<html> <head> <base href="https://bugs.freedesktop.org/"> </head> <body> <div> <a class="bz_bug_link bz_status_CLOSED bz_closed" title="CLOSED WORKSFORME - GPU Hang with single-channel RAM configuration" href="https://bugs.freedesktop.org/show_bug.cgi?id=105064#c21">Comment # 21</a> on <a class="bz_bug_link bz_status_CLOSED bz_closed" title="CLOSED WORKSFORME - GPU Hang with single-channel RAM configuration" href="https://bugs.freedesktop.org/show_bug.cgi?id=105064">bug 105064</a> from <a class="email" href="mailto:chris@chris-wilson.co.uk" title="Chris Wilson <chris@chris-wilson.co.uk>"> Chris Wilson</a> <pre>Likely fix: commit ba74cb10c775c839f6e1d0fabd1e772eabd9c43f Author: Michel Thierry <<a href="mailto:michel.thierry@intel.com">michel.thierry@intel.com</a>> Date: Mon Nov 20 12:34:58 2017 +0000 drm/i915/execlists: Delay writing to ELSP until HW has processed the previous write The hardware needs some time to process the information received in the ExecList Submission Port, and expects us to not write anything more until it has 'acknowledged' this new submission by sending an IDLE_ACTIVE or PREEMPTED CSB event. If we do not follow this, the driver could write new data into the ELSP before HW had finishing fetching the previous one, putting us in 'undefined behaviour' space. This seems to be the problem causing the spurious PREEMPTED & COMPLETE events after a COMPLETE like the one below: [] vcs0: sw rd pointer = 2, hw wr pointer = 0, current 'head' = 3. [] vcs0: Execlist CSB[0]: 0x00000018 _ 0x00000007 [] vcs0: Execlist CSB[1]: 0x00000001 _ 0x00000000 [] vcs0: Execlist CSB[2]: 0x00000018 _ 0x00000007 <<< COMPLETE [] vcs0: Execlist CSB[3]: 0x00000012 _ 0x00000007 <<< PREEMPTED & COMPLETE [] vcs0: Execlist CSB[4]: 0x00008002 _ 0x00000006 [] vcs0: Execlist CSB[5]: 0x00000014 _ 0x00000006 The ELSP writes that lead to this CSB sequence show that the HW hadn't started executing the previous execlist (the one with only ctx 0x6) by the time the new one was submitted; this is a bit more clear in the data show in the EXECLIST_STATUS register at the time of the ELSP write. [] vcs0: ELSP[0] = 0x0_0 [execlist1] - status_reg = 0x0_302 [] vcs0: ELSP[1] = 0x6_fedb2119 [execlist0] - status_reg = 0x0_8302 [] vcs0: ELSP[2] = 0x7_fedaf119 [execlist1] - status_reg = 0x0_8308 [] vcs0: ELSP[3] = 0x6_fedb2119 [execlist0] - status_reg = 0x7_8308 Note that having to wait for this ack does not disable lite-restores, although it may reduce their numbers. Bugzilla: <a class="bz_bug_link bz_status_CLOSED bz_closed" title="CLOSED FIXED - [BAT][CI] Incomplete : kernel BUG at drivers/gpu/drm/i915/intel_lrc.c:880!" href="show_bug.cgi?id=102035">https://bugs.freedesktop.org/show_bug.cgi?id=102035</a> Signed-off-by: Michel Thierry <<a href="mailto:michel.thierry@intel.com">michel.thierry@intel.com</a>> Link: <a href="https://patchwork.freedesktop.org/patch/msgid/">https://patchwork.freedesktop.org/patch/msgid/</a><<a href="mailto:20171118003038.7935-1-michel.thierry@intel.com">20171118003038.7935-1-michel.thierry@intel.com</a>> Link: <a href="https://patchwork.freedesktop.org/patch/msgid/20171120123458.23242-4-chris@chris-wilson.co.uk">https://patchwork.freedesktop.org/patch/msgid/20171120123458.23242-4-chris@chris-wilson.co.uk</a> Reviewed-by: Chris Wilson <<a href="mailto:chris@chris-wilson.co.uk">chris@chris-wilson.co.uk</a>> Tested-by: Chris Wilson <<a href="mailto:chris@chris-wilson.co.uk">chris@chris-wilson.co.uk</a>> Signed-off-by: Chris Wilson <<a href="mailto:chris@chris-wilson.co.uk">chris@chris-wilson.co.uk</a>> which was asked to be tested in c3.</pre> </div> <hr> You are receiving this mail because: <ul> <li>You are the assignee for the bug.</li> <li>You are the QA Contact for the bug.</li> <li>You are on the CC list for the bug.</li> </ul> </body> </html>