From ba74cb10c775c839f6e1d0fabd1e772eabd9c43f Mon Sep 17 00:00:00 2001 From: Michel Thierry Date: Mon, 20 Nov 2017 12:34:58 +0000 Subject: [PATCH] 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: https://bugs.freedesktop.org/show_bug.cgi?id=102035 Signed-off-by: Michel Thierry Link: https://patchwork.freedesktop.org/patch/msgid/<20171118003038.7935-1-michel.thierry@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20171120123458.23242-4-chris@chris-wilson.co.uk Reviewed-by: Chris Wilson Tested-by: Chris Wilson Signed-off-by: Chris Wilson --- drivers/gpu/drm/i915/intel_lrc.c | 22 ++++++++++++++++++++++ drivers/gpu/drm/i915/intel_ringbuffer.h | 1 + 2 files changed, 23 insertions(+) diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c index 4aed00323780..8d0c49388863 100644 --- a/drivers/gpu/drm/i915/intel_lrc.c +++ b/drivers/gpu/drm/i915/intel_lrc.c @@ -477,6 +477,7 @@ static void execlists_submit_ports(struct intel_engine_cs *engine) elsp_write(desc, elsp); } + execlists_clear_active(&engine->execlists, EXECLISTS_ACTIVE_HWACK); } static bool ctx_single_port_submission(const struct i915_gem_context *ctx) @@ -529,6 +530,7 @@ static void inject_preempt_context(struct intel_engine_cs *engine) elsp_write(0, elsp); elsp_write(ce->lrc_desc, elsp); + execlists_clear_active(&engine->execlists, EXECLISTS_ACTIVE_HWACK); } static void execlists_dequeue(struct intel_engine_cs *engine) @@ -575,9 +577,20 @@ static void execlists_dequeue(struct intel_engine_cs *engine) * know the next preemption status we see corresponds * to this ELSP update. */ + GEM_BUG_ON(!port_count(&port[0])); if (port_count(&port[0]) > 1) goto unlock; + /* + * If we write to ELSP a second time before the HW has had + * a chance to respond to the previous write, we can confuse + * the HW and hit "undefined behaviour". After writing to ELSP, + * we must then wait until we see a context-switch event from + * the HW to indicate that it has had a chance to respond. + */ + if (!execlists_is_active(execlists, EXECLISTS_ACTIVE_HWACK)) + goto unlock; + if (HAS_LOGICAL_RING_PREEMPTION(engine->i915) && rb_entry(rb, struct i915_priolist, node)->priority > max(last->priotree.priority, 0)) { @@ -871,6 +884,15 @@ static void execlists_submission_tasklet(unsigned long data) GEM_TRACE("%s csb[%dd]: status=0x%08x:0x%08x\n", engine->name, head, status, buf[2*head + 1]); + + if (status & (GEN8_CTX_STATUS_IDLE_ACTIVE | + GEN8_CTX_STATUS_PREEMPTED)) + execlists_set_active(execlists, + EXECLISTS_ACTIVE_HWACK); + if (status & GEN8_CTX_STATUS_ACTIVE_IDLE) + execlists_clear_active(execlists, + EXECLISTS_ACTIVE_HWACK); + if (!(status & GEN8_CTX_STATUS_COMPLETED_MASK)) continue; diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h index c00804ed64c6..e437a9556c08 100644 --- a/drivers/gpu/drm/i915/intel_ringbuffer.h +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h @@ -251,6 +251,7 @@ struct intel_engine_execlists { unsigned int active; #define EXECLISTS_ACTIVE_USER 0 #define EXECLISTS_ACTIVE_PREEMPT 1 +#define EXECLISTS_ACTIVE_HWACK 2 /** * @port_mask: number of execlist ports - 1 -- 2.11.0