Re: [PATCH 2/2] drm/nouveau: Don't signal when killing the fence context

[Danilo Krummrich <dakr@kernel.org>]

On Thu, May 22, 2025 at 02:34:33PM +0200, Christian König wrote:
> See all the functions inside include/linux/dma-fence.h can be used by everybody. It's basically the public interface of the dma_fence object.

As you write below, in certain cases it is valid to call this from drivers, so
it's not unreasonable to have it as part of the public API.

> So testing if a fence is signaled without calling the callback is only allowed by whoever implemented the fence.
> 
> In other words nouveau can test nouveau fences, i915 can test i915 fences, amdgpu can test amdgpu fences etc... But if you have the wrapper that makes it officially allowed that nouveau starts testing i915 fences and that would be problematic.

In general, I like the  __dma_fence_is_signaled() helper, because this way we
can document in which cases it is allowed to be used, i.e. the ones you descibe
above.

test_bit() can be called by anyone and there is no documentation comment
explaining that it is only allowed under certain conditions.

Having the __dma_fence_is_signaled() helper properly documented could get you
rid of having to explain in which case the test_bit() dance is allowed to do
over and over again. :-)

I also think the name is good, since the '__' prefix already implies that there
are some restrictions on the use of this helper.