Re: [PATCH v2 1/2] drm/i915: Avoid BIT(max) - 1 and use GENMASK(max - 1, 0)

[Paulo Zanoni <paulo.r.zanoni@intel.com>]

Em Qua, 2017-02-08 às 15:12 +0200, Joonas Lahtinen escreveu:
> "BIT(max) - 1" will overflow when max = 32, and GCC will complain.
> We already have GENMASK for generating the mask, use it!
> 
> v2: Majestic off by one spotted (Chris)
> 
> Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Chris Wilson <chris@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..0891cc0 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 - 1, 0);
>  	sseu->subslice_mask &= ~((fuse2 & GEN8_F2_SS_DIS_MASK) >>
>  				 GEN8_F2_SS_DIS_SHIFT);
>  
> diff --git a/drivers/gpu/drm/i915/intel_fbdev.c
> b/drivers/gpu/drm/i915/intel_fbdev.c
> index 281c5c4..e6f3eb2d 100644
> --- a/drivers/gpu/drm/i915/intel_fbdev.c
> +++ b/drivers/gpu/drm/i915/intel_fbdev.c
> @@ -369,7 +369,7 @@ static bool intel_fb_initial_config(struct
> drm_fb_helper *fb_helper,
>  		return false;
>  
>  	memcpy(save_enabled, enabled, count);
> -	mask = BIT(count) - 1;
> +	mask = GENMASK(count - 1, 0);

Due to some debugging accident I ended up with a machine where count is
zero.

In this case:

"BIT(count) - 1" is 0
"GENMASK(count - 1, 0)" is 0xFFFFFFFFFFFFFFFF

The consequence is that the machine freezes after i915.ko is loaded.

So we have a "short blanket" problem here: one solution is wrong for
the maximum value while the other solution is wrong for the minimum
value. Either your chest or your feet gets warm, not both.

I see that "count" comes from fb_helper->connector_count. The
drm_fb_helper.h documentation says that this is "number of connected
connectors". So now I'm wondering that maybe zero is actually a
possible value (outside of my accident), in which case this patch would
be considered a regression. Maybe for those PCH_NONE/PCH_NOP cases?

Perhaps we could only revert this specific chunk and keep the power
domain chunk using GENMASK? Any other solutions?

Also, I have no idea if the subslice_mask case would accept zero as
input. I'll let you do the analysis of this piece of the code.

Thanks,
Paulo

>  	conn_configured = 0;
>  retry:
>  	for (i = 0; i < count; i++) {
> diff --git a/drivers/gpu/drm/i915/intel_runtime_pm.c
> b/drivers/gpu/drm/i915/intel_runtime_pm.c
> index 66aa1bb..94df466 100644
> --- a/drivers/gpu/drm/i915/intel_runtime_pm.c
> +++ b/drivers/gpu/drm/i915/intel_runtime_pm.c
> @@ -1249,7 +1249,7 @@ static void
> vlv_dpio_cmn_power_well_disable(struct drm_i915_private *dev_priv,
>  	vlv_set_power_well(dev_priv, power_well, false);
>  }
>  
> -#define POWER_DOMAIN_MASK (BIT(POWER_DOMAIN_NUM) - 1)
> +#define POWER_DOMAIN_MASK (GENMASK(POWER_DOMAIN_NUM - 1, 0))
>  
>  static struct i915_power_well *lookup_power_well(struct
> drm_i915_private *dev_priv,
>  						 int power_well_id)
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