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This was confusing at best. Change it to output the actual choices.
(Seems like in the end it's always me who has to clean up other people's
bullshit.)
Context names were not unique - but they should be, so fix it. The whole
point of the original --opengl-backend option was to side-step the
tricky auto-detection, so you know exactly what you get. The goal of
this commit is to make --gpu-context work the same way. Fix the
non-unique names by appending "vk" to the names.
Keep in mind that this was not suitable for slecting the "UI" backend
anyway, since "x11" would force GLX, whereas people on not-NVIDIA
actually want "x11egl". Users trying to use --gpu-context=x11 to force
the X11 backend would always end up with GLX, which would at least break
VAAPI hardware decoding for them. Basically the idea that this option
could select the "UI" type is completely broken - it selects an
implementation, which implies a UI. Selecting the UI type This would
require a separate mechanism. (Although in theory this separate
mechanism could be part of the --gpu-context option - in any case,
someone would have to implement it.)
To achieve help output that can actually be understood, just duplicate
the code. Most of that code is duplicated anyway, and trying to share
just the list code with the result of making the output unreadable
doesn't make too much sense. If we wanted to save code/effort, we could
just remove the help output altogether.
--gpu-api has non-unique entries, and it would be nice to group them
(e.g. list all OpenGL capable contexts with "opengl"), but C makes this
simple idea too much of a pain, so don't do it.
Also remove a stray tab from the android entry on the manpage.
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This time based on ra/vo_gpu. 2017 is the year of the vulkan desktop!
Current problems / limitations / improvement opportunities:
1. The swapchain/flipping code violates the vulkan spec, by assuming
that the presentation queue will be bounded (in cases where rendering
is significantly faster than vsync). But apparently, there's simply
no better way to do this right now, to the point where even the
stupid cube.c examples from LunarG etc. do it wrong.
(cf. https://github.com/KhronosGroup/Vulkan-Docs/issues/370)
2. The memory allocator could be improved. (This is a universal
constant)
3. Could explore using push descriptors instead of descriptor sets,
especially since we expect to switch descriptors semi-often for some
passes (like interpolation). Probably won't make a difference, but
the synchronization overhead might be a factor. Who knows.
4. Parallelism across frames / async transfer is not well-defined, we
either need to use a better semaphore / command buffer strategy or a
resource pooling layer to safely handle cross-frame parallelism.
(That said, I gave resource pooling a try and was not happy with the
result at all - so I'm still exploring the semaphore strategy)
5. We aggressively use pipeline barriers where events would offer a much
more fine-grained synchronization mechanism. As a result of this, we
might be suffering from GPU bubbles due to too-short dependencies on
objects. (That said, I'm also exploring the use of semaphores as a an
ordering tactic which would allow cross-frame time slicing in theory)
Some minor changes to the vo_gpu and infrastructure, but nothing
consequential.
NOTE: For safety, all use of asynchronous commands / multiple command
pools is currently disabled completely. There are some left-over relics
of this in the code (e.g. the distinction between dev_poll and
pool_poll), but that is kept in place mostly because this will be
re-extended in the future (vulkan rev 2).
The queue count is also currently capped to 1, because of the lack of
cross-frame semaphores means we need the implicit synchronization from
the same-queue semantics to guarantee a correct result.
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