Re: [PATCH] drm/i915: Silence build error with UBSAN

[Chris Wilson]

Quoting Jani Nikula (2018-10-16 10:59:42)
> On Mon, 15 Oct 2018, Stephen Boyd <swboyd@xxxxxxxxxxxx> wrote:
> > When I enable UBSAN and compile this driver with clang I get the
> > following build error:
> >
> > drivers/gpu/drm/i915/intel_engine_cs.o: In function `intel_engine_init_execlist':
> > drivers/gpu/drm/i915/intel_engine_cs.c:411: undefined reference to `__compiletime_assert_411'
> >
> > from what I can figure out, the compiler can't optimize
> > execlists_num_ports() sufficiently enough at compile time to figure out
> > that the 'execlists->port_mask = 1' assignment one line above the
> > BUILD_BUG_ON_NOT_POWER_OF_2 check will make execlists_num_ports() return
> > 2. Most likely that's because UBSAN is going to check the load inside
> > execlists_num_ports() and that check isn't omitted so the optimizer
> > can't optimize away the whole function.
> 
> See [1] for a better explanation.
> 
> [1] http://mid.mail-archive.com/20181009171401.14980-1-natechancellor@xxxxxxxxx
> 
> > So let's just change this check to cause a build error when the maximum
> > number of ports isn't a power of two. It looks like this is similar to
> > what's being checked here so this might work well enough.
> 
> That's not the same thing. I guess I'd go with the below instead,
> similar to the check on the next line. I guess both of the checks could
> be static on gcc.
> 
> BR,
> Jani.
> 
> 
> diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
> index f27dbe26bcc1..897d5a557d88 100644
> --- a/drivers/gpu/drm/i915/intel_engine_cs.c
> +++ b/drivers/gpu/drm/i915/intel_engine_cs.c
> @@ -461,12 +461,14 @@ static void intel_engine_init_batch_pool(struct intel_engine_cs *engine)
>         i915_gem_batch_pool_init(&engine->batch_pool, engine);
>  }
>  
> +#define IS_POWER_OF_2(n) ((n) != 0 && ((n) & ((n) - 1)) == 0)
> +
>  static void intel_engine_init_execlist(struct intel_engine_cs *engine)
>  {
>         struct intel_engine_execlists * const execlists = &engine->execlists;
>  
>         execlists->port_mask = 1;
> -       BUILD_BUG_ON_NOT_POWER_OF_2(execlists_num_ports(execlists));
> +       GEM_BUG_ON(!IS_POWER_OF_2(execlists_num_ports(execlists)));

That should be happy with is_power_of_2() from log2.h
-Chris