Re: [PATCH] drm/i915: Fix 32b overflow check in gen8_ppgtt_alloc_page_directories

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

On Thu, Apr 30, 2015 at 02:59:34PM +0100, Michel Thierry wrote:
> The patch 69876bed7e008f5fe01538a2d47c09f2862129d0: "drm/i915/gen8:
> page directories rework allocation" added an overflow warning, but the
> mask had an extra 0. Use typo-prone option suggested by Dave instead.
> 
> This check will be unnecessary after gen8_alloc_va_range handles more
> than 4 PDPs (48b addressing).
> 
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Cc: Dave Gordon <david.s.gordon@intel.com>
> Signed-off-by: Michel Thierry <michel.thierry@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem_gtt.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index 6fae6bd..6d894fc 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -756,8 +756,8 @@ static int gen8_ppgtt_alloc_page_directories(struct i915_hw_ppgtt *ppgtt,
>  
>  	WARN_ON(!bitmap_empty(new_pds, GEN8_LEGACY_PDPES));
>  
> -	/* FIXME: PPGTT container_of won't work for 64b */
> -	WARN_ON((start + length) > 0x800000000ULL);
> +	/* FIXME: upper bound must not overflow 31 bits  */
> +	WARN_ON((start + length) & (~0ULL << 31));

Why is it 31 and not 32?

>  
>  	gen8_for_each_pdpe(pd, pdp, start, length, temp, pdpe) {
>  		if (pd)
> -- 
> 2.1.1
> 
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-- 
Ville Syrjälä
Intel OTC
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