[PATCH RFC 14/14] drm/msm/a6xx: Poll for GBIF unhalt status in hw_init

[Konrad Dybcio <konrad.dybcio@linaro.org>]

Some GPUs - particularly A7xx ones - are really really stubborn and
sometimes take a longer-than-expected time to finish unhalting GBIF.

Note that this is not caused by the request a few lines above.

Poll for the unhalt ack to make sure we're not trying to write bits to
an essentially dead GPU that can't receive data on its end of the bus.
Failing to do this will result in inexplicable GMU timeouts or worse.

This is a rather ugly hack which introduces a whole lot of latency.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
---
 drivers/gpu/drm/msm/adreno/a6xx_gpu.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
index fac325217a7e..b0fa764ba299 100644
--- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
+++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
@@ -1629,6 +1629,10 @@ static int hw_init(struct msm_gpu *gpu)
 		mb();
 	}
 
+	/* Some GPUs are stubborn and take their sweet time to unhalt GBIF! */
+	if (adreno_is_a7xx(adreno_gpu) && a6xx_has_gbif(adreno_gpu))
+		spin_until(!gpu_read(gpu, REG_A6XX_GBIF_HALT_ACK));
+
 	gpu_write(gpu, REG_A6XX_RBBM_SECVID_TSB_CNTL, 0);
 
 	if (adreno_is_a619_holi(adreno_gpu))

-- 
2.41.0