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

[Chris Wilson]

On Fri, Feb 24, 2017 at 06:03:09PM -0300, Paulo Zanoni wrote:
> 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 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..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.

It's already been fixed (hopefully).

commit 754a76591b12c88f57ad8b4ca533a5c9566a1922
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Fri Feb 24 11:43:06 2017 +0000

    drm/i915/fbdev: Stop repeating tile configuration on stagnation

-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre