[PATCH 11/12] swiotlb: move the SWIOTLB config symbol to lib/Kconfig

On Tue, Apr 24, 2018 at 08:55:49AM +0200, Christoph Hellwig wrote:

On Tue, Apr 24, 2018 at 12:52:05AM +0100, Russell King - ARM Linux wrote:

quoted

On Mon, Apr 23, 2018 at 07:04:18PM +0200, Christoph Hellwig wrote:

quoted

This way we have one central definition of it, and user can select it as
needed.  Note that we also add a second ARCH_HAS_SWIOTLB symbol to
indicate the architecture supports swiotlb at all, so that we can still
make the usage optional for a few architectures that want this feature
to be user selectable.

Signed-off-by: Christoph Hellwig <hch@lst.de>

Hmm, this looks like we end up with NEED_SG_DMA_LENGTH=y on ARM by
default, which probably isn't a good idea - ARM pre-dates the dma_length
parameter in scatterlists, and I don't think all code is guaranteed to
do the right thing if this is enabled.

We shouldn't end up with NEED_SG_DMA_LENGTH=y on ARM by default.

Your patch as sent would end up with:

ARM selects ARCH_HAS_SWIOTLB
SWIOTLB is defaulted to ARCH_HAS_SWIOTLB
SWIOTLB selects NEED_SG_DMA_LENGTH

due to:

@@ -106,6 +106,7 @@ config ARM
        select REFCOUNT_FULL
        select RTC_LIB
        select SYS_SUPPORTS_APM_EMULATION
+       select ARCH_HAS_SWIOTLB

and:

+config SWIOTLB
+       bool "SWIOTLB support"
+       default ARCH_HAS_SWIOTLB
+       select NEED_SG_DMA_LENGTH

Therefore, the default state for SWIOTLB and hence NEED_SG_DMA_LENGTH
becomes 'y' on ARM, and any defconfig file that does not mention SWIOTLB
explicitly ends up with both these enabled.

It is only select by ARM_DMA_USE_IOMMU before the patch, and it will
now also be selected by SWIOTLB, which for arm is never used or seleted
directly by anything but xen-swiotlb.

See above.

Then again looking at the series there shouldn't be any need to
even select NEED_SG_DMA_LENGTH for swiotlb, as we'll never merge segments,
so I'll fix that up.

That would help to avoid any regressions along the lines I've spotted
by review.

It does look a bit weird though - patch 10 arranged stuff so that we
didn't end up with SWIOTLB always enabled, but this patch reintroduces
that with the allowance that the user can disable if so desired.

-- 
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