Am 08.11.2016 um 12:45 schrieb Chris Wilson:
On Tue, Nov 08, 2016 at 12:32:56PM +0100, Daniel Vetter wrote:
On Tue, Nov 08, 2016 at 10:35:08AM +0000, Chris Wilson wrote:
On Tue, Nov 08, 2016 at 03:54:47PM +0900, Gustavo Padovan wrote:
From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
Hi,
This is yet another version of the DRM fences patches. Please refer
to the cover letter[1] in a previous version to check for more details.
Explicit fencing is not a superset of the implicit fences. The driver
may be using implicit fences (on a reservation object) to serialise
asynchronous operations wrt to each other (such as dispatching threads
to flush cpu caches to memory, manipulating page tables and the like
before the flip). Since the user doesn't know about these operations,
they are not included in the explicit fence they provide, at which point
we can't trust their fence to the exclusion of the implicit fences...
My thoughts are that in atomic_check drivers just fill in the fence from
the reservation_object (i.e. the uapi implicit fencing part). If there's
any additional work that's queued up in ->prepare_fb then I guess the
driver needs to track that internally, but _only_ for kernel-internally
queued work.
That's not a trivial task to work out which of the fence contexts within
the reservation object are required and which are to be replaced by the
explicit fence, esp. when you have to consider external fences.
The reason for that is that with explicit fencing we want to allow
userspace to overwrite any existing implicit fences that might hang
around.
I'm just suggesting the danger of that when userspace doesn't know
everything and the current interfaces do not allow for userspace to know,
we only tell userspace about its own action (more or less).
It's even worse than that. See the kernel can for example swap out
objects any time it wants.
Userspace doesn't know about such operations and so can't provide them
as explicit fence.
Same is true for example in situations where one userspace process
doesn't know about operations another process does. E.g. for backward
compatibility with DRI2/3 for example.
So we will always have a mixture of implicit fences and explicit fences.
The approach we used for amdgpu is that we implicit wait for all fences
which the initiator of an operation can't know about (e.g. from another
process or kernel internally) and explicitly wait for all additional
fences provided by the initiator or an operation.
Regards,
Christian.
-Chris
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