Hi, Daniel,
On 6/21/19 5:57 PM, Daniel Vetter wrote:
On Fri, Jun 21, 2019 at 05:12:19PM +0200, Thomas Hellström (VMware) wrote:
On 6/21/19 1:57 PM, Gerd Hoffmann wrote:
Aargh. Please don't do this. Multiple reasons:
1) I think It's bad to dump all buffer object functionality we can possibly
think of in a single struct and force that on all (well at least most)
users. It's better to isolate functionality in structs, have utility
functions for those and let the drivers derive their buffer objects from
whatever functionality they actually need.
2) vmwgfx is not using gem and we don't want to carry that extra payload in
the buffer object.
3) TTM historically hasn't been using the various drm layers except for
later when common helpers have been used, (like the vma manager and the
cache utilities). It's desirable to keep that layer distinction. (which is
really what I'm saying in 1.)
Now if more and more functionality that originated in TTM is moving into GEM
we need to find a better way to do that without duplicating functionality. I
suggest adding pointers in the TTM structs and defaulting those pointers to
the member in the TTM struct. Optionally to to the member in the GEM struct.
If we need to migrate those members out of the TTM struct, vmwgfx would have
to provide them in its own buffer class.
NAK from the vmwgfx side.
It's 59 DRIVER_GEM vs 1 which is not. I think the verdict is clear what
the reasonable thing to do is here, and this will allow us to
substantially improve code and concept sharing across drm drivers.
10 years ago it was indeed not clear whether everyone doing the same is a
bright idea, but that's no more. If you want I guess you can keep a
private copy of ttm in vmwgfx, but not sure that's really worth it
long-term.
-Daniel
It's not a question about whether GEM or TTM, or even the number of
drivers using one or the other. (GEM would actually be a good choice for
the latter vmwgfx device versions). But this is going against all recent
effort to make different parts of drm functionality recently self-contained.
Just stop and think for a while what would happen if someone would
suggest doing the opposite: making a gem object a derived class of a TTM
object, arguing that a lot of GEM drivers are using TTM as a backend.
There would probably be a lot of people claiming "we don't want to
unnecesarily carry that stuff". That's because that would also be a poor
design.
What I'm suggesting is, build that improved code and concept sharing around
struct gem_ttm_object {
struct gem_object;
struct ttm_object;
};
And lets work toghether to eliminate what's duplicated.
The vmwgfx driver is doing what it does mostly because all buffer
objects do not need to be user-space visible, and do not need to be
mapped by user-space. And there are other types of objects that DO need
to be user-space visible, and that do need to be shared by processes.
Hence user-space visibility is something that should be abstracted and
made available to those objects. Not lumped together with all other
potential buffer object functionality.
/Thomas
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