Re: [PATCH 04/10] drm/ttm: change fence_lock to inner lock, v3

[Maarten Lankhorst <maarten.lankhorst@canonical.com>]

Hey,

Op 20-11-12 16:08, Thomas Hellstrom schreef:
> On 11/20/2012 02:13 PM, Maarten Lankhorst wrote:
>> Op 20-11-12 13:03, Thomas Hellstrom schreef:
>>> On 11/20/2012 12:33 PM, Maarten Lankhorst wrote:
>>>> Op 20-11-12 08:48, Thomas Hellstrom schreef:
>>>>> On 11/19/2012 04:33 PM, Maarten Lankhorst wrote:
>>>>>> Op 19-11-12 16:04, Thomas Hellstrom schreef:
>>>>>>> On 11/19/2012 03:17 PM, Thomas Hellstrom wrote:
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> This patch looks mostly good, although I think ttm_bo_cleanup_refs becomes overly complicated:
>>>>>>>> Could this do, or am I missing something?
>>>>>>>>
>>>>>>> Actually, my version is bad, because ttm_bo_wait() is called with the lru lock held.
>>>>>>>
>>>>>>> /Thomas
>>>>>> Oh digging through it made me remember why I had to release the reservation early and
>>>>>> had to allow move_notify to be called without reservation.
>>>>>>
>>>>>> Fortunately move_notify has a NULL parameter, which is the only time that happens,
>>>>>> so you can still check do BUG_ON(mem != NULL && !ttm_bo_reserved(bo)); in your
>>>>>> move_notify handler.
>>>>>>
>>>>>> 05/10 removed the loop and assumed no new fence could be attached after the driver has
>>>>>> declared the bo dead.
>>>>>>
>>>>>> However, at that point it may no longer hold a reservation to confirm this, that's why
>>>>>> I moved the cleanup to be done in the release_list handler. It could still be done in
>>>>>> ttm_bo_release, but we no longer have a reservation after we waited. Getting
>>>>>> a reservation can fail if the bo is imported for example.
>>>>>>
>>>>>> While it would be true that in that case a new fence may be attached as well, that
>>>>>> would be less harmful since that operation wouldn't involve this device, so the
>>>>>> ttm bo can still be removed in that case. When that time comes I should probably
>>>>>> fix up that WARN_ON(ret) in ttm_bo_cleanup_refs. :-)
>>>>>>
>>>>>> I did add a WARN_ON(!atomic_read(&bo->kref.refcount)); to
>>>>>> ttm_bo_reserve and ttm_eu_reserve_buffers to be sure nothing is done on the device
>>>>>> itself. If that is too paranoid, those WARN_ON's could be dropped. I prefer to leave them
>>>>>> in for a kernel release or 2. But according to the rules that would be the only time you
>>>>>> could attach a new fence and trigger the WARN_ON for now..
>>>>> Hmm, I'd appreciate if you could group patches with functional changes that depend on eachother togeteher,
>>>>> and "this is done because ...", which makes it much easier to review, (and to follow the commit history in case
>>>>> something goes terribly wrong and we need to revert).
>>>>>
>>>>> Meanwhile I'll take a look at the final ttm_bo.c and see if I can spot any culprits.
>>>>>
>>>>> In general, as long as a bo is on a LRU list, we must be able to attach fences because of accelerated eviction.
>>>> I thought it was deliberately designed in such a way that it was kept on the lru list,
>>>> but since it's also on the ddestroy list it won't start accelerated eviction,
>>>> since it branches into cleanup_refs early, and lru_lock still protects all the list entries.
>>> I used bad wording. I meant that unbinding might be accelerated, but  currently (quite inefficiently)
>>> do synchronized unbinding, assuming that only the CPU can do that. When we start to support
>>> unsynchronized moves, we need to be able to attach fences at least at the last move_notify(bo, NULL);
>> Would you need to wait in that case on fence_wait being completed before calling move_notify?
>>
>> If not, you would still only need to perform one wait, but you'd have to make sure move_notify only gets
>> called by 1 thread before checking the fence pointer and performing a wait. At that point you still hold the
>> lru_lock though, so it shouldn't be too hard to make something safe.
>
> I think typically a driver that wants to implement asynchronous moves don't want to wait before calling
> move_notify, but may wait in move_notify or move. Typically (upcoming vmwgfx) it would invalidate the buffer in move_notify(bo, NULL), attach a fence and then use the normal delayed destroy to wait on that fence before destroying the buffer.
>
> Otherwise, since binds / unbinds are handled in the GPU command stream there's never any need to wait for moves except when there's a CPU
> access.
Well, nouveau actually needs fence_wait to finish first, since vm changes are out of band.
But I guess it should be possible to attach it as work to the fence when it's signaled, and I
may want to do something like that already for performance reasons in a different place,
so I guess it doesn't matter.

Is calling move_notify(bo, NULL) legal and a noop the second time? That would save a flag in the bo to check if it's called already,
although I suppose we could always define a TTM_BO_PRIV_FLAG_* for it otherwise.

move_notify might end up being called with the lru_lock held, but that shouldn't be a problem.

~Maarten