[Xen-devel] drm_gem_get_pages and proper flushing/coherency

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

Hello, all!

My driver (Xen para-virtualized frontend) in some scenarios uses
drm_gem_get_pages for allocating backing storage for dumb buffers.
There are use-cases which showed some artifacts on the screen
(modetest, other) which were worked around by flushing pages of the
buffer on page flip with drm_clflush_pages. But, the problem here
is that drm_clflush_pages is not available on ARM platforms (it is a NOP)
and doing flushes on every page flip seems to be non-optimal.

Other drivers that use drm_gem_get_pages seem to use DMA map/unmap
on the shmem backed buffer (this is from where drm_gem_get_pages
allocates the pages) and this is an obvious approach as the buffer needs
to be shared with real HW for DMA - please correct me if my understanding
here is wrong.

This is the part I missed in my implementation as I don't really have a
HW device which needs DMA, but a backend running in a different Xen domain.

Thus, as the buffer is backed with cachable pages the backend may see

artifacts on its side.


I am looking for some advices on what would be the best option to
make sure dumb buffers are not flushed every page flip and still
the memory remains coherent to the backend. I have implemented a
DMA map/unmap of the shmem pages on GEM object creation/destruction
and this does solve the problem, but as the backend is not really
a DMA device this is a bit misleading.

Is there any other (more?) suitable/preferable way(s) to achieve the same?

Thank you,
Oleksandr


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel