Re: [PATCH v2] drivers: hv: vmbus: Fix build on Clang 12 failed by DMA_BIT_MASK(64) expansion

[Wei Liu <wei.liu@kernel.org>]

On Fri, Feb 04, 2022 at 10:39:46AM +0100, Vitaly Kuznetsov wrote:
> Vitaly Chikunov <vt@altlinux.org> writes:
> 
> > Clang 12.0.1 cannot understand that value 64 is excluded from the shift
> > at compile time (for use in global context) resulting in build error[1]:
> >
> >   drivers/hv/vmbus_drv.c:2082:29: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
> >   static u64 vmbus_dma_mask = DMA_BIT_MASK(64);
> > 			      ^~~~~~~~~~~~~~~~
> >   ./include/linux/dma-mapping.h:76:54: note: expanded from macro 'DMA_BIT_MASK'
> >   #define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
> > 						       ^ ~~~
> >
> > Avoid using DMA_BIT_MASK macro for that corner case.
> 
> Was this reported to Clang already? It looks like a clear bug on their side.

This is already known.

I dealt with a similar report before. At the time I decided nothing's
wrong with the code.

https://lore.kernel.org/linux-kernel/YcC1CobR%2Fn0tJhdV@archlinux-ax161/
https://github.com/ClangBuiltLinux/linux/issues/92

I can certainly live with a workaround like this.

> 
> >
> > Cc: Tianyu Lan <Tianyu.Lan@microsoft.com>
> > Cc: Michael Kelley <mikelley@microsoft.com>
> > Cc: Long Li <longli@microsoft.com>
> > Cc: Wei Liu <wei.liu@kernel.org>
> > Link: https://lore.kernel.org/linux-hyperv/20220203235828.hcsj6najsl7yxmxa@altlinux.org/
> > Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
> > ---
> > Changes since v1:
> > - Patch description updated, s/GCC 11/Clang 12/.
> >
> >  drivers/hv/vmbus_drv.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> > index 17bf55fe3169..2376ee484362 100644
> > --- a/drivers/hv/vmbus_drv.c
> > +++ b/drivers/hv/vmbus_drv.c
> > @@ -2079,7 +2079,7 @@ struct hv_device *vmbus_device_create(const guid_t *type,
> >  	return child_device_obj;
> >  }
> >  
> > -static u64 vmbus_dma_mask = DMA_BIT_MASK(64);

Please add a comment here

  // Use ~0ULL instead of DMA_BIT_MASK(64) to work around a bug in Clang.

> > +static u64 vmbus_dma_mask = ~0ULL;
> >  /*
> >   * vmbus_device_register - Register the child device
> >   */
> 
> -- 
> Vitaly
>