b5734cc1c4
When handle->type is WINSYS_HANDLE_TYPE_FD, the caller wants a file descriptor
for the BO backing the resource. We previously had two paths for this:
1. If rsrc->scanout is available, we prime the GEM handle from the KMS device
(rsrc->scanout->handle) to a file descriptor via the KMS device.
2. If rsrc->scanout is not available, we prime the GEM handle from the GPU
(bo->gem_handle) to a file descriptor via the GPU device.
In both cases, the caller passes in a resource (with BO) and expects out a file
descriptor. There are no direct GEM handles in the function signature; the
caller doesn't care which GEM handle we prime to get the file descriptor. In
principle, both paths produce the same file descriptor for the same BO, since
both GEM handles represent the same underlying resource (viewed from different
devices).
On grounds of redundancy alone, it makes sense to remove the rsrc->scanout path.
Why have a path that only works sometimes, when we have another path that works
always?
In fact, the issues with the rsrc->scanout path are deeper. rsrc->scanout is
populated by renderonly_create_gpu_import_for_resource, which does the
following:
1. Get a file descriptor for the resource by resource_get_handle with
WINSYS_HANDLE_TYPE_FD
2. Prime the file descriptor to a GEM handle via the KMS device.
Here comes strike number 2: in order to get a file descriptor via the KMS
device, we had to /already/ get a file descriptor via the GPU device. If we go
down the KMS device path, we effectively round trip:
GPU handle -> fd -> KMS handle -> fd
There is no good reason to do this; if everything works, the fd is the same in
each case. If everything works. If.
The lifetimes of the GPU handle and the KMS handle are not necessarily bound. In
principle, a resource can be created with scanout (constructing a KMS handle).
Then the KMS view can be destroyed (invalidating the GEM handle for the KMS
device), even though the underlying resource is still valid. Notice the GPU
handle is still valid; its lifetime is tied to the resource itself. Then a
caller can ask for the FD for the resource; as the resource is still valid, this
is sensible. Under the scanout path, we try to get the FD by priming the GEM
handle on the KMS device... but that GEM handle is no longer valid, causing the
PRIME ioctl to fail with ENOENT. On the other hand, if we primed the GPU GEM
handle, everything works as expected.
These edge cases are not theoretical; recent versions of Xwayland trigger this
ENOENT, causing issue #5758 on all Panfrost devices. As far as I can tell, no
other kmsro driver has this 'special' kmsro path; the only part of
resource_get_handle that needs special handling for kmsro is getting a KMS
handle.
Let's remove the broken, useless path, fix Xwayland, bring us in line with other
drivers, and delete some code.
Thank you for coming to my ted talk.
Closes: #5758
Fixes:
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| android | ||
| bin | ||
| build-support | ||
| docs | ||
| include | ||
| src | ||
| subprojects | ||
| .dir-locals.el | ||
| .editorconfig | ||
| .gitattributes | ||
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| .gitlab-ci.yml | ||
| .mailmap | ||
| .travis.yml | ||
| CODEOWNERS | ||
| README.rst | ||
| VERSION | ||
| meson.build | ||
| meson_options.txt | ||
README.rst
`Mesa <https://mesa3d.org>`_ - The 3D Graphics Library ====================================================== Source ------ This repository lives at https://gitlab.freedesktop.org/mesa/mesa. Other repositories are likely forks, and code found there is not supported. Build & install --------------- You can find more information in our documentation (`docs/install.rst <https://mesa3d.org/install.html>`_), but the recommended way is to use Meson (`docs/meson.rst <https://mesa3d.org/meson.html>`_): .. code-block:: sh $ mkdir build $ cd build $ meson .. $ sudo ninja install Support ------- Many Mesa devs hang on IRC; if you're not sure which channel is appropriate, you should ask your question on `OFTC's #dri-devel <irc://irc.oftc.net/dri-devel>`_, someone will redirect you if necessary. Remember that not everyone is in the same timezone as you, so it might take a while before someone qualified sees your question. To figure out who you're talking to, or which nick to ping for your question, check out `Who's Who on IRC <https://dri.freedesktop.org/wiki/WhosWho/>`_. The next best option is to ask your question in an email to the mailing lists: `mesa-dev\@lists.freedesktop.org <https://lists.freedesktop.org/mailman/listinfo/mesa-dev>`_ Bug reports ----------- If you think something isn't working properly, please file a bug report (`docs/bugs.rst <https://mesa3d.org/bugs.html>`_). Contributing ------------ Contributions are welcome, and step-by-step instructions can be found in our documentation (`docs/submittingpatches.rst <https://mesa3d.org/submittingpatches.html>`_). Note that Mesa uses gitlab for patches submission, review and discussions.