[Intel-gfx] [PATCH] drm/i915: Avoid BIT(max) - 1 and use GENMASK(max, 0)

[Chris Wilson]

On Tue, Feb 07, 2017 at 03:51:41PM +0200, Joonas Lahtinen wrote:
> "BIT(max) - 1" will overflow when max = 32, and GCC will complain.
> We already have GENMASK for generating the mask, use it!
> 
> Signed-off-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/intel_device_info.c | 2 +-
>  drivers/gpu/drm/i915/intel_fbdev.c       | 2 +-
>  drivers/gpu/drm/i915/intel_runtime_pm.c  | 2 +-
>  3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_device_info.c b/drivers/gpu/drm/i915/intel_device_info.c
> index fcf8181..1c06fbb 100644
> --- a/drivers/gpu/drm/i915/intel_device_info.c
> +++ b/drivers/gpu/drm/i915/intel_device_info.c
> @@ -234,7 +234,7 @@ static void broadwell_sseu_info_init(struct drm_i915_private *dev_priv)
>  	 * The subslice disable field is global, i.e. it applies
>  	 * to each of the enabled slices.
>  	 */
> -	sseu->subslice_mask = BIT(ss_max) - 1;
> +	sseu->subslice_mask = GENMASK(ss_max, 0);

Too simple!

	BUILD_BUG_ON(BIT(ss_max)-1 != GENMASK(ss_max, 0));

Needs to be GENMASK(ss_max - 1, 0); the high bit is inclusive.

Otherwise, the list matches up with a quick grep for BIT*(x)-1.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre