linux/Documentation/gpu/xen-front.rst
Oleksandr Andrushchenko 4394e96423 drm/xen-front: Remove CMA support
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
2018-04-18 10:18:27 +03:00

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ReStructuredText

====================================================
drm/xen-front Xen para-virtualized frontend driver
====================================================
This frontend driver implements Xen para-virtualized display
according to the display protocol described at
include/xen/interface/io/displif.h
Driver modes of operation in terms of display buffers used
==========================================================
.. kernel-doc:: drivers/gpu/drm/xen/xen_drm_front.h
:doc: Driver modes of operation in terms of display buffers used
Buffers allocated by the frontend driver
----------------------------------------
.. kernel-doc:: drivers/gpu/drm/xen/xen_drm_front.h
:doc: Buffers allocated by the frontend driver
Buffers allocated by the backend
--------------------------------
.. kernel-doc:: drivers/gpu/drm/xen/xen_drm_front.h
:doc: Buffers allocated by the backend
Driver limitations
==================
.. kernel-doc:: drivers/gpu/drm/xen/xen_drm_front.h
:doc: Driver limitations