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

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Date:   Tue, 11 Sep 2018 16:48:20 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.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 04:37:48PM -0700, Nick Desaulniers wrote:
> 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.
> 

Thanks for the review Nick. If that is indeed the case, I will spin up a v2.

> 
> >
> >  /**
> >   * struct zynqmp_dma_desc_sw - Per Transaction structure
> > --
> > 2.18.0
> >
> 
> 
> -- 
> Thanks,
> ~Nick Desaulniers