From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Fri, 15 Jun 2018 10:31:37 +0100
Subject: drm/i915/execlists: Reset the CSB head tracking on reset/sanitization
Git-commit: b2209e62a4507f6649233eaf0675de3ad86bd868
Patch-mainline: v4.19-rc1
References: FATE#326289 FATE#326079 FATE#326049 FATE#322398 FATE#326166

We can avoid the mmio read of the CSB pointers after reset based on the
knowledge that the HW always start writing at entry 0 in the CSB buffer.
We need to reset our CSB head tracking after GPU reset (and on
sanitization after resume) so that we are expecting to read from entry
0, hence we reset our head tracking back to the entry before (the last
entry in the ring).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180615093137.14270-2-chris@chris-wilson.co.uk

Acked-by: Petr Tesarik <ptesarik@suse.com>
---
 drivers/gpu/drm/i915/intel_lrc.c |   15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -970,22 +970,19 @@ static void process_csb(struct intel_eng
 			&engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX];
 		unsigned int head, tail;
 
-		if (unlikely(execlists->csb_use_mmio)) {
-			buf = (u32 * __force)
-				(i915->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_BUF_LO(engine, 0)));
-			execlists->csb_head = -1; /* force mmio read of CSB */
-		}
-
 		/* Clear before reading to catch new interrupts */
 		clear_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted);
 		smp_mb__after_atomic();
 
-		if (unlikely(execlists->csb_head == -1)) { /* after a reset */
+		if (unlikely(execlists->csb_use_mmio)) {
 			if (!fw) {
 				intel_uncore_forcewake_get(i915, execlists->fw_domains);
 				fw = true;
 			}
 
+			buf = (u32 * __force)
+				(i915->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_BUF_LO(engine, 0)));
+
 			head = readl(i915->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_PTR(engine)));
 			tail = GEN8_CSB_WRITE_PTR(head);
 			head = GEN8_CSB_READ_PTR(head);
@@ -1970,7 +1967,7 @@ static void execlists_reset(struct intel
 	spin_unlock(&engine->timeline.lock);
 
 	/* Following the reset, we need to reload the CSB read/write pointers */
-	engine->execlists.csb_head = -1;
+	engine->execlists.csb_head = GEN8_CSB_ENTRIES - 1;
 
 	local_irq_restore(flags);
 
@@ -2478,7 +2475,7 @@ static int logical_ring_init(struct inte
 			upper_32_bits(ce->lrc_desc);
 	}
 
-	engine->execlists.csb_head = -1;
+	engine->execlists.csb_head = GEN8_CSB_ENTRIES - 1;
 
 	return 0;