Re: [PATCH 1/5] drm/i915/gem: fix bitwise and logical AND mixup

[Nathan Chancellor <nathan@kernel.org>]

On Wed, Sep 18, 2024 at 02:17:44PM +0300, Jani Nikula wrote:
> CONFIG_DRM_I915_USERFAULT_AUTOSUSPEND is an int, defaulting to 250. When
> the wakeref is non-zero, it's either -1 or a dynamically allocated
> pointer, depending on CONFIG_DRM_I915_DEBUG_RUNTIME_PM. It's likely that
> the code works by coincidence with the bitwise AND, but with
> CONFIG_DRM_I915_DEBUG_RUNTIME_PM=y, there's the off chance that the
> condition evaluates to false, and intel_wakeref_auto() doesn't get
> called. Switch to the intended logical AND.
> 
> Fixes: ad74457a6b5a ("drm/i915/dgfx: Release mmap on rpm suspend")
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Cc: Anshuman Gupta <anshuman.gupta@intel.com>
> Cc: Andi Shyti <andi.shyti@linux.intel.com>
> Cc: <stable@vger.kernel.org> # v6.1+
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_ttm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> index 5c72462d1f57..c157ade48c39 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> @@ -1131,7 +1131,7 @@ static vm_fault_t vm_fault_ttm(struct vm_fault *vmf)
>  		GEM_WARN_ON(!i915_ttm_cpu_maps_iomem(bo->resource));
>  	}
>  
> -	if (wakeref & CONFIG_DRM_I915_USERFAULT_AUTOSUSPEND)
> +	if (wakeref && CONFIG_DRM_I915_USERFAULT_AUTOSUSPEND)

This is going to result in a clang warning:

  drivers/gpu/drm/i915/gem/i915_gem_ttm.c:1134:14: error: use of logical '&&' with constant operand [-Werror,-Wconstant-logical-operand]
   1134 |         if (wakeref && CONFIG_DRM_I915_USERFAULT_AUTOSUSPEND)
        |                     ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  drivers/gpu/drm/i915/gem/i915_gem_ttm.c:1134:14: note: use '&' for a bitwise operation
   1134 |         if (wakeref && CONFIG_DRM_I915_USERFAULT_AUTOSUSPEND)
        |                     ^~
        |                     &
  drivers/gpu/drm/i915/gem/i915_gem_ttm.c:1134:14: note: remove constant to silence this warning
   1134 |         if (wakeref && CONFIG_DRM_I915_USERFAULT_AUTOSUSPEND)
        |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

Consider adding the explicit '!= 0' to make it clear this should be a
boolean expression.

Cheers,
Nathan

>  		intel_wakeref_auto(&to_i915(obj->base.dev)->runtime_pm.userfault_wakeref,
>  				   msecs_to_jiffies_timeout(CONFIG_DRM_I915_USERFAULT_AUTOSUSPEND));
>  
> -- 
> 2.39.2
>