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

[Thomas Hellstrom <thellstrom@vmware.com>]

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.

/Thomas


>
> ~Maarten
>