[Intel-gfx] [PATCH] drm/i915: limit PPGTT size to 2GB in 32-bit platforms

[Chris Wilson]

On Thu, May 28, 2015 at 04:24:47PM +0100, Michel Thierry wrote:
> We already set this limit for the GGTT.
> 
> This is a temporary patch until a full replacement of size_t variables
> (inadequate in 32-bit kernel) is in place.
> 
> Regression from:
> 	commit a4e0bedca678c81eea4cd79a4bd502335639f73a
> 	Author: Michel Thierry <michel.thierry at intel.com>
> 	Date:   Wed Apr 8 12:13:35 2015 +0100
> 
> 		drm/i915: Use complete address space in true PPGTT
> 
> Suggested-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> Signed-off-by: Michel Thierry <michel.thierry at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem_gtt.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index 17b7df0..ffecb87 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -951,7 +951,15 @@ static int gen8_ppgtt_init(struct i915_hw_ppgtt *ppgtt)
>  	gen8_initialize_pd(&ppgtt->base, ppgtt->scratch_pd);
>  
>  	ppgtt->base.start = 0;
> +
> +#ifdef CONFIG_X86_32
> +	/* Limit 32b platforms to GGTT size (2GB) */

Forgot the comment about why the limit is inplace and when it can be
lifted. Remember comments are for *why* not *what*.

> +	struct drm_i915_private *dev_priv = ppgtt->base.dev->dev_private;

Fancy new fangled C.

> +	ppgtt->base.total = dev_priv->gtt.base.total;
	ppgtt->base.total = to_i915(ppgtt->base.dev)->gtt.base.total;

> +#else
>  	ppgtt->base.total = 1ULL << 32;
> +#endif

Better yet would be:
	ppgtt->base.total = 1ULL << 32;
	if (IS_ENABLED(CONFIG_X86_32)) {
		/* While we have a proliferation of size_t variables
		 * we cannot represent the full ppgtt size on 32bit,
		 * so limit it to the same size as the GGTT (currently
		 * 2GiB).
		 */
		 ppgtt->base.total = to_i915(ppgtt->base.dev)->gtt.base.total;
	}

However, I am afraid that it will become obsolete but be forgotten to be
removed.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre