drm/i915: Record ring->start address in error state

This is mostly useful for execlists where the rings switch between
contexts (and so checking that the ring's start register matches the
context is important).
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -455,6 +455,7 @@ struct drm_i915_error_state {
 		u32 semaphore_seqno[I915_NUM_RINGS - 1];
 
 		/* Register state */
+		u32 start;
 		u32 tail;
 		u32 head;
 		u32 ctl;

drivers/gpu/drm/i915/i915_gpu_error.c

@@ -251,6 +251,7 @@ static void i915_ring_error_state(struct drm_i915_error_state_buf *m,
 		return;
 
 	err_printf(m, "%s command stream:\n", ring_str(ring_idx));
+	err_printf(m, "  START: 0x%08x\n", ring->start);
 	err_printf(m, "  HEAD:  0x%08x\n", ring->head);
 	err_printf(m, "  TAIL:  0x%08x\n", ring->tail);
 	err_printf(m, "  CTL:   0x%08x\n", ring->ctl);
@@ -883,6 +884,7 @@ static void i915_record_ring_state(struct drm_device *dev,
 	ering->instpm = I915_READ(RING_INSTPM(ring->mmio_base));
 	ering->seqno = ring->get_seqno(ring, false);
 	ering->acthd = intel_ring_get_active_head(ring);
+	ering->start = I915_READ_START(ring);
 	ering->head = I915_READ_HEAD(ring);
 	ering->tail = I915_READ_TAIL(ring);
 	ering->ctl = I915_READ_CTL(ring);