drm/i915/execlists: Reset CSB write pointer after reset

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

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 81f000c245ab956c4c508099e0c1e79e6cd5ea8d..88289d1d07e720a42fc3a1fadb020f346be5e3cd 100644 (file)
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -885,6 +885,21 @@ static void reset_irq(struct intel_engine_cs *engine)
        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;
@@ -1971,7 +1986,7 @@ static void execlists_reset(struct intel_engine_cs *engine,
        __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);
 
@@ -2470,7 +2485,6 @@ static int logical_ring_init(struct intel_engine_cs *engine)
                        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)) {
@@ -2478,13 +2492,18 @@ static int logical_ring_init(struct intel_engine_cs *engine)
                        (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;
 

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 25792889dbf4b00cfe0f4367e199532515f38410..b0262099085978fe49e57ea60dc917a643c54db5 100644 (file)
--- 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
         */