, I did a lot of searching on this a couple of months ago, and the above is what I found. I hope it's accurate and helps some people. If anyone spots any errors please tell me and I'll correct them. :)

Important Edit: See also jkoljo's post here: http://xdaforums.com/showthread.php?p=14746145#post14746145

Where he says "U" gives faster results than "P". As I said, the info above was from reading not testing, so it seems it may be more complicated than it appeared at first and I've changed some of the text above. I hope it still explains some of the issues though!