Op 28-09-12 21:42, Thomas Hellstrom schreef:
On 09/28/2012 04:14 PM, Maarten Lankhorst wrote:
Hey,
Op 28-09-12 14:41, Maarten Lankhorst schreef:
Documentation says that code requiring dma-buf should add it to select, so inline fallbacks are not going to be used. A link error will make it obvious what went wrong, instead of silently doing nothing at runtime.
The whole patch series is in my tree, I use stg so things might move around, do not use for merging currently:
http://cgit.freedesktop.org/~mlankhorst/linux/log/?h=v10-wip
It contains everything in here plus the patches for ttm to make it work, I use a old snapshot of drm-next + merge of nouveau as base. Description of what the parts do:
Series to fix small api issues when moving over:
drm/ttm: Remove cpu_writers related code drm/ttm: Add ttm_bo_is_reserved function drm/radeon: Use ttm_bo_is_reserved drm/vmwgfx: use ttm_bo_is_reserved
drm/vmwgfx: remove use of fence_obj_args drm/ttm: remove sync_obj_arg drm/ttm: remove sync_obj_arg from ttm_bo_move_accel_cleanup drm/ttm: remove sync_arg entirely
drm/nouveau: unpin buffers before releasing to prevent lockdep warnings drm/nouveau: add reservation to nouveau_bo_vma_del drm/nouveau: add reservation to nouveau_gem_ioctl_cpu_prep
Hey great, now we only have one user left for fence waiting before reserving, lets fix that and remove fence lock: ttm_bo_cleanup_refs_or_queue and ttm_bo_cleanup_refs have to reserve before waiting, lets do it in the squash commit so we don't have to throw lock order around everywhere:
drm/ttm: remove fence_lock
-- Up to this point should be mergeable now
Then we start working on lru_lock removal slightly, this means the lru list no longer is empty but can contain only reserved buffers:
drm/ttm: do not check if list is empty in ttm_bo_force_list_clean drm/ttm: move reservations for ttm_bo_cleanup_refs
-- Still mergeable up to this point, just fixes
Patch series from this email: dma-buf: remove fallback for !CONFIG_DMA_SHARED_BUFFER fence: dma-buf cross-device synchronization (v9) seqno-fence: Hardware dma-buf implementation of fencing (v3) reservation: cross-device reservation support reservation: Add lockdep annotation and selftests
Now hook it up to drm/ttm in a few steps: usage around reservations: drm/ttm: make ttm reservation calls behave like reservation calls drm/ttm: use dma_reservation api dma-buf: use reservations drm/ttm: allow drivers to pass custom dma_reservation_objects for a bo
then kill off the lru lock around reservation: drm/ttm: remove lru_lock around ttm_bo_reserve drm/ttm: simplify ttm_eu_*
The lru_lock removal patch removes the lock around lru_lock around the reservation, this will break the assumption that items on the lru list and swap list can always be reserved, and this gets patched up too. Is there any part in ttm you disagree with? I believe that this is all mergeable, the lru_lock removal patch could be moved to before the reservation parts, this might make merging easier, but I don't think there is any ttm part of the series that are wrong on a conceptual level.
~Maarten
....From another email
As previously discussed, I'm unfortunately not prepared to accept removal of the reserve-lru atomicity into the TTM code at this point. The current code is based on this assumption and removing it will end up with efficiencies, breaking the delayed delete code and probably a locking nightmare when trying to write new TTM code.
The lru lock removal patch fixed the delayed delete code, it really is not different from the current situation. In fact it is more clear without the guarantee what various parts are trying to protect.
Nothing prevents you from holding the lru_lock while trylocking,
[1] While this would not cause any deadlocks, Any decent lockdep code would establish lru->reserve as the locking order once a lru- reserve trylock succeeds, but the locking order is really reserve->lru for obvious reasons, which means we will get a lot of lockdep errors? Yes, there are a two reversals like these already in the TTM code, and I'm not very proud of them.
I was doing a evil hack where I 'released' lru_lock to lockdep before doing the annotation for a blocking acquire, and left trylock annotations as they were. This made lockdep do the right thing.
And this is even before it starts to get interesting, like how you guarantee that when you release a buffer from the delayed delete list, you're the only process having a reference?
l thought list_kref made sure of that? Even if not the only one with a reference, the list_empty check would make sure it would only run once. I'l fix it up again so it doesn't become a WARN_ON_ONCE, I didn't know it could run multiple times at the time, so I'll change it back to unlikely.
Now, it's probably possible to achieve what you're trying to do, if we accept the lock reversal in [1], but since I have newborn twins and I have about one hour of spare time a week with I now spent on this review and I guess there are countless more hours before this can work. (These code paths were never tested, right?) One of the biggest TTM reworks was to introduce the atomicity assumption and remove a lot of code that was prone to deadlocks, races and buffer leaks. I'm not prepared to revert that work without an extremely good reason, and "It can be done" is not such a reason.
Deepest apologies, I only did a quick glance at the code part of this email, overlooked this part since I was swamped with other things and meant to do a full reply on monday. I didn't mean to make it sound like I only cared blindly about merging my code, just wanted to find a good solution.
We *need* to carefully weigh it against any benefits you have in your work, and you need to test these codepaths in parallell cases subject to heavy aperture / vram thrashing and frequent signals causing interrupted waits.
Agreed, is there already a tester for this or should I write my own?
And I think you need to present the gains in your work that can motivate the testing-and review time for this.
Agreed.
~Maarten