linux-kernel - Re: [Linaro-mm-sig] [PATCH v4 0/2] Add p2p via dmabuf to habanalabs

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Date:   Wed, 7 Jul 2021 14:17:11 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Christoph Hellwig <hch@....de>, Oded Gabbay <ogabbay@...nel.org>,
        linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        sumit.semwal@...aro.org, christian.koenig@....com,
        galpress@...zon.com, sleybo@...zon.com,
        dri-devel@...ts.freedesktop.org, jgg@...pe.ca,
        linux-rdma@...r.kernel.org, linux-media@...r.kernel.org,
        dledford@...hat.com, airlied@...il.com, alexander.deucher@....com,
        leonro@...dia.com, amd-gfx@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org
Subject: Re: [Linaro-mm-sig] [PATCH v4 0/2] Add p2p via dmabuf to habanalabs

Am 06.07.21 um 14:23 schrieb Daniel Vetter:
> On Tue, Jul 06, 2021 at 02:21:10PM +0200, Christoph Hellwig wrote:
>> On Tue, Jul 06, 2021 at 10:40:37AM +0200, Daniel Vetter wrote:
>>>> Greg, I hope this will be good enough for you to merge this code.
>>> So we're officially going to use dri-devel for technical details review
>>> and then Greg for merging so we don't have to deal with other merge
>>> criteria dri-devel folks have?
>>>
>>> I don't expect anything less by now, but it does make the original claim
>>> that drivers/misc will not step all over accelerators folks a complete
>>> farce under the totally-not-a-gpu banner.
>>>
>>> This essentially means that for any other accelerator stack that doesn't
>>> fit the dri-devel merge criteria, even if it's acting like a gpu and uses
>>> other gpu driver stuff, you can just send it to Greg and it's good to go.
>>>
>>> There's quite a lot of these floating around actually (and many do have
>>> semi-open runtimes, like habanalabs have now too, just not open enough to
>>> be actually useful). It's going to be absolutely lovely having to explain
>>> to these companies in background chats why habanalabs gets away with their
>>> stack and they don't.
>> FYI, I fully agree with Daniel here.  Habanlabs needs to open up their
>> runtime if they want to push any additional feature in the kernel.
>> The current situation is not sustainable.
> Before anyone replies: The runtime is open, the compiler is still closed.
> This has become the new default for accel driver submissions, I think
> mostly because all the interesting bits for non-3d accelerators are in the
> accel ISA, and no longer in the runtime. So vendors are fairly happy to
> throw in the runtime as a freebie.

Well a compiler and runtime makes things easier, but the real question 
is if they are really required for upstreaming a kernel driver?

I mean what we need is to be able to exercise the functionality. So 
wouldn't (for example) an assembler be sufficient?

> It's still incomplete, and it's still useless if you want to actually hack
> on the driver stack.

Yeah, when you want to hack on it in the sense of extending it then this 
requirement is certainly true.

But as far as I can see userspace don't need to be extendable to justify 
a kernel driver. It just needs to have enough glue to thoughtfully 
exercise the relevant kernel interfaces.

Applying that to GPUs I think what you need to be able to is to write 
shaders, but that doesn't need to be in a higher language requiring a 
compiler and runtime. Released opcodes and a low level assembler should 
be sufficient.

Regards,
Christian.

> -Daniel