[PATCH] dma-buf/fence: make fence context 64 bit

[Daniel Vetter]

On Thu, May 19, 2016 at 11:30 AM, Christian König
<deathsimple at vodafone.de> wrote:
> Am 19.05.2016 um 11:14 schrieb Daniel Vetter:
>>
>> On Thu, May 19, 2016 at 11:00:36AM +0200, Christian König wrote:
>>>
>>> From: Christian König <christian.koenig at amd.com>
>>>
>>> Fence contexts are created on the fly (for example) by the GPU scheduler
>>> used
>>> in the amdgpu driver as a result of an userspace request. Because of this
>>> userspace could in theory force a wrap around of the 32bit context number
>>> if it doesn't behave well.
>>>
>>> Avoid this by increasing the context number to 64bits. This way even when
>>> userspace manages to allocate a billion contexts per second it takes more
>>> than 500 years for the context number to wrap around.
>>>
>>> Signed-off-by: Christian König <christian.koenig at amd.com>
>>
>> Hm, I think it'd be nice to wrap this up into a real struct and then
>> manage them with some idr or whatever. For debugging we might also want to
>> keep track of all fences on a given timeline and similar things, so
>> there will be a need for this in the future.
>>
>> So if you go through every driver I think it's better to replace the type
>> with struct fence_context *context while we're at it. Makes it a notch
>> bigger since we need to add a little bit of error handling to all callers
>> of fence_context_alloc.
>>
>> Volunteered? ;-)
>
>
> Well, that's exactly what I wanted to avoid. 64bit numbers are fast to
> allocate and easy to compare.
>
> If I make it a structure then we would need to kmalloc() it and make sure it
> is reference counted so it stays alive as long as any fence structure is
> alive which is referring to it.
>
> The overhead sounds to much to me, especially since we currently don't have
> a real use for that right now.

Hm, I guess if you're worried about the kmalloc we could make
fence_context embeddable. At least I assume you have to allcate
something somewhere already to store the u64, and that something also
needs to be refcounted already (or cleaned up suitably) to make sure
it doesn't disappear before the fences go away.

I'm just raising this because the longer we wait with redoing this
interface the more painful it'll be. Android at least does have a
full-blown struct, and the reason is exclusively for debugging. And
from what I've heard from android devs debugging fence lockups is a
complete pain. That's why I think sooner or later there's no way
around a full blown struct.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch