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

[Danilo Krummrich <dakr@redhat.com>]

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