Re: [PATCH 1/3] habanalabs: implement dma-fence mechanism

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Jul 13, 2020 at 09:08:55PM +0200, Daniel Vetter wrote:
> On Mon, Jul 13, 2020 at 9:03 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> >
> > On Mon, Jul 13, 2020 at 08:34:12PM +0200, Daniel Vetter wrote:
> > > On Mon, Jul 13, 2020 at 5:57 PM Greg Kroah-Hartman
> > > <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > On Mon, Jul 13, 2020 at 06:54:22PM +0300, Oded Gabbay wrote:
> > > > > From: Ofir Bitton <obitton@habana.ai>
> > > > >
> > > > > Instead of using standard dma-fence mechanism designed for GPU's, we
> > > > > introduce our own implementation based on the former one. This
> > > > > implementation is much more sparse than the original, contains only
> > > > > mandatory functionality required by the driver.
> > > >
> > > > Sad you can't use the in-kernel code for this, I really don't understand
> > > > what's wrong with using it as-is.
> > > >
> > > > Daniel, why do we need/want duplicate code floating around in the tree
> > > > like this?
> > >
> > > The rules around dma-fence are ridiculously strict, and it only makes
> > > sense to inflict that upon you if you actually want to participate in
> > > the cross driver uapi built up around dma-buf and dma-fence.
> > >
> > > I've recently started some lockdep annotations to better enforce these
> > > rules (and document them), and it's finding tons of subtle bugs even
> > > in drivers/gpu (and I only just started with annotating drivers:
> > >
> > > https://lore.kernel.org/dri-devel/20200707201229.472834-1-daniel.vetter@ffwll.ch/
> > >
> > > You really don't want to deal with this if you don't have to. If
> > > drivers/gpu folks (who created this) aren't good enough to understand
> > > it, maybe it's not a good idea to sprinkle this all over the tree. And
> > > fundamentally all this is is a slightly fancier struct completion. Use
> > > that one instead, or a wait_queue.
> > >
> > > I discussed this a bit with Oded, and he thinks it's easier to
> > > copypaste and simplify, but given that all other drivers seem to get
> > > by perfectly well with completion or wait_queue, I'm not sure that's a
> > > solid case.
> > >
> > > Also adding Jason Gunthorpe, who very much suggested this should be
> > > limited to dma-buf/gpu related usage only.
> >
> > Without all the cross-driver stuff dma_fence is just a
> > completion. Using dma_fence to get a completion is big abuse of what
> > it is intended for.
> >
> > I think the only problem with this patch is that it keeps too much of
> > the dma_fence stuff around. From what I could tell it really just
> > wants to add a kref and completion to struct hl_cs_compl and delete
> > everything to do with dma_fence.
> 
> Yeah, that's what I recommended doing too. error flag might be needed
> too I think, but that's it.

Ok, so this should be made much simpler and not use this copy/paste code
at all.  I can accept that :)

Ofir, care to redo this?

thanks,

greg k-h