There has been a lot of activity in V3DV, the Vulkan driver for Raspberry Pi 4, over the last year: we have significantly reworked our synchronization code, obtained Vulkan 1.1 conformance, implemented Vulkan 1.2 support, continued to work on compiler optimizations and more. In this talk I would like to go through the main development milestones and changes we implemented in the driver as well as discussing some limitations of the underlying hardware platform that have discouraged us from implementing features such as scalar block layout or fp16. (c) X.Org Developer Conference (XDC) 2022 October 4-6, 2022 Minneapolis, Minnesota, USA https://indico.freedesktop.org/event/2/

1. Status of Vulkan on Raspberry Pi
XDC 2022
Iago Toral Quiroga <itoral@igalia.com>

2. Contents
1. Current status.
2. Development highlights.
3. Pain points.
4. Q&A.
X Developers Conference
October 2022

3. Current Status
• Vulkan 1.1 conformance (October 2021).
• Vulkan 1.2 conformance (July 2022).
• Implemented 35 extensions (core and optional).
• New multisync kernel interface (thanks to Melissa Wen!)
• Improved performance (mostly compiler & sync improvements)
• Using common synchronization infrastructure in Mesa (thanks to Jason Ekstrand!).
• Android support (thanks to Roman Stratiienko!).
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October 2022

4. Development Highlights
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October 2022

5. Synchronization
• Single-sync and Multi-sync paths available.
• Undesirable, single-sync to be deprecated in the future.
• Multi-sync is more flexible and a better match for Vulkan.
• Ported to use common synchronization framework in Mesa.
• Gained (emulated) timeline semaphores for free in the process!
• Dropped driver submit threading in favor of threaded submit mode.
• Incidentally, found about CPU throttling issues with drmSyncObjWait.
• Reworked barriers to better match Vulkan semantics and optimize some cases.
• Tried harder to skip binning syncs in favor of render syncs when possible.
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October 2022

6. nir_address_format_2x32bit_global
• Needed for VK_KHR_buffer_device_address.
• Existing nir_address_format_{64|32}bit_global not useful for us.
• The 64bit version will inject 64-bit cast and pack/unpack instructions.
• The 32bit version won’t match Vulkan’s explicit 64-bit addresses.
• The new format can honor Vulkan’s 64-bit semantics without requiring 64-bit
instructions.
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October 2022

7. Double-Buffer Mode
• Help hide tile store latency at the expense of reducing tile size.
• May cause additional shader invocations in the geometry part of the pipeline though.
• Needs heuristics to decide when to enable.
• Experimental feature, enable via V3D_DEBUG=db.
• We probably need to tune the heuristics further.
• We may want to port to GL driver to have a larger testing ground.
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October 2022

8. Double-Buffer Mode
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October 2022
• Some workloads improved:
• Serious Sam: +7.95% fps.
• Quake: +6.32% fps.
• Quake3e: +4.03% fps.
• RbDoom3: +3.59% fps.

9. Compiler
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October 2022
• Stopped being so aggressive trying to hide latency.
• Thread switching and TMU pipelining are quite effective at hiding latency already.
• Over-estimating latency can delay critical paths in shaders.
• Slight performance improvement in pretty much all workloads we tested.
• Stopped rebuilding interference graph after each spill.
• Massive improvement for compile times in spilling shaders.
• We still recompute liveness so we don’t hurt spilling quality.
• We use this mostly to reduce spilling by testing multiple compile strategies.

10. Pain points
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October 2022

11. CPU jobs
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October 2022
• A few things in a command buffer may need CPU intervention.
• Threaded submit helps a bit.
• This is the reason we cannot reliably support SYNC_FD exports.
• I hope we can find ways to get rid of this in the future.

12. fp16
X Developers Conference
October 2022
• Hardware design based on emitting 2x16-bit instructions.
• Significant register allocation constraints.
• Difficult to exploit optimally in practice.
• We do support VK_KHR_16bit_storage though.

13. Zink
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October 2022
• It requires VK_EXT_scalar_block_layout.
• We cannot implement this optimally due to hardware restrictions.
• Alternatively, we could lower all load/store to scalar.
• Not great for performance though.
• … but may be worthwhile to expand testing grounds for Vulkan on Raspberry Pi.
• Maybe expose this feature under a V3D_DEBUG setting?

14. Q&A
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October 2022
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