linux-kernel - RE: [PATCH 1/3] habanalabs: implement dma-fence mechanism

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Date:   Tue, 14 Jul 2020 12:07:29 +0000
From:   Ofir Bitton <obitton@...ana.ai>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>
CC:     Jason Gunthorpe <jgg@...pe.ca>,
        Oded Gabbay <oded.gabbay@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        SW_Drivers <SW_Drivers@...ana.ai>
Subject: RE: [PATCH 1/3] habanalabs: implement dma-fence mechanism

Sure,
I will send a new patch using completion instead of dma-fence

Thanks,
Ofir

-----Original Message-----
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> 
Sent: Tuesday, July 14, 2020 09:37
To: Daniel Vetter <daniel.vetter@...ll.ch>
Cc: Jason Gunthorpe <jgg@...pe.ca>; Oded Gabbay <oded.gabbay@...il.com>; Linux Kernel Mailing List <linux-kernel@...r.kernel.org>; SW_Drivers <SW_Drivers@...ana.ai>; Ofir Bitton <obitton@...ana.ai>
Subject: Re: [PATCH 1/3] habanalabs: implement dma-fence mechanism

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@...pe.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@...uxfoundation.org> wrote:
> > > >
> > > > On Mon, Jul 13, 2020 at 06:54:22PM +0300, Oded Gabbay wrote:
> > > > > From: Ofir Bitton <obitton@...ana.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.v
> > > etter@...ll.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