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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <567dfcac-165d-5b8b-8179-6a981973111f@redhat.com>
Date: Wed, 30 Jan 2019 10:57:55 +0100
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Subject: Re: [Qemu-devel] [RFC PATCH 1/5] ppc64: Express dependencies of
 'pseries' and 'powernv' machines with kconfig
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To: Thomas Huth <thuth@redhat.com>, qemu-devel@nongnu.org, yang.zhong@intel.com
Cc: qemu-ppc@nongnu.org, =?UTF-8?Q?C=c3=a9dric_Le_Goater?= <clg@kaod.org>

On 30/01/19 10:43, Thomas Huth wrote:
> CONFIG_SPAPR_RNG should stay in the ppc-softmmu.mak file since this
> is a completely optional device.

What about making it

    default y

and adding to the .mak file this:

#CONFIG_SPAPR_RNG=3Dn

I think the two approaches are more or less equivalent, but
"#CONFIG_FOO=3Dn" has a small advantage when the feature has a build-time
dependency, such as CONFIG_MILKYMIST_TMU2's dependency on OpenGL.  In
that case,

   CONFIG_MILKYMIST_TMU2=3Dy

would report a contradiction if OpenGL is not available at build time, wh=
ile

   default y
   ...
   #CONFIG_MILKYMIST_TMU2=3Dn

would not.

In any case, there are many many such cases in x86 (QXL, HYPERV, FDC,
SEV, ISA_IPMI_*, APPLESMC, SGA, HPET, VTD, AMD_IOMMU, PVPANIC, TPM_TIS,
TPM_CRB) and they should be handled in the same way---and especially
they should be mentioned in the default-configs/ file under "Optional
devices".  So thanks for noticing this case!

Paolo