Re: [PATCH 15/21] drm/i915/gtt: Fill scratch page

From: Michel Thierry <michel.thierry@intel.com>
To: Mika Kuoppala <mika.kuoppala@linux.intel.com>,
	"intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>
Subject: Re: [PATCH 15/21] drm/i915/gtt: Fill scratch page
Date: Wed, 03 Jun 2015 15:03:15 +0100	[thread overview]
Message-ID: <556F0923.3070400@intel.com> (raw)
In-Reply-To: <1432314314-23530-16-git-send-email-mika.kuoppala@intel.com>

On 5/22/2015 6:05 PM, Mika Kuoppala wrote:
> During review of dynamic page tables series, I was able
> to hit a lite restore bug with execlists. I assume that
> due to incorrect pd, the batch run out of legit address space
> and into the scratch page area. The ACTHD was increasing
> due to scratch being all zeroes (MI_NOOPs). And as gen8
> address space is quite large, the hangcheck happily waited
> for a long long time, keeping the process effectively stuck.

FYI, it is probably safe to assume that the only thing updated in a 
lite-restore is the ring tail. I didn't realize that until recently.

This issue is more frequent in GuC submission mode, and we are currently 
evaluation 2 alternatives to trigger the re-reading of the page tables.

>
> According to Chris Wilson any modern gpu will grind to halt
> if it encounters commands of all ones. This seemed to do the
> trick and hang was declared promptly when the gpu wandered into
> the scratch land.
>
> v2: Use 0xffff00ff pattern (Chris)
>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_gem_gtt.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index 43fa543..a2a0c88 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -2168,6 +2168,8 @@ void i915_global_gtt_cleanup(struct drm_device *dev)
>          vm->cleanup(vm);
>   }
>
> +#define SCRATCH_PAGE_MAGIC 0xffff00ffffff00ffULL
> +
>   static int alloc_scratch_page(struct i915_address_space *vm)
>   {
>          struct i915_page_scratch *sp;
> @@ -2185,6 +2187,7 @@ static int alloc_scratch_page(struct i915_address_space *vm)
>                  return ret;
>          }
>
> +       fill_px(vm->dev, sp, SCRATCH_PAGE_MAGIC);
>          set_pages_uc(px_page(sp), 1);
>
>          vm->scratch_page = sp;
> --
> 1.9.1
>
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