Re: [PATCH drm-next v4 00/14] [RFC] DRM GPUVA Manager & Nouveau VM_BIND UAPI

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On 6/14/23 09:58, Donald Robson wrote:
On Tue, 2023-06-13 at 16:20 +0200, Danilo Krummrich wrote:

I'm definitely up improving the existing documentation. Anything in
particular you think should be described in more detail?

- Danilo

Hi Danilo,

As I said, with inexperience it's possible I missed what I was
looking for in the existing documentation, which is highly detailed
in regard to how it deals with operations, but usage was where I fell
down.

If I understand there are three ways to use this, which are:
1) Using drm_gpuva_insert() and drm_gpuva_remove() directly using
    stack va objects.

What do you mean with stack va objects?

2) Using drm_gpuva_insert() and drm_gpuva_remove() in a callback
    context, after having created ops lists using
    drm_gpuva_sm_[un]map_ops_create().
3) Using drm_gpuva_[un]map() in callback context after having
    prealloced a node and va objects for map/remap function use,
    which must be forwarded in as the 'priv' argument to
    drm_gpuva_sm_[un]map().

Right, and I think it might be worth concretely mentioning this in the documentation.


The first of these is pretty self-explanatory.  The second was also
fairly easy to understand, it has an example in your own driver, and
since it takes care of allocs in drm_gpuva_sm_map_ops_create() it
leads to pretty clean code too.

The third case, which I am using in the new PowerVR driver did not
have an example of usage and the approach is quite different to 2)
in that you have to prealloc everything explicitly.  I didn't realise
this, so it led to a fair amount of frustration.

Yeah, I think this is not entirely obvious why this is the case. I should maybe add a comment on how the callback way of using this interface is motivated.

The requirement of pre-allocation arises out of two circumstances.
First, having a single callback for every drm_gpuva_op on the GPUVA space implies that we're not allowed to fail the operation, because processing the drm_gpuva_ops directly implies that we can't unwind them on failure.

I know that the API functions the documentation guides you to use in this case actually can return error codes, but those are just range checks. If they fail, it's clearly a bug. However, I did not use WARN() for those cases, since the driver could still decide to use the callbacks to keep track of the operations in a driver specific way, although I would not recommend doing this and rather like to try to cover the drivers use case within the regular way of creating a list of operations.

Second, most (other) drivers when using the callback way of this interface would need to execute the GPUVA space updates asynchronously in a dma_fence signalling critical path, where no memory allocations are permitted.


I think if you're willing, it would help inexperienced implementers a
lot if there were some brief 'how to' snippets for each of the three
use cases.

Yes, I can definitely add some.


Thanks,
Donald




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