drm/msm/a6xx: Ensure clean GMU state in a6xx_gmu_fw_start

While it's not very well understood, there is some sort of a fault
handler implemented in the GMU firmware which triggers when a certain
bit is set, resulting in the M3 core not booting up the way we expect
it to.

Write a magic value to a magic register to hopefully prevent that
from happening.
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/543335/Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/adreno/a6xx_gmu.c

@@ -790,6 +790,12 @@ static int a6xx_gmu_fw_start(struct a6xx_gmu *gmu, unsigned int state)
 	gmu_write(gmu, REG_A6XX_GMU_AHB_FENCE_RANGE_0,
 		(1 << 31) | (0xa << 18) | (0xa0));
 
+	/*
+	 * Snapshots toggle the NMI bit which will result in a jump to the NMI
+	 * handler instead of __main. Set the M3 config value to avoid that.
+	 */
+	gmu_write(gmu, REG_A6XX_GMU_CM3_CFG, 0x4052);
+
 	/*
 	 * Note that the GMU has a slightly different layout for
 	 * chip_id, for whatever reason, so a bit of massaging