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drm/todo: Better defio support in the generic fbdev emulation

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The current one essentially means you need CMA or a vmalloc backed
object, which makes fbdev emulation a special case.

Since implementing this will be quite a bit of work, capture the idea
in a TODO.

Cc: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190107102238.7789-1-daniel.vetter@ffwll.ch
This commit is contained in:
Daniel Vetter 2019-01-07 11:22:38 +01:00
parent 1c95f662fc
commit be5cadc7e7
1 changed files with 30 additions and 0 deletions
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30
Documentation/gpu/todo.rst
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@ -209,6 +209,36 @@ Would be great to refactor this all into a set of small common helpers.
Contact: Daniel Vetter
Generic fbdev defio support
---------------------------
The defio support code in the fbdev core has some very specific requirements,
which means drivers need to have a special framebuffer for fbdev. Which prevents
us from using the generic fbdev emulation code everywhere. The main issue is
that it uses some fields in struct page itself, which breaks shmem gem objects
(and other things).
Possible solution would be to write our own defio mmap code in the drm fbdev
emulation. It would need to fully wrap the existing mmap ops, forwarding
everything after it has done the write-protect/mkwrite trickery:
- In the drm_fbdev_fb_mmap helper, if we need defio, change the
default page prots to write-protected with something like this::
vma->vm_page_prot = pgprot_wrprotect(vma->vm_page_prot);
- Set the mkwrite and fsync callbacks with similar implementions to the core
fbdev defio stuff. These should all work on plain ptes, they don't actually
require a struct page. uff. These should all work on plain ptes, they don't
actually require a struct page.
- Track the dirty pages in a separate structure (bitfield with one bit per page
should work) to avoid clobbering struct page.
Might be good to also have some igt testcases for this.
Contact: Daniel Vetter, Noralf Tronnes
Put a reservation_object into drm_gem_object
--------------------------------------------