It wasn't purely an X problem; if I enabled gpm, the mouse failed in the console as well as in X. And it wasn't hardware, because if I used Ubuntu 9.10's standard kernel, my mouse worked every time.
After much poking around with kernel options, I discovered that if I tunred off the Direct Rendering manager ("Intel 830M, 845G, 852GM, 855GM, 865G (i915 driver)"), my mouse would work. But that wasn't a satisfactory solution; aside from not being able to run Google Earth, it seems that Intel graphics needs DRM even to get reasonable performance redrawing windows. Without it, every desktop switch means watching windows slowly redraw over two or three seconds.
(Aside: why is it that Intel cards with shared CPU memory need DRM to draw basic 2-D windows, when my ancient ATI Radeon cards without shared memory had no such problems?)
But I think I finally have it nailed. In the kernel's Direct Rendering Manager options (under Graphics), the "Intel 830M, 845G, 852GM, 855GM, 865G (i915 driver)" using its "i915 driver" option has a new sub-option: "Enable modesetting on intel by default".
The help says:
Choose this option if you want kernel modesetting enabled by default, and you have a new enough userspace to support this. Running old userspaces with this enabled will cause pain. Note that this causes the driver to bind to PCI devices, which precludes loading things like intelfb.
Sounds optional, right? Sounds like, if I want to build a kernel that will work on both karmic and jaunty, I should leave that off so as not to "cause pain".
But no. It turns out it's actually mandatory on karmic. Without it, there's a race condition where about 80-90% of the time, hal won't see a mouse device at all, so the mouse won't work either in X or even on the console with gpm.
It's sort of the opposite of the "Remove sysfs features which may confuse old userspace tools" in General Setup, where the name implies that it's optional on new distros like Karmic, but in fact, if you leave it on, the kernel won't work reliably.
So be warned when configuring a kernel for brand-new distros. There are some new pitfalls, and options that worked in the past may not work any longer!
Update: see also the followup post for two more non-optional options.
[ 23:34 Nov 10, 2009 More linux/kernel | permalink to this entry | ]