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

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Message-ID: <CAKwvOdmJ1NGmFi8Vcv5oPr7828X3tbOZeUainPhtC4HnTK_xjw@mail.gmail.com>
Date:   Wed, 12 Sep 2018 10:00:57 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.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 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>

>
>         /* 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