On 07/06/2015 11:24 AM, Daniel Vetter wrote: > On Fri, Jul 03, 2015 at 02:11:37PM -0400, Michael Kaminsky wrote: >> I few days ago I built a kernel from git (commit 6aaf0da872), and >> noticed a couple of NULL pointer deferences. These seem to be >> regressions as they aren't present in v4.1. >> >> I did a bisect between v4.1 and 6aaf0da872, and came up with the >> following commit as the first bad one: >> >> d5432a9d drm/i915: Stage new modeset state straight into atomic state >> >> My laptop is a Thinkpad T540p. The bug manifests itself specifically >> when I'm connected to my dock. Starting with this commit, when I plug >> an external monitor into the dock and then unplug it, I get the NULL >> pointer dereference in drm_mode_copy (see kernel trace #1 below). The >> bug happens during unplug. >> >> Plugging/unplugging the same monitor directly into my laptop doesn't >> seem tickle the bug. It also doesn't seem to matter which connector I >> plug/unplug into on the dock (VGA, DP, etc.). >> >> This laptop/dock uses DP MST, so wonder if that's the problem. An >> external VGA monitor connected directly to my laptop shows up as output >> VGA1, but when that same monitor is hooked up to the dock's VGA port, it >> shows up as output DP2-3 (for example). >> >> That commit the first place where things seem to go wrong, but later >> commits actually show a different, but possibly related NULL pointer >> dereference in drm_crtc_index (see kernel trace #2 below). In these >> kernels, I don't even get to the point where I can unplug the monitor. >> Instead, as soon as I connect two external monitors to my dock, a >> NULL dereference occurs. My initial tests show that it seems to >> happen specifically with 2 external monitors, not 1, and when they are >> connected to the dock, not the laptop itself. This bug occurs in commit >> 6aaf0da872 (my starting point), and I noticed it during my bisect in at >> least commit 27a1b688, though it might first start occurring earlier. >> I know that 0f63cca already has the first bug above (unplugging >> monitor problem). I suspect that the new problem probably starts >> between those two commits, but I haven't had the chance to pinpoint >> it--perhaps this info will be enough to identify the source of both >> problems, but if not, I can try to dig deeper. > > Yeah mst dp hotplugs connectors, and we've changed a few things in there. > Can you please boot with drm.debug=0xe added to your kernel cmdline, > reproduce each issue and the grab the complete kernel log for each case? > It'll be really big but should help figuring out what's amiss. > > Also please retest with latest drm-next or upstream linus, we've just > merged a few patches to close some dp mst races. > > Thanks, Daniel Daniel, I was able to do some quick testing with a recent upstream linus kernel (commit 1c4c7159 -- basically one commit after v4.2-rc1). To keep things simple, I just tested with this one kernel for now. This kernel basically exhibits the second case I described above, but does so even after attaching a single monitor. I can trigger the bug as follows: I boot with my laptop docked, but no monitors attached to the dock. Once the machine has booted fully, I switch to a console. (Switching to the console is just a convenience so that I can see the kernel messages immediately; it doesn't seem to affect the results.) Then, I plug an external monitor into the VGA port on the dock. (I also tried plugging a DP monitor into the dock and get similar results.) As soon as I plug in that single external monitor, the NULL pointer dereference occurs. It seems repeatable with this kernel. The log is here: http://pastebin.com/dKgxfz4y Thanks! Michael _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
