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It turns out this was only needed to paper over a bug in the CMA
helpers, which was addressed in
commit 998fb1a0f4
Author: Liviu Dudau <Liviu.Dudau@arm.com>
Date: Fri Nov 10 13:33:10 2017 +0000
drm: gem_cma_helper.c: Allow importing of contiguous scatterlists with nents > 1
Without this the following pipeline didn't work:
domU:
1. xen-front allocates a non-contig buffer
2. creates grants out of it
dom0:
3. converts the grants into a dma-buf. Since they're non-contig, the
scatter-list is huge.
4. imports it into rcar-du, which requires dma-contig memory for
scanout.
-> On this given platform there's an IOMMU, so in theory this should
work. But in practice this failed, because of the huge number of sg
entries, even though the IOMMU driver mapped it all into a dma-contig
range.
With a guest-contig buffer allocated in step 1, this problem doesn't
exist. But there's technically no reason to require guest-contig
memory for xen buffer sharing using grants.
Given all that, the xen-front cma support is not needed and should be
removed.
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180417074012.21311-1-andr2000@gmail.com
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====================================================
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drm/xen-front Xen para-virtualized frontend driver
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====================================================
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This frontend driver implements Xen para-virtualized display
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according to the display protocol described at
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include/xen/interface/io/displif.h
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Driver modes of operation in terms of display buffers used
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==========================================================
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.. kernel-doc:: drivers/gpu/drm/xen/xen_drm_front.h
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:doc: Driver modes of operation in terms of display buffers used
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Buffers allocated by the frontend driver
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----------------------------------------
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.. kernel-doc:: drivers/gpu/drm/xen/xen_drm_front.h
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:doc: Buffers allocated by the frontend driver
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Buffers allocated by the backend
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--------------------------------
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.. kernel-doc:: drivers/gpu/drm/xen/xen_drm_front.h
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:doc: Buffers allocated by the backend
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Driver limitations
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==================
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.. kernel-doc:: drivers/gpu/drm/xen/xen_drm_front.h
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:doc: Driver limitations
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