linux-kernel - Re: [PATCH] dmaengine: ep93xx: Return proper enum in ep93xx_dma_chan

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Date:   Wed, 12 Sep 2018 11:36:19 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     dan.j.williams@...el.com, vkoul@...nel.org,
        dmaengine@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dmaengine: ep93xx: Return proper enum in
 ep93xx_dma_chan_direction

On Wed, Sep 12, 2018 at 10:00:57AM -0700, Nick Desaulniers wrote:
> On Tue, Sep 11, 2018 at 4:40 PM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > Clang warns when implicitly converting from one enumerated type to
> > another. Avoid this by using the equivalent value from the expected
> > type.
> >
> > In file included from drivers/dma/ep93xx_dma.c:30:
> > ./include/linux/platform_data/dma-ep93xx.h:88:10: warning: implicit
> > conversion from enumeration type 'enum dma_data_direction' to different
> > enumeration type 'enum dma_transfer_direction' [-Wenum-conversion]
> >                 return DMA_NONE;
> >                 ~~~~~~ ^~~~~~~~
> > 1 warning generated.
> >
> > Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > ---
> >  include/linux/platform_data/dma-ep93xx.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/platform_data/dma-ep93xx.h b/include/linux/platform_data/dma-ep93xx.h
> > index f8f1f6b952a6..eb9805bb3fe8 100644
> > --- a/include/linux/platform_data/dma-ep93xx.h
> > +++ b/include/linux/platform_data/dma-ep93xx.h
> > @@ -85,7 +85,7 @@ static inline enum dma_transfer_direction
> >  ep93xx_dma_chan_direction(struct dma_chan *chan)
> >  {
> >         if (!ep93xx_dma_chan_is_m2p(chan))
> > -               return DMA_NONE;
> > +               return DMA_TRANS_NONE;
> 
> Looks like this changes the value of the returned enum from:
> DMA_NONE: 3
> DMA_TRANS_NONE: 4
> 
> Hopefully the DMA maintainer can review if this was ever correct?
> Looks like the callers of ep93xx_dma_chan_direction were comparing
> this return type, so those comparisons would likely always be failing
> if `!ep93xx_dma_chan_is_m2p(chan)`.
> 
> Normally this warning (-Wenum-conversion) is kind of noisey, but it's
> concerning when the enumerated values evaluate to different integers.
> I think it's found an actual bug here.
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> 

I should have highlighted this in the actual commit message but I
checked all of the callsites of ep93xx_dma_chan_direction and one
directly refers to an enum value, DMA_MEM_TO_DEV; every other one
just checks if the value is or isn't equal to another value of this
enum type (DMA_NONE isn't referenced anywhere else in this driver).
I don't believe this results in a funcitonal change.

Thanks for the review!
Nathan

> >
> >         /* even channels are for TX, odd for RX */
> >         return (chan->chan_id % 2 == 0) ? DMA_MEM_TO_DEV : DMA_DEV_TO_MEM;
> > --
> > 2.18.0
> >
> 
> 
> -- 
> Thanks,
> ~Nick Desaulniers