On 01/03/2017 03:38 PM, Marek Olšák wrote:
On Thu, Oct 20, 2016 at 8:31 AM, Daniel Vetter <daniel@xxxxxxxx> wrote:
On Wed, Oct 19, 2016 at 6:46 PM, Marek Olšák <maraeo@xxxxxxxxx> wrote:
We've had per buffer metadata in Radeon since KMS, which I believe first
appeared in 2009. It's 4 bytes large and is used to communicate tiling
flags between Mesa, DDX, and the kernel display code. It was a widely
accepted solution back then and Red Hat was the main developer. So yeah,
pretty much all people except Intel were collaborating on "sneaking" this
in in 2009. I think radeon driver developers deserve an apology for that
language.
Amdgpu extended that metadata to 8 bytes and it's used in the same way as
radeon. Additionally, amdgpu added opaque metadata having 256 bytes for use
by userspace drivers only. The kernel driver isn't supposed to read it or
parse it. The format is negotiated between userspace driver developers for
sharing of more complex allocations than 2D displayable surfaces.
Metadata needed for kms (what Christian also pointed out) is what everyone
did (intel included) and I think that's perfectly reasonable. And I was
aware of that radeon is doing that since the dawn of ages since forever.
What I think is not really ok is opaque metadata blobs that the kernel
never ever inspect, but just carries around. That essentially means you're
reimplementing some bad form of IPC, and I dont think that's something the
drm subsystem (or dma-buf) really should be doing. Because you still have
that real protocol in userspace (dri2/3, wayland, whatever), but now with
a side channel with no documented ordering and synchronization. It gets
the job done for single-vendor buffer metadata transport, but as soon as
there's more than one vendor, or as soon as you need to reallocate buffers
dynamically because the usage changes it gets bad imo (and I've seen what
The metadata is immutable after allocation, so it's not a
communication channel. There is no synchronization or ordering needed
for immutable metadata. That implies that a shared buffer can't be
reused for an entirely different purpose. It can only be used as-is or
freed.
For suballocated memory, the idea is to reallocate it as a separate
buffer on the first "handle" export, so that shared suballocated
buffers don't exist.
Yeah, once it becomes mutable the fun starts imo. I didn't realize
that you're treating it strictly immutable since at least the kernel
ioctl has both set and get (and that's the thing I looked at).
Immutable stuff shouldn't be any problem (except that of course it
won't work cross-driver in any fashion)
that looks like on android in various forms). And that consensus (at least
among folks involved in dma-buf) goes back to the dma-buf kickoff 3-day
meeting we've had over 5 years ago. Not sure we're gaining anything with a
"who's older" competition.
Anyways it's there and it's uabi so will never disappear. Just wanted to
make sure it's clear that for dma-buf we've discussed this years ago, and
decided it wasn't a great idea. And I think that's still correct.
The arguments against blob metadata sound reasonable to me. I'm pretty
sceptic that window system protocols will make driver-specific
metadata blobs redundant anytime soon though. It seems the protocols
don't get much attention nowadays and there is no incentive to do
things differently in that area. At least that's how it appears to me,
but I'm not involved in that.
Folks are working on protocols again, at least I think the plan is to
make all that shared buffer allocation dance also work over
compositor/client situation (would be a bit pointless without that).
And agreed there'll always be driver-specific stuff which is opaque to
everyone else, but I hope at least in the future that all gets
shuffled around through protocol extensions. And not in the way every
Android gfx stack seems to work, where everyone has their own
vendor-private ipc-over-dma-buf thing. Wayland definitely got this
right, both protocol versioning and being able to add any kind of
new/vendor-private protocol endpoints to any wayland protocol. X is a
lot more pain, but since it finally looks like the world is switching
away from it we might get away with a simpler protocol there. At
least all the tricky reallocation dances seem to matter a lot more on
mobile/tablets/phones, and there Wayland starts to rule.
I've been thinking about it, and it looks like we're gonna continue
using immutable per-BO metadata (buffer layout, tiling description,
compression flags). The reasons are that everything else is less
economical, and the current "modifier" work done in EGL/GBM is
insufficient for our hardware - we need approx. 96 bytes of metadata
for proper buffer sharing (not just for display, but also 3D interop -
MSAA, mipmapping, compression), while EGL modifiers only support 8
bytes of metadata. However, that doesn't matter, because:
These are the components that need to work with the BO metadata:
- Mesa driver backend
- AMDGPU kernel driver
These are the components that should never know about the BO metadata:
- Any Mesa shared code
- EGL
- GBM
- Window system protocols
- Display servers
- DDXs
The more components you need to change when the requirements change,
the less economical the whole thing is, and the more painful the
deployment is.
Interop with other vendors would be trivial - the kernel drivers can
exchange buffer layouts, and DRM can have an interface for it.
Userspace doesn't have to know about any of that. (It also seems kinda
dangerous to use userspace as a middle man for passing the
metadata/modifiers around)
Could you elaborate one what seems dangerous about it?
Thanks,
-James
Speaking of compression for display, especially the separate
compression buffer: That should be fully contained in the main DMABUF
and described by the per-BO metadata. Some other drivers want to use a
separate DMABUF for the compression buffer - while that may sound good
in theory, it's not economical for the reason described above.
Marek
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