Unix Device Memory Allocation project

[Christian König]

Am 19.10.2016 um 08:23 schrieb Daniel Vetter:
> On Wed, Oct 19, 2016 at 1:40 AM, Marek Olšák <maraeo at gmail.com> wrote:
>> - The producer-consumer interop API doesn't know about the metadata.
>> All you need to pass around is a buffer handle. (KMS, DMABUF, etc.)
>>    * There was a note during the talk that DMABUF doesn't have any
>> metadata. Well, I just told you that it has, but it's private to
>> amdgpu and possibly accessible to other kernel drivers too.
>>    * We can build upon this idea. I think the worst thing to do would
>> be to add metadata handling to driver-agnostic userspace APIs. Really,
>> driver-agnostic APIs shouldn't know about that, because they can't
>> understand all the hw-specific information encoded in the metadata.
>> Also, when you want to change the metadata format, you only have to
>> update the affected drivers, not userspace APIs.
> That's a bit a surprise to hear, since "can't we just add a bit of
> opaque metadata to dma-buf" came up all the time over the past years,
> and died all the time again. dma-buf shouldn't imo be just yet another
> linux IPC mechanism and protocol, which is pretty much what you end up
> doing when you add this stuff. DRM runs all kinds of compositors with
> all kinds of existing userspace proto, and with reasonable ones like
> Wayland vendors can add whatever extensions they want. Plus there's
> all the interop with v4l and every other kernel subsytem. Trying to
> standardize that into some blob that works for everyone is imo nigh
> impossible.
>
> On top of that dma-buf is the wrong thing - you don't want this on
> buffers, but on surfaces. At least when it's time to reallocate. And
> oh dear I have seen what happens when soc vendors extend this design
> to cover that use-case, plus dynamic reallocation and all that. Imo
> there should be no way at all this ever comes close to dma-buf itself.
>
> And tbh I think it's a bit silly that amd snuck this in through
> amdgpu. But as long as you don't expect this to spread I guess it'll
> be fine.

Actually we didn't started to do it like this with amdgpu. Radeon works 
exactly the same way.

Additional to that there is a really good reason to do it like this: The 
kernel needs this information to program the CRTC for scanning out 
tilled surfaces as well.

So you either end up with additions to the mode setting call to 
transport all the device specific information about the data layout in 
the buffer or you attach this information directly to your buffer handle.

We choose the later and I think that this is a rather nice solution 
compared to all the headache you run into pushing this information 
through all the different protocols and APIs (DRI2,DRI3,Wayland,Modeset).

Regards,
Christian.

> -Daniel