linux-kernel - Re: [PATCH] vfio: Fix build break when SPAPR_TCE

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Date:   Mon, 7 Nov 2016 17:30:09 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...abs.org, aik@...abs.ru, gwshan@...ux.vnet.ibm.com
Subject: Re: [PATCH] vfio: Fix build break when SPAPR_TCE_IOMMU=n



On 07/11/2016 17:25, Alex Williamson wrote:
> On Mon, 07 Nov 2016 19:34:42 +1100
> Michael Ellerman <mpe@...erman.id.au> wrote:
> 
>> Paolo Bonzini <pbonzini@...hat.com> writes:
>>> On 04/11/2016 06:48, Michael Ellerman wrote:  
>>>> diff --git a/drivers/vfio/Kconfig b/drivers/vfio/Kconfig
>>>> index da6e2ce77495..6b51a4ebed8a 100644
>>>> --- a/drivers/vfio/Kconfig
>>>> +++ b/drivers/vfio/Kconfig
>>>> @@ -6,12 +6,12 @@ config VFIO_IOMMU_TYPE1
>>>>  config VFIO_IOMMU_SPAPR_TCE
>>>>  	tristate
>>>>  	depends on VFIO && SPAPR_TCE_IOMMU
>>>> -	default n
>>>> +	default VFIO  
>>>
>>> No need to depend on VFIO since you already have it in "default".  
> 
> depends and defaults are different beasts though, if VFIO is not
> enabled and we're not on a powerpc system with SPAPR,
> VFIO_IOMMU_SPAPR_TCE should not be selectable, not just default to 'n'.

AFAIU without a prompt nothing is selectable anyway (hence my preference
for a shorthand).

Paolo

>>
>> True, I can take that out.
>>
>>> (I assume you cannot use "default y" because "depends on" doesn't downgrade
>>> "y" to "m" when VFIO is a module.  
>>
>> Correct.
>>
>>> A shorthand is
>>>
>>> 	def_tristate VFIO && SPAPR_TCE_IOMMU  
>>
>> Yep. My experience though is that a lot of folks don't really know what
>> that means. So I prefer to spell it out with an explicit type, depends
>> and default.
>>
>> But I'll respin it that way if Alex prefers the shorter style.
> 
> Perhaps I'm one of those people.  Non-powerpc archs should not have an
> option to select this, which is why the depends is there, AIUI.  So
> long as we don't start exposing options that aren't relevant to a
> platform, I'm flexible on what shorthands we use, but you may need to
> teach me about them first.  Thanks,
> 
> Alex
>