During IGT testing it has been shown that the specification defined polling time of 1 us for FCLK_DONE, is sometimes not enough. The issue is still reproducible while disabling C-states through the PM QoS framework and also while disabling preemtion. From this the most plausible explanation is that the issue is due to a firmware flaw. As a workaround, it is better to wait a little bit longer for the FCLK_DONE to come around, than to leave with an DRM_ERROR and having FCLK_DONE at a randome time after. While spinning a list of igt tests prone to reproduce the issue the FCLK_DONE poll failed at approximately 2% of the invocations of the bdw_set_cdclk function. The longest poll time during this testing was measured to ~7us. So, the suggested new poll time of 100us is on the safe side. v2: Added more documentation about investigations done. Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102243 Signed-off-by: Marta Lofstedt <marta.lofstedt@xxxxxxxxx> Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx> --- drivers/gpu/drm/i915/intel_cdclk.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/drivers/gpu/drm/i915/intel_cdclk.c b/drivers/gpu/drm/i915/intel_cdclk.c index d32911816fc2..f89232e0f6fa 100644 --- a/drivers/gpu/drm/i915/intel_cdclk.c +++ b/drivers/gpu/drm/i915/intel_cdclk.c @@ -669,8 +669,11 @@ static void bdw_set_cdclk(struct drm_i915_private *dev_priv, val |= LCPLL_CD_SOURCE_FCLK; I915_WRITE(LCPLL_CTL, val); + /* According to the spec, it should be enough to poll for this 1 us. + * However, extensive testing shows that this can take longer. + */ if (wait_for_us(I915_READ(LCPLL_CTL) & - LCPLL_CD_SOURCE_FCLK_DONE, 1)) + LCPLL_CD_SOURCE_FCLK_DONE, 100)) DRM_ERROR("Switching to FCLK failed\n"); val = I915_READ(LCPLL_CTL); -- 2.11.0 _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
