Re: [PATCH] dma-fence: allow dma fence to have their own lock

From: "Christian König" <christian.koenig@amd.com>
To: Sergey Senozhatsky <senozhatsky@chromium.org>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	Gustavo Padovan <gustavo@padovan.org>
Cc: linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org,
	dri-devel@lists.freedesktop.org, Tomasz Figa <tfiga@chromium.org>,
	Christoph Hellwig <hch@infradead.org>,
	Ricardo Ribalda <ribalda@chromium.org>,
	linux-media@vger.kernel.org
Subject: Re: [PATCH] dma-fence: allow dma fence to have their own lock
Date: Mon, 30 May 2022 16:55:41 +0200	[thread overview]
Message-ID: <7eee4274-bd69-df8d-9067-771366217804@amd.com> (raw)
In-Reply-To: <20220530142232.2871634-1-senozhatsky@chromium.org>

Hi Sergey,

I'm removing most of the mail because you have a very fundamental 
misunderstanding about what this dma_fence lock is all about.

Am 30.05.22 um 16:22 schrieb Sergey Senozhatsky:
> [SNIP]
> So the `lock` should have at least same lifespan as the DMA fence
> that borrows it, which is impossible to guarantee in our case.

Nope, that's not correct. The lock should have at least same lifespan as 
the context of the DMA fence.

The idea here is that DMA fence signaling and callback calling 
serializes. E.g. when you have fence a,b,c,d... they must signal in the 
order a,b,c,d... and that's what this lock is good for.

If you just want to create a single dma_fence which is also only bound 
to a single context you can embed the lock into the fence without much 
problem.

See how the dma_fence_array does that for example: 
https://elixir.bootlin.com/linux/latest/source/include/linux/dma-fence-array.h#L37

Regards,
Christian.