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51 lines
2.3 KiB
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51 lines
2.3 KiB
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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 and Raspberry Pi 5.
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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 and 5, 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 (Raspberry
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Pi 4), and V3D 7.1 (Raspberry Pi 5). When the support for V3D 7.1
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landed, the support for 3.3 and 4.1 was dropped as it was not tested
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anymore (see `MR#25851
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<https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25851>`__)
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Broadcom's reference software platforms do not make use of the open
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source V3D stack, but porting a particular hardware implementation to
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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 synced up with
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whatever version Mesa was last being developed against.
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