dma-buf non-coherent mmap

[Jakob Bornecrantz]

On Fri, Nov 1, 2013 at 1:25 AM, Rob Clark <robdclark at gmail.com> wrote:
> On Thu, Oct 31, 2013 at 8:17 PM, Jakob Bornecrantz <wallbraker at gmail.com> wrote:
>> On Fri, Nov 1, 2013 at 12:00 AM, Daniel Vetter <daniel at ffwll.ch> wrote:
>>> On Thu, Oct 31, 2013 at 10:07:25PM +0100, Thomas Hellstrom wrote:
>>>> On 10/31/2013 09:48 PM, Dave Airlie wrote:
>>>> >On Fri, Nov 1, 2013 at 6:40 AM, Thomas Hellstrom <thellstrom at vmware.com> wrote:
>>>> >>Well, I'd be happy to avoid mmap, but then what does optional mean in this
>>>> >>context?
>>>> >>That all generic user-space apps *must* implement a workaround if mmap isn't
>>>> >>implemented?
>>>> >>
>>>> >>It's unfortunate a bit like implicit synchronization mentioned in section 3)
>>>> >>in Direct Userspace Access/mmap Support
>>>> >>in the kernel dma-buf doc: It should be avoided, otherwise it might be
>>>> >>relied upon by userspace and exporters
>>>> >>not implementing it will suffer.
>>>> >>
>>>> >>In reality, people will start using mmap() and won't care to implement
>>>> >>workarounds if it's not supported, and drivers like
>>>> >>vmwgfx and non-coherent architectures will suffer.
>>>> >>
>>>> >>I haven't looked closely at how DRI3 or Wayland/weston use or will use
>>>> >>dma-buf, but if they rely on mmap, we're sort
>>>> >>of lost. MIR uses the following scheme:
>>>> >DRI3 and wayland won't use dma-buf mmap directly,
>>>> >
>>>> >using dma-buf mmap directly is wrong for anything that shares objects
>>>> >with itself.
>>>>
>>>> That sounds good to hear. Perhaps we should add that to the dma-buf docs.
>>>
>>> Userspace mmap was essentially added as a concession to the android ion
>>> guys since they really, really wanted it. We've tried to tell them that
>>> it's a horrible idea (see all the fun with coherency and syncing), but
>>> they said that they have userspace for it already and so we let it be.
>>>
>>> Imo if you're not running Android userspace there's no need for this at
>>> all.
>>
>> But now it turns out that gstreamer is using it and our life is hell. We should
>> have made it not work for _any_ driver if CONFIG_ANDRIOD wasn't set.
>
> well, at the moment mmap is only implemented by a few of the arm
> drivers, from the looks of it.  So I guess in the near term it is
> mostly going to be of interest to the SoC crowd.

Ah okay, I thought this was desktop gstreamer.

> Not sure about the long term, perhaps we should see about
> turning gallium pipe_transfer/map stuff into a kernel interface
> (ioctl's directly on the dmabuf fd, perhaps?)

Or they could just create a OpenGL context, I know it sounds heavy weight.
But somebody will eventually want to synchronize this with a different client
API in a non-blocking way. And OpenGL or some other Khronos API already
have that integration, no real need to reinvent the wheel. Then again I think
both GBM and DRI have hooks for mapping and unmapping buffers, so those
might suffice.

Cheers, Jakob.