[RFC] drm/radeon: userfence IOCTL

[Christian König]

Hello everyone,

we have a requirement for a bit different kind of fence handling. Currently we handle fences completely inside the kernel, but in the future we would like to emit multiple fences inside the same IB as well.

This works by adding multiple fence commands into an IB which just write their value to a specific location inside a BO and trigger the appropriate hardware interrupt.

The user part of the driver stack should then be able to call an IOCTL to wait for the interrupt and block for the value (or something larger) to be written to the specific location.

This has the advantage that you can have multiple synchronization points in the same IB and don't need to split up your draw commands over several IBs so that the kernel can insert kernel fences in between.

The following set of patches tries to implement exactly this IOCTL. The big problem with that IOCTL is that TTM needs the BO to be kept in the same place while it is mapped inside the kernel page table. So this requires that we pin down the BO for the duration of the wait IOCTL.

This practically gives userspace a way of pinning down BOs for as long as it wants, without the ability for the kernel for intervention.

Any ideas how to avoid those problems? Or better ideas how to handle the new requirements?

Please note that the patches are only hacked together quick&dirty to demonstrate the problem and not very well tested.

Best regards,
Christian.