Re: dma-buf non-coherent mmap

[Jakob Bornecrantz <wallbraker@gmail.com>]

On Fri, Nov 1, 2013 at 12:00 AM, Daniel Vetter <daniel@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@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.

Cheers, Jakob.