[PATCH 02/25] dma-fence: prime lockdep annotations

[Jason Gunthorpe]

On Fri, Jul 10, 2020 at 04:02:35PM +0200, Daniel Vetter wrote:

> > dma_fence only possibly makes some sense if you intend to expose the
> > completion outside a single driver.
> >
> > The prefered kernel design pattern for this is to connect things with
> > a function callback.
> >
> > So the actual use case of dma_fence is quite narrow and tightly linked
> > to DRM.
> >
> > I don't think we should spread this beyond DRM, I can't see a reason.
> 
> Yeah v4l has a legit reason to use dma_fence, android wants that
> there. 

'legit' in the sense the v4l is supposed to trigger stuff in DRM when
V4L DMA completes? I would still see that as part of DRM

Or is it building a parallel DRM like DMA completion graph?

> > Trying to improve performance of limited HW by using sketchy
> > techniques at the cost of general system stability should be a NAK.
>
> Well that's pretty much gpu drivers, all the horrors for a bit more speed :-)
> 
> On the text itself, should I upgrade to "must not" instead of "should
> not"? Or more needed?

Fundamentally having some unknowable graph of dependencies where parts
of the graph can be placed in critical regions like notifiers is a
complete maintenance nightmare.

I think building systems like this should be discouraged :\

Jason