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

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Message-ID: <52AC58E3.5030209@vmware.com>
Date:	Sat, 14 Dec 2013 14:10:59 +0100
From:	Thomas Hellstrom <thellstrom@...are.com>
To:	Rob Clark <robdclark@...il.com>
CC:	Thomas Hellstrom <thomas@...pmail.org>,
	Thierry Reding <thierry.reding@...il.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 12/14/2013 02:02 PM, Rob Clark wrote:
> 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?

Yes, I think that's needed for both the memory-shortage case where we 
want to unpin, and I guess it would desirable for
iommu space management as well.

/Thomas

>
> BR,
> -R
>
>
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