Am 26.11.20 um 13:14 schrieb Thomas Zimmermann:
Hi
Am 26.11.20 um 13:08 schrieb Christian König:
[...]
Yeah, I remember a bug report about frequent page-table
modifications wrt to vram helpers. So we implemented the lazy
unmapping / vmap caching, as suggested by Christian back them. My
guess is that anything TTM-based can use a similar pattern.
Christian probably knows the corner cases.
My memory is failing me, what corner case are you talking about?
Haha! :D What I meant was that if there were corner cases, you'd know
about them. From your comment, I guess there are none.
Ah, ok :) I was wondering for a moment if I have missed something.
Cheers,
Christian.
Best regards
Thomas
Christian.
CMA seems obviously working correctly already.
For SHMEM, I'd have to figure out the reference counting of the
pages involved. My guess is that the vunmap in
drm_gem_shmem_vunmap() could be moved into the free callback, plus a
few other modifications.
Best regards
Thomas
But if you're willing to do all that conversion of callers, then of
course I'm not stopping you. Not at all, it's great to see that kind
of maze untangled.
-Daniel
Best regards
Thomas
That should give us at least some way forward to gradually conver
all the
drivers and helpers over to dma_resv locking.
-Daniel
The pin count is currently maintained by the vmap implementation
in vram
helpers. Calling vmap is an implicit pin; calling vunmap is an
implicit
unpin. This prevents eviction in the damage worker. But now I
was told than
pinning is only for BOs that are controlled by userspace and
internal users
should acquire the resv lock. So vram helpers have to be fixed,
actually.
In vram helpers, unmapping does not mean eviction. The unmap
operation only
marks the BO as unmappable. The real unmap happens when the
eviction takes
place. This avoids many modifications to the page tables. IOW an
unpinned,
unmapped BO will remain in VRAM until the memory is actually
needed.
Best regards
Thomas
So I'm still not seeing how this can go boom.
Now long term it'd be nice to cut everything over to dma_resv
locking, but
the issue there is that beyond ttm, none of the helpers (and
few of the
drivers) use dma_resv. So this is a fairly big uphill battle.
Quick
interim fix seems like the right solution to me.
-Daniel
Regards,
Christian.
Best regards
Thomas
There's no recursion taking place, so I guess the reservation
lock could be
acquired/release in drm_client_buffer_vmap/vunmap(), or a
separate pair of
DRM client functions could do the locking.
Given how this "do the right locking" is a can of worms (and
I think
it's
worse than what you dug out already) I think the
fb_helper.lock hack is
perfectly good enough.
I'm also somewhat worried that starting to use dma_resv lock
in generic
code, while many helpers/drivers still have their
hand-rolled locking,
will make conversion over to dma_resv needlessly more
complicated.
-Daniel
Best regards
Thomas
[1]
https://cgit.freedesktop.org/drm/drm-tip/tree/drivers/gpu/drm/drm_fb_helper.c?id=ac60f3f3090115d21f028bffa2dcfb67f695c4f2#n394
Please note that the reservation lock you need to take
here is part of
the GEM object.
Usually we design things in the way that the code needs to
take a lock
which protects an object, then do some operations with the
object and
then release the lock again.
Having in the lock inside the operation can be done as
well, but
returning with it is kind of unusual design.
Sorry for the noob questions. I'm still trying to
understand the
implications of acquiring these locks.
Well this is the reservation lock of the GEM object we are
talking about
here. We need to take that for a couple of different
operations,
vmap/vunmap doesn't sound like a special case to me.
Regards,
Christian.
Best regards
Thomas
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--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer
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