Re: [PATCH] drm/i915: Restore GT performance in headless mode with DMC loaded

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, May 05, 2017 at 12:43:21PM +0100, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> 
> It seems that the DMC likes to transition between the DC states
> a lot when there are no connected displays (no active power
> domains) during simple command submission.

Is it trapping on some interrupt register accesses or what? And
if so which registers are affected?

> 
> This frantic activity on DC states has a terrible impact on the
> performance of the overall chip with huge latencies observed in
> the interrupt handlers and elsewhere. Simple tests like
> igt/gem_latency -n 0 are slowed down by a factor of eight.
> 
> Work around it by grabbing a modeset display power domain whilst
> there is any GT activity. This seems to be effective in making
> the DMC keep its paws off the chip.
> 
> On the other hand this may have a negative impact on the overall
> power budget of the chip and so could still affect performance.
> 
> This version limits the workaround got SKL GT3 and GT4 parts but
> this is just due the absence of testing on other platforms. It
> is possible we will have to apply it wider.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100572
> Testcase: igt/gem_exec_nop/headless
> Cc: Imre Deak <imre.deak@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_drv.h         | 5 +++++
>  drivers/gpu/drm/i915/i915_gem.c         | 4 ++++
>  drivers/gpu/drm/i915/i915_gem_request.c | 3 +++
>  3 files changed, 12 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 320c16df1c9c..4d58e2e28c2f 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2990,6 +2990,11 @@ intel_info(const struct drm_i915_private *dev_priv)
>  
>  #define HAS_DECOUPLED_MMIO(dev_priv) (INTEL_INFO(dev_priv)->has_decoupled_mmio)
>  
> +#define NEEDS_CSR_GT_PERF_WA(dev_priv) \
> +	HAS_CSR(dev_priv) && \
> +	(IS_SKL_GT3(dev_priv) || IS_SKL_GT4(dev_priv)) && \
> +	(dev_priv)->csr.dmc_payload
> +
>  #include "i915_trace.h"
>  
>  static inline bool intel_scanout_needs_vtd_wa(struct drm_i915_private *dev_priv)
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index b2727905ef2b..c52d863f409c 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3200,7 +3200,11 @@ i915_gem_idle_work_handler(struct work_struct *work)
>  
>  	if (INTEL_GEN(dev_priv) >= 6)
>  		gen6_rps_idle(dev_priv);
> +
>  	intel_runtime_pm_put(dev_priv);
> +
> +	if (NEEDS_CSR_GT_PERF_WA(dev_priv))
> +		intel_display_power_put(dev_priv, POWER_DOMAIN_MODESET);
>  out_unlock:
>  	mutex_unlock(&dev->struct_mutex);
>  
> diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
> index 10361c7e3b37..10a3b51f6362 100644
> --- a/drivers/gpu/drm/i915/i915_gem_request.c
> +++ b/drivers/gpu/drm/i915/i915_gem_request.c
> @@ -873,6 +873,9 @@ static void i915_gem_mark_busy(const struct intel_engine_cs *engine)
>  
>  	GEM_BUG_ON(!dev_priv->gt.active_requests);
>  
> +	if (NEEDS_CSR_GT_PERF_WA(dev_priv))
> +		intel_display_power_get(dev_priv, POWER_DOMAIN_MODESET);
> +
>  	intel_runtime_pm_get_noresume(dev_priv);
>  	dev_priv->gt.awake = true;
>  
> -- 
> 2.9.3
> 
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-- 
Ville Syrjälä
Intel OTC
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