linux-kernel - Re: [PATCH] dmaengine: xilinx_dma: Fix __aligned attribute on zynqmp_dma_desc

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Message-ID: <CAKwvOd=KaX4+3J4WGOcWBAfhHEyytXKKCLHFdCuDAr5McQ-g=g@mail.gmail.com>
Date:   Tue, 11 Sep 2018 16:37:48 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     dan.j.williams@...el.com, vkoul@...nel.org,
        appana.durga.rao@...inx.com, dmaengine@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dmaengine: xilinx_dma: Fix __aligned attribute on zynqmp_dma_desc_ll

On Tue, Sep 11, 2018 at 4:06 PM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> Clang warns:
>
> drivers/dma/xilinx/zynqmp_dma.c:166:4: warning: attribute 'aligned' is
> ignored, place it after "struct" to apply attribute to type declaration
> [-Wignored-attributes]
> }; __aligned(64)
>    ^
> ./include/linux/compiler_types.h:200:38: note: expanded from macro
> '__aligned'
> #define __aligned(x)            __attribute__((aligned(x)))
>                                                ^
> 1 warning generated.
>
> Place __aligned before the semicolon.
>
> Fixes: b0cc417c1637 ("dmaengine: Add Xilinx zynqmp dma engine driver support")
> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>  drivers/dma/xilinx/zynqmp_dma.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/dma/xilinx/zynqmp_dma.c b/drivers/dma/xilinx/zynqmp_dma.c
> index c74a88b65039..dc19d67cb8c1 100644
> --- a/drivers/dma/xilinx/zynqmp_dma.c
> +++ b/drivers/dma/xilinx/zynqmp_dma.c
> @@ -163,7 +163,7 @@ struct zynqmp_dma_desc_ll {
>         u32 ctrl;
>         u64 nxtdscraddr;
>         u64 rsvd;
> -}; __aligned(64)
> +} __aligned(64);

Thanks for this patch Nathan.  Thinking more about this...the integer
passed to __attribute__((aligned(x))) should be in terms of bytes.  64
bytes seems kind of high.  Maybe they meant 64 *bits* thus 8 *bytes*
which already the default alignment of the struct:
https://godbolt.org/z/7vW6E3

In which case, the correct fix is to remove the `__aligned(64);`
outright.  Since that doesn't change anything (thanks to clang's
helpful -Wignored-attributes), such a patch would be "No Functional
Change" (does not change the status quo).  Still, it might be good for
the maintainer to remark if 64 *byte* alignment was intentional (I
would think not, but I don't have the datasheet for this piece of
hardware in front of me; never say never) before sending such a patch.

If the 64 *byte* (512 bit) alignment was intentional (again, which I
doubt), then this patch is good to go, but that would then be a
functional change and should be tested by someone with hardware.

>
>  /**
>   * struct zynqmp_dma_desc_sw - Per Transaction structure
> --
> 2.18.0
>

-- 
Thanks,
~Nick Desaulniers