On 10/31/2013 06:52 PM, Rob Clark wrote:
On Thu, Oct 31, 2013 at 1:00 PM, Thomas Hellstrom <thellstrom@xxxxxxxxxx> wrote:
Hi!
I'm just looking over what's needed to implement drm Prime / dma-buf exports
+ imports in the vmwgfx driver. It seems like most dma-bufs ops are quite
straightforward to implement except user-space mmap().
The reason being that vmwgfx dma-bufs will be using completely non-coherent
memory, whenever there needs to be CPU accesses.
The accelerated contents resides in an opaque structure on the device into
which we can DMA to and from, so for mmap to work we need to zap ptes and
DMA to the device when doing something accelerated, and on the first
page-fault DMA data back and wait for idle if the device did a write to the
dma-buf.
Now this shouldn't really be a problem if dma-bufs were only used for
cross-device sharing, but since people apparently want to use dma-buf file
handles to share CPU data between processes it really becomes a serious
problem.
Needless to say we'd want to limit the size of the DMAs, and have mmap users
request regions for read, and mark regions dirty for write, something
similar to gallium's texture transfer objects.
Any ideas?
well, I think vmwgfx is part of the reason we decided mmap would be
optional for dmabuf. So perhaps it is an option to simply ignore
mmap?
BR,
-R
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:
1) Create a GBM buffer
2) Get a Prime handle, export to client
3) Client imports prime handle, casts into a GBM buffer which is
typecast to a dumb buffer
4) Client uses DRM dumb mmap(). :(.
Now, step 3) and step 4) are clearly API violations, but it works by
coincidence on major drivers.
Now this usage-pattern could easily (and IMHO should) be blocked in DRM
by refusing to map dumb buffers
that haven't been created as dumb buffers, but If dma-bufs are to be
shared by user-space apps to transfer
contents in this manner, we need a reliable replacement that works also
for non-coherent situations.
Ideally I'd like to see tex[sub]image-like operations, but with dma-buf
mmap in place, I'm afraid that's what will
be used...
Regars,
Thomas
/Thomas
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