From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Thu, 28 Jun 2018 21:12:08 +0100
Subject: drm/i915/execlists: Reset CSB write pointer after reset
Git-commit: f4b58f0438ba53002e2032e3817c8e847da9a9e1
Patch-mainline: v4.19-rc1
References: FATE#326289 FATE#326079 FATE#326049 FATE#322398 FATE#326166

On HW reset, the HW clears the write pointer (to 0). But since it also
writes its first CSB entry to slot 0, we need to reset the write pointer
back to the element before (so the first entry we read is 0).

This is required for the next patch, where we trust the CSB completely!

v2: Use _MASKED_FIELD
v3: Store the reset value, so that we differentiate between mmio/hwsp
transparently and without pretense.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180628201211.13837-6-chris@chris-wilson.co.uk

Acked-by: Petr Tesarik <ptesarik@suse.com>
---
 drivers/gpu/drm/i915/intel_lrc.c        |   23 +++++++++++++++++++++--
 drivers/gpu/drm/i915/intel_ringbuffer.h |    9 +++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)

--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -884,6 +884,21 @@ static void reset_irq(struct intel_engin
 	clear_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted);
 }
 
+static void reset_csb_pointers(struct intel_engine_execlists *execlists)
+{
+	/*
+	 * After a reset, the HW starts writing into CSB entry [0]. We
+	 * therefore have to set our HEAD pointer back one entry so that
+	 * the *first* entry we check is entry 0. To complicate this further,
+	 * as we don't wait for the first interrupt after reset, we have to
+	 * fake the HW write to point back to the last entry so that our
+	 * inline comparison of our cached head position against the last HW
+	 * write works even before the first interrupt.
+	 */
+	execlists->csb_head = execlists->csb_write_reset;
+	WRITE_ONCE(*execlists->csb_write, execlists->csb_write_reset);
+}
+
 static void execlists_cancel_requests(struct intel_engine_cs *engine)
 {
 	struct intel_engine_execlists * const execlists = &engine->execlists;
@@ -1970,7 +1985,7 @@ static void execlists_reset(struct intel
 	__unwind_incomplete_requests(engine);
 
 	/* Following the reset, we need to reload the CSB read/write pointers */
-	engine->execlists.csb_head = GEN8_CSB_ENTRIES - 1;
+	reset_csb_pointers(&engine->execlists);
 
 	spin_unlock_irqrestore(&engine->timeline.lock, flags);
 
@@ -2469,7 +2484,6 @@ static int logical_ring_init(struct inte
 			upper_32_bits(ce->lrc_desc);
 	}
 
-	execlists->csb_head = GEN8_CSB_ENTRIES - 1;
 	execlists->csb_read =
 		i915->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_PTR(engine));
 	if (csb_force_mmio(i915)) {
@@ -2477,13 +2491,18 @@ static int logical_ring_init(struct inte
 			(i915->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_BUF_LO(engine, 0)));
 
 		execlists->csb_write = (u32 __force *)execlists->csb_read;
+		execlists->csb_write_reset =
+			_MASKED_FIELD(GEN8_CSB_WRITE_PTR_MASK,
+				      GEN8_CSB_ENTRIES - 1);
 	} else {
 		execlists->csb_status =
 			&engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX];
 
 		execlists->csb_write =
 			&engine->status_page.page_addr[intel_hws_csb_write_index(i915)];
+		execlists->csb_write_reset = GEN8_CSB_ENTRIES - 1;
 	}
+	reset_csb_pointers(execlists);
 
 	return 0;
 
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -325,6 +325,15 @@ struct intel_engine_execlists {
 	u32 preempt_complete_status;
 
 	/**
+	 * @csb_write_reset: reset value for CSB write pointer
+	 *
+	 * As the CSB write pointer maybe either in HWSP or as a field
+	 * inside an mmio register, we want to reprogram it slightly
+	 * differently to avoid later confusion.
+	 */
+	u32 csb_write_reset;
+
+	/**
 	 * @csb_head: context status buffer head
 	 */
 	u8 csb_head;