[Intel-gfx] [PATCH] drm/i915: Prevent machine hang from Broxton's vtd w/a and error capture

[Daniel Vetter]

On Wed, Dec 06, 2017 at 02:19:03PM +0000, Chris Wilson wrote:
> Since capturing the error state requires fiddling around with the GGTT
> to read arbitrary buffers and is itself run under stop_machine(), it
> deadlocks the machine (effectively a hard hang) when run in conjunction
> with Broxton's VTd workaround to serialize GGTT access.
> 
> Fixes: 0ef34ad6222a ("drm/i915: Serialize GTT/Aperture accesses on BXT")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Jon Bloomfield <jon.bloomfield at intel.com>
> Cc: John Harrison <john.C.Harrison at intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gpu_error.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
> index 48418fb81066..e6c7e8e53815 100644
> --- a/drivers/gpu/drm/i915/i915_gpu_error.c
> +++ b/drivers/gpu/drm/i915/i915_gpu_error.c
> @@ -1813,6 +1813,10 @@ void i915_capture_error_state(struct drm_i915_private *dev_priv,
>  	if (!i915_modparams.error_capture)
>  		return;
>  
> +	/* Prevent recursively calling stop_machine() and deadlocking. */
> +	if (intel_ggtt_update_needs_vtd_wa(dev_priv))
> +		return;

I'd put this closer to the stop machine, at the head of
i915_capture_gpu_state(). If the bogus debug output annoys then we could
switch that to an PTR_ERR return value I guess. But I guess this here is
ok too, so either way:

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> +
>  	if (READ_ONCE(dev_priv->gpu_error.first_error))
>  		return;
>  
> -- 
> 2.15.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch