Re: [Linaro-mm-sig] [PATCH 1/2] dma-buf.rst: Document why indefinite fences are a bad idea

From: "Thomas Hellström (Intel)" <thomas_os@shipmail.org>
To: christian.koenig@amd.com, Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Felix Kuehling <Felix.Kuehling@amd.com>,
	Daniel Stone <daniels@collabora.com>,
	linux-rdma <linux-rdma@vger.kernel.org>,
	Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	"moderated list:DMA BUFFER SHARING FRAMEWORK" 
	<linaro-mm-sig@lists.linaro.org>,
	Steve Pronovost <spronovo@microsoft.com>,
	amd-gfx mailing list <amd-gfx@lists.freedesktop.org>,
	Jason Ekstrand <jason@jlekstrand.net>,
	Jesse Natalie <jenatali@microsoft.com>,
	Daniel Vetter <daniel.vetter@intel.com>,
	Thomas Hellstrom <thomas.hellstrom@intel.com>,
	Linux Media Mailing List <linux-media@vger.kernel.org>,
	Dave Airlie <airlied@gmail.com>,
	Mika Kuoppala <mika.kuoppala@intel.com>
Subject: Re: [Linaro-mm-sig] [PATCH 1/2] dma-buf.rst: Document why indefinite fences are a bad idea
Date: Wed, 22 Jul 2020 16:30:09 +0200	[thread overview]
Message-ID: <a046ea93-b303-17d4-add4-efd0a709cfd2@shipmail.org> (raw)
In-Reply-To: <c742c557-f48f-1591-1d1d-a5181b408a67@gmail.com>


On 2020-07-22 16:23, Christian König wrote:
> Am 22.07.20 um 16:07 schrieb Daniel Vetter:
>> On Wed, Jul 22, 2020 at 3:12 PM Thomas Hellström (Intel)
>> <thomas_os@shipmail.org> wrote:
>>> On 2020-07-22 14:41, Daniel Vetter wrote:
>>>> I'm pretty sure there's more bugs, I just haven't heard from them yet.
>>>> Also due to the opt-in nature of dma-fence we can limit the scope of
>>>> what we fix fairly naturally, just don't put them where no one cares
>>>> :-) Of course that also hides general locking issues in dma_fence
>>>> signalling code, but well *shrug*.
>>> Hmm, yes. Another potential big problem would be drivers that want to
>>> use gpu page faults in the dma-fence critical sections with the
>>> batch-based programming model.
>> Yeah that's a massive can of worms. But luckily there's no such driver
>> merged in upstream, so hopefully we can think about all the
>> constraints and how to best annotate&enforce this before we land any
>> code and have big regrets.
>
> Do you want a bad news? I once made a prototype for that when Vega10 
> came out.
>
> But we abandoned this approach for the the batch based approach 
> because of the horrible performance.

In context of the previous discussion I'd consider the fact that it's 
not performant in the batch-based model good news :)

Thomas


>
> KFD is going to see that, but this is only with user queues and no 
> dma_fence involved whatsoever.
>
> Christian.
>
>> -Daniel
>>
>>
>>
>> -- 
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> http://blog.ffwll.ch
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