Re: [PATCH] drm/i915: Reword warning for missing cases

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Mar 13, 2018 at 01:06:49PM +0000, Chris Wilson wrote:
> Quoting Ville Syrjälä (2018-03-13 13:01:42)
> > On Tue, Mar 13, 2018 at 12:17:13AM +0000, Chris Wilson wrote:
> > > Quoting Lucas De Marchi (2018-03-13 00:03:12)
> > > > In some places we end up converting switch statements to a series of
> > > > if/else, particularly when introducing helper functions to handle a
> > > > group of cases. It's tempting to either leave a wrong warning (since now
> > > > we don't have a switch case anymore) or to convert to WARN(1, ...),
> > > > losing what MISSING_CASE() provides: source location and id number.
> > > > 
> > > > Fix the message to allow reusing MISSING_CASE() - it may not always be
> > > > correct (e.g. if you are not checking an id anymore), but it avoids
> > > > useless conversions. A quick grep reveals at least a few users in
> > > > drivers/gpu/drm/i915/intel_csr.c and drivers/gpu/drm/i915/intel_ddi.c.
> > > > 
> > > > Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/i915_utils.h | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
> > > > index 51dbfe5bb418..8cdc21b92f5f 100644
> > > > --- a/drivers/gpu/drm/i915/i915_utils.h
> > > > +++ b/drivers/gpu/drm/i915/i915_utils.h
> > > > @@ -40,7 +40,7 @@
> > > >  #undef WARN_ON_ONCE
> > > >  #define WARN_ON_ONCE(x) WARN_ONCE((x), "%s", "WARN_ON_ONCE(" __stringify(x) ")")
> > > >  
> > > > -#define MISSING_CASE(x) WARN(1, "Missing switch case (%lu) in %s\n", \
> > > > +#define MISSING_CASE(x) WARN(1, "Missing case (%lu) in %s\n", \
> > > >                              (long)(x), __func__)
> > > 
> > > Whilst here you could make this more informative by:
> > > "Missing case (%s = %lu) in %s\n", __stringify(x), (long)(x), __func__
> > 
> > The backtrace isn't enough?
> 
> Not if we have more than one in a function.

I was just commenting on the __func__ part actually. That seems pretty
much redundant to me. The stringify part does seem like a decent idea,
and matches our WARN() trickery pretty well.

> (Why would we do that, you
> might ask, and I'd answer if the point is to make this more generic and
> versatile, then do so. We already have the value, why not then explain
> what that value is.) And give me a single sentence identifying the missing
> case makes it much more pleasant.
> -Chris

-- 
Ville Syrjälä
Intel OTC
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