[Intel-gfx] [PATCH] drm/i915: Consider HW CSB write pointer before resetting the sw read pointer

[Michel Thierry]

A previous commit resets the Context Status Buffer (CSB) read pointer in
ring init
    commit c0a03a2e4c4e ("drm/i915: Reset CSB read pointer in ring init")

This is generally correct, but this pointer is not reset after
suspend/resume in some platforms (cht). In this case, the driver should
read the register value instead of resetting the sw read counter to 0.
Otherwise we process old events, leading to unwanted pre-emptions or
something worse.

But in other platforms (bdw) and also during GPU reset or power up, the
CSBWP is reset to 0x7 (an invalid number), and in this case the read
pointer should be set to 0.

Signed-off-by: Lei Shen <lei.shen at intel.com>
Signed-off-by: Deepak S <deepak.s at intel.com>
Signed-off-by: Michel Thierry <michel.thierry at intel.com>
---
 drivers/gpu/drm/i915/intel_lrc.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index ff9a481..dd87812 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1479,6 +1479,7 @@ static int gen8_init_common_ring(struct intel_engine_cs *ring)
 {
 	struct drm_device *dev = ring->dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
+	u8 next_context_status_buffer_hw;
 
 	lrc_setup_hardware_status_page(ring,
 				ring->default_context->engine[ring->id].state);
@@ -1496,7 +1497,28 @@ static int gen8_init_common_ring(struct intel_engine_cs *ring)
 		   _MASKED_BIT_DISABLE(GFX_REPLAY_MODE) |
 		   _MASKED_BIT_ENABLE(GFX_RUN_LIST_ENABLE));
 	POSTING_READ(RING_MODE_GEN7(ring));
-	ring->next_context_status_buffer = 0;
+
+	/*
+	 * Instead of resetting the Context Status Buffer (CSB) read pointer to
+	 * zero, we need to read the write pointer from hardware and use its
+	 * value because "this register is power context save restored".
+	 * Effectively, these states have been observed:
+	 *
+	 *      | Suspend-to-idle (freeze) | Suspend-to-RAM (mem) |
+	 * BDW  | CSB regs not reset       | CSB regs reset       |
+	 * CHT  | CSB regs not reset       | CSB regs not reset   |
+	 */
+	next_context_status_buffer_hw = I915_READ(RING_CONTEXT_STATUS_PTR(ring)) & 0x07;
+
+	/*
+	 * When the CSB registers are reset (also after power-up / gpu reset),
+	 * CSB write pointer is set to all 1's, which is not valid, use 0 in
+	 * this special case.
+	 */
+	if (next_context_status_buffer_hw == 0x7)
+		next_context_status_buffer_hw = 0;
+
+	ring->next_context_status_buffer = next_context_status_buffer_hw;
 	DRM_DEBUG_DRIVER("Execlists enabled for %s\n", ring->name);
 
 	memset(&ring->hangcheck, 0, sizeof(ring->hangcheck));
-- 
2.5.3