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]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: b877a609-cb6e-44c4-812b-a33a369ef542

📥 Commits

Reviewing files that changed from the base of the PR and between 45fcccd and 63d85bd.

📒 Files selected for processing (2)

• src/mips/common/hardware/spu.h
• src/mips/psyqo/src/spu.cpp

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📝 Walkthrough

Walkthrough

Adds two new SPU hardware register address macros (SPU_ENDX_LOW and SPU_ENDX_HIGH). Updates SPU voice state handling in multiple functions: adjusts channel silence sample rates, reconfigures SPU controller initialization values and output volumes, changes register write behavior in audio playback, and refactors free channel detection to use the new endx bitmask instead of volume checking.

Changes

Cohort / File(s)	Summary
Hardware Register Definitions src/mips/common/hardware/spu.h	Added two new SPU hardware register address macros for SPU_ENDX_LOW (0x1f801d9c) and SPU_ENDX_HIGH (0x1f801d9e).
SPU Voice State Logic src/mips/psyqo/src/spu.cpp	Updated channel silence handling to set sample rate to 0x1000 instead of zeroing. Modified initialize() to configure SPU_CTRL with specific bitmask and reduce output volumes from 0x7fff to 0x3fff. Changed playADPCM() key-on/key-off behavior from OR-ing to direct register overwrites. Refactored getNextFreeChannel() to detect free channels via endx bitmask instead of checking current volume.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Poem

🐰 Registers shift in silicon streams,
Where endx bits unlock the channel's dreams,
Silence whispers at 0x1000,
SPU's heart beats steady, control set just right—
A rabbit's tune now rings so bright! 🎵

🚥 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' directly relates to the main changes in the PR, which involve fixing SPU hardware register handling and behavior on actual hardware.
Description check	✅ Passed	The description comprehensively covers the changeset, detailing four specific fixes: SPU not being enabled for DMA transfer, volume exceeding maximum values, readonly SPU_KEY register issues, and ENDX register-based channel detection.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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