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48 lines
2.2 KiB
48 lines
2.2 KiB
4 months ago
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V3D
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===
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Mesa's ``v3d`` graphics driver stack includes a `conformant GLES3.1
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driver
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<https://www.khronos.org/conformance/adopters/conformant-products/opengles#submission_882>`__
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called ``v3d`` and a Vulkan graphics driver called ``v3dv``, notably
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used on the Raspberry Pi 4.
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The v3d Mesa drivers communicate directly with the `v3d
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<https://www.kernel.org/doc/html/latest/gpu/v3d.html>`__ kernel DRM
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driver for scheduling GPU commands. Additionally, on the Raspberry Pi
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4, the kernel uses the vc4 DRM driver for display support, so Mesa
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exposes a ``vc4_dri.so`` using the kmsro helpers to do
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behind-the-scenes buffer management between the two kernel drivers,
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while executing rendering on the v3d kernel module.
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Initial development work was done on the Broadcom 7268 (V3D 3.3) and
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7278 (V3D 4.1). Development since then has been on V3D 4.2, and
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though the 3.3 and 4.1 support continues to remain in tree, it is not
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tested any more. Broadcom's reference software platforms do not make
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use of the open source v3d stack, but porting a particular hardware
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implementation to use it would still be possible.
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Hardware Documentation
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----------------------
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Broadcom never released a public specification for the V3D 3.x or 4.x
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series.
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For driver developers, Broadcom publicly released a `specification
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<https://docs.broadcom.com/doc/12358545>`__ PDF for the 21553, which
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is closely related to the vc4 GPU present in the Raspberry Pi. They
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also released a `snapshot <https://docs.broadcom.com/docs/12358546>`__
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of a corresponding Android graphics driver. That graphics driver was
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ported to Raspbian for a demo, but was not expected to have ongoing
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development.
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Developers with NDA access with Broadcom or Raspberry Pi can get
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access to the V3D architecture specification for documentation of the
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GPU's programming model. There is also a C++ software simulator
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called simpenrose, and the Mesa driver includes a backend
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(``src/broadcom/drm-shim/``) to use simpenrose from an x86 system with
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the i915 graphics driver with all of the vc4 rendering commands
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emulated on simpenrose and memcpyed to the real GPU. Note that
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simpenrose's API drifts over time, so you need to be syned up with
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whatever version Mesa was last being developed against.
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