Fixed SPU code on real hardware

[CloudMracek]

SPU fixes

• SPU not being on for dummy block DMA transfer
• Volume set to channels was over the maximum volume value for fixed volume mode
• SPU_KEY_LOW and HIGH are readonly. Using a bitmask on them caused a read which caused undefined volume behavior on real hardware
• Added a more robust detection of free channels using ENDX registers, which have also been added to common/hardware/spu.h

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Review profile: CHILL

Plan: Pro

Run ID: 1790048e-e326-4528-88c8-20fffd710a66

📥 Commits

Reviewing files that changed from the base of the PR and between 63d85bd and 0b31743.

📒 Files selected for processing (1)

• src/mips/psyqo/src/spu.cpp

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📝 Walkthrough

Walkthrough

Single-file update to SPU audio driver adjusting voice sample rate values, output volume registers, hardware register operation sequencing during initialization, and key on/off register write behavior from bitwise OR operations to direct assignment.

Changes

Cohort / File(s)	Summary
SPU Audio Driver src/mips/psyqo/src/spu.cpp	Modified channel silencing to set sample rate to 0x1000 instead of 0; reduced main output volume from 0x7fff to 0x3fff; reordered dmaWrite to execute after SPU_CTRL setup; changed SPU_KEY_OFF_* and SPU_KEY_ON_* register writes from OR-ing bits to overwriting with direct assignment (1 << bit).

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 Whiskers twitch with audio cheer,
Sample rates and volumes clear,
Register writes now stand alone,
No more OR-ing—assignment's shown,
SPU sings with voice so true! 🎵

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Fixed SPU code on real hardware' accurately summarizes the main purpose of the changeset—fixing SPU-related issues for real hardware compatibility.
Description check	✅ Passed	The description provides specific details about SPU fixes including DMA transfers, volume settings, and SPU_KEY register handling, all of which relate directly to the changes in the code.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[nicolasnoble]

Changes look correct. The init sequence now properly disables the SPU first, configures registers, then enables before DMA - fixing the DMA-with-SPU-off issue while preserving the clean-slate guarantee for unknown power-on state. KEY_ON/KEY_OFF plain writes are correct per hardware behavior (registers are consumed by the SPU and don't retain readable state, so OR-ing with the read was meaningless). Sample rate 0x1000 for silenced voices makes sense to let them process the dummy sample.