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

From: "Christian König" <christian.koenig@amd.com>
To: Jason Gunthorpe <jgg@mellanox.com>
Cc: "Daniel Vetter" <daniel@ffwll.ch>,
	"DRI Development" <dri-devel@lists.freedesktop.org>,
	"Intel Graphics Development" <intel-gfx@lists.freedesktop.org>,
	linux-rdma@vger.kernel.org,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>,
	"Felix Kuehling" <Felix.Kuehling@amd.com>,
	"kernel test robot" <lkp@intel.com>,
	"Thomas Hellström" <thomas.hellstrom@intel.com>,
	"Mika Kuoppala" <mika.kuoppala@intel.com>,
	linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org,
	amd-gfx@lists.freedesktop.org,
	"Chris Wilson" <chris@chris-wilson.co.uk>,
	"Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>,
	"Daniel Vetter" <daniel.vetter@intel.com>
Subject: Re: [PATCH 02/25] dma-fence: prime lockdep annotations
Date: Fri, 10 Jul 2020 15:01:10 +0200	[thread overview]
Message-ID: <4f4a2cf7-f505-8494-1461-bd467870481e@amd.com> (raw)
In-Reply-To: <20200710125453.GC23821@mellanox.com>

Am 10.07.20 um 14:54 schrieb Jason Gunthorpe:
> On Fri, Jul 10, 2020 at 02:48:16PM +0200, Christian König wrote:
>> Am 10.07.20 um 14:43 schrieb Jason Gunthorpe:
>>> On Thu, Jul 09, 2020 at 10:09:11AM +0200, Daniel Vetter wrote:
>>>> Hi Jason,
>>>>
>>>> Below the paragraph I've added after our discussions around dma-fences
>>>> outside of drivers/gpu. Good enough for an ack on this, or want something
>>>> changed?
>>>>
>>>> Thanks, Daniel
>>>>
>>>>> + * Note that only GPU drivers have a reasonable excuse for both requiring
>>>>> + * &mmu_interval_notifier and &shrinker callbacks at the same time as having to
>>>>> + * track asynchronous compute work using &dma_fence. No driver outside of
>>>>> + * drivers/gpu should ever call dma_fence_wait() in such contexts.
>>> I was hoping we'd get to 'no driver outside GPU should even use
>>> dma_fence()'
>> My last status was that V4L could come use dma_fences as well.
> I'm sure lots of places *could* use it, but I think I understood that
> it is a bad idea unless you have to fit into the DRM uAPI?

It would be a bit questionable if you use the container objects we came 
up with in the DRM subsystem outside of it.

But using the dma_fence itself makes sense for everything which could do 
async DMA in general.

> You are better to do something contained in the single driver where
> locking can be analyzed.
>
>> I'm not 100% sure, but wouldn't MMU notifier + dma_fence be a valid use case
>> for things like custom FPGA interfaces as well?
> I don't think we should expand the list of drivers that use this
> technique.
> Drivers that can't suspend should pin memory, not use blocked
> notifiers to created pinned memory.

Agreed totally, it's a complete pain to maintain even for the GPU drivers.

Unfortunately that doesn't change users from requesting it. So I'm 
pretty sure we are going to see more of this.

Christian.

>
> Jason