Re: [PATCH v2 4/4] drm/i915: Codify our assumption that the Global GTT is <= 4GiB

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Joonas Lahtinen <joonas.lahtinen@linux.intel.com> writes:

> [ text/plain ]
> From: Chris Wilson <chris@chris-wilson.co.uk>
>
> Throughout the code base, we use u32 for offsets into the global GTT. If
> we ever see any hardware with a larger GGTT, then we run the real risk
> of silent corruption. So test for our assumption up front so that we
> have a nice reminder should the time come when it fails.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> [Rebased and changed 1ull -> 1ULL]
> ---
>  drivers/gpu/drm/i915/i915_gem_gtt.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index 8a9fa03..ba144ba 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -3191,6 +3191,13 @@ int i915_gem_gtt_init(struct drm_device *dev)
>  	if (ret)
>  		return ret;
>  
> +	if ((ggtt->base.total - 1) >> 32) {
> +		DRM_ERROR("We never expected a Global GTT with more than 32bits of address space! Found %lldM!\n",
> +			  ggtt->base.total >> 20);
> +		ggtt->base.total = 1ULL << 32;
> +		ggtt->mappable_end = min(ggtt->mappable_end,
> ggtt->base.total);

mappable_end is not u64 so perhaps 
s/min/min_t(u64,

The mappable_end is really confusing after you realize it is used in 2
different contexts where only in the other, it holds true. In the pa
space, it definitely is not end but len. If we would rename it to
mappable_len it would be ambivalent to context and more clear.

With min_t change,
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

> +	}
> +
>  	/*
>  	 * Initialise stolen early so that we may reserve preallocated
>  	 * objects for the BIOS to KMS transition.
> -- 
> 2.5.0
>
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