On Sat, Dec 20, 2025 at 10:50 AM Nicolas Frattaroli
<[email protected]> wrote:
>
> The MediaTek MT8196 SoC's Mali SHADER_PRESENT register does not list
> only functional shader cores, but also those that are fused off to
> improve yield.
>
> The SHADER_PRESENT bitmask with the one fused off core omitted is to be
> found in an efuse. However, the efuse address is considered
> confidential, and is not public knowledge.
>
> The MT8196 GPUEB MCU, which does the power management for the Mali GPU
> on this SoC, knows and reads the efuse however, and exposes it in the
> shared memory intended to communicate state to the application
> processor. Reading the bitmask from this shared memory area is the
> vendor's intended solution.
>
> This series models this in the binding and implements it in the
> corresponding Linux drivers:
> - the mali-valhall-csf binding gets an nvmem-cells/nvmem-cell-names
> property to declare that shader-present is in a different castle
> - the mt8196-gpufreq binding requires nodes to expose the shader-present
> cell
> - panthor checks for the presence of the shader-present cell and uses it
> as the shader-present value if it's found, instead of the Mali GPU
> register contents
> - mtk-mfg-pmdomain becomes an nvmem provider and will happily serve
> queries for the shader-present cell
>
> While it would be preferable if we could read the efuse directly, it's
> not possible as things stand, and insisting on it will just keep this
> hardware from working in mainline. Running a GPU workload with a
> SHADER_PRESENT bitmask that includes a faulty core results in corrupt
> GPU rendering output.
>
> Modelling the mt8196-gpufreq device as a nvmem-cell provider however is
> not lying about the hardware's capabilities, as it truly does provide
> access to the nvmem-cell, even if it acts as a proxy.
>
> From a bindings and panthor perspective, this is also generic enough to
> where hypothetical other vendors doing the same thing (even with direct
> efuse access) can rely on the same cell name and implementation.
>
> Signed-off-by: Nicolas Frattaroli <[email protected]>
Series is Reviewed-by: Chia-I Wu <[email protected]>
> ---
> Changes in v2:
> - panthor: move to nvmem_cell_read_variable_le_u64
> - mtk-mfg-pmdomain: put of_node in error path
> - mtk-mfg-pmdomain: remove leftover stray of_node_put on NULL
> - Link to v1:
> https://lore.kernel.org/r/[email protected]
>
> ---
> Nicolas Frattaroli (4):
> dt-bindings: gpu: mali-valhall-csf: Add shader-present nvmem cell
> dt-bindings: power: mt8196-gpufreq: Describe nvmem provider ability
> drm/panthor: Implement reading shader_present from nvmem
> pmdomain: mediatek: mtk-mfg: Expose shader_present as nvmem cell
>
> .../bindings/gpu/arm,mali-valhall-csf.yaml | 14 +++++
> .../bindings/power/mediatek,mt8196-gpufreq.yaml | 13 +++++
> drivers/gpu/drm/panthor/panthor_hw.c | 36 ++++++++++---
> drivers/pmdomain/mediatek/mtk-mfg-pmdomain.c | 59
> ++++++++++++++++++++++
> 4 files changed, 116 insertions(+), 6 deletions(-)
> ---
> base-commit: 638148c6ffa31d2e4958d51708ff3675221e2da7
> change-id: 20251217-mt8196-shader-present-e2dc3f97fc74
>
> Best regards,
> --
> Nicolas Frattaroli <[email protected]>
>
