linux-kernel - Re: [RFC] dma-buf: Implement test module

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Date:	Sat, 14 Dec 2013 08:02:08 -0500
From:	Rob Clark <robdclark@...il.com>
To:	Thomas Hellstrom <thomas@...pmail.org>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	Thomas Hellstrom <thellstrom@...are.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>
Subject: Re: [RFC] dma-buf: Implement test module

On Sat, Dec 14, 2013 at 7:47 AM, Thomas Hellstrom <thomas@...pmail.org> wrote:
> On 12/14/2013 01:37 PM, Thierry Reding wrote:
>>
>> On Thu, Dec 12, 2013 at 11:30:23PM +0100, Daniel Vetter wrote:
>>>
>>> On Thu, Dec 12, 2013 at 8:34 PM, Thomas Hellstrom <thellstrom@...are.com>
>>> wrote:
>>>>
>>>> On 12/12/2013 03:36 PM, Thierry Reding wrote:
>>>>>
>>>>> This is a simple test module that can be used to allocate, export and
>>>>> delete DMA-BUF objects. It can be used to test DMA-BUF sharing in
>>>>> systems that lack a real second driver.
>>>>>
>>>>>
>>>> Looks nice. I wonder whether this could be extended to create a
>>>> "streaming"
>>>> dma-buf from a user space mapping. That could be used as a generic way
>>>> to
>>>> implement streaming (user) buffer objects, rather than to add explicit
>>>> support for those in, for example, TTM.
>>>
>>> Atm there's no way to get gpus to unbind their dma-buf mappings, so
>>> their essentially pinned forever from first use on.
>>
>> Shouldn't this work by simply calling the GEM_CLOSE IOCTL on the handle
>> returned by drmPrimeFDToHandle()? I mean that should drop the last
>> reference on the GEM object and cause it to be cleaned up (which should
>> include detaching the DMA-BUF).
>
>
> Actually, while the GEM prime implementation appears to pin an exported
> dma-buf on first attach, from the dma-buf documentation it seems sufficient
> to pin it on map or cpu access.
>
> But what I assume Daniel is referring to is that there is no way for
> exporters to tell importers to force unmap() the dma-buf, so that it can be
> unpinned?

yeah, or some way for importers to opportunistically keep around a
mapping rather than map/unmap on each use..

maybe we need something shrinker-ish for dmabuf?

BR,
-R

> Daniel, maybe you could elaborate a bit on this?
>
> Thomas
>
>>
>> Thierry
>>
>>
>>
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>> dri-devel@...ts.freedesktop.org
>> http://lists.freedesktop.org/mailman/listinfo/dri-devel
>
>
>
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