ALSA: compress: add mmap buffer support and SNDRV_COMPRESS_ACK ioctl

[lgirdwood]

Overview

This series adds memory-mapped (MMAP) buffer support to the ALSA compress-offload subsystem and implements the corresponding callbacks for ASoC and the Sound Open Firmware (SOF) IPC4 driver.

Currently, ALSA compressed streams only support copy-based data transfer (snd_compr_write_data() / snd_compr_read()), and snd_compr_mmap() unconditionally returns -ENXIO. For high-throughput, low-latency audio offload engines (such as hardware DSP spatial audio, voice processing, and adjacent metadata/PCM streaming in Steam Audio offload), user space needs direct, zero-copy MMAP access to the DMA buffer ring.

Alignment with Regular ALSA PCM Flow

This implementation directly mirrors the established ALSA PCM MMAP architectural pattern:

1. Commit / Progress Notification:

   • In ALSA PCM (pcm_lib_apply_appl_ptr()), when user space writes to the mapped buffer and calls snd_pcm_mmap_commit(), the core advances appl_ptr and invokes the optional driver callback substream->ops->ack(substream).
   • In ALSA Compress, SNDRV_COMPRESS_ACK serves the exact same role: user space commits written bytes, the core advances runtime->total_bytes_available (the compress equivalent of appl_ptr), and calls stream->ops->ack(stream, bytes).

2. Why SOF's ack is a No-Op:

   • On Intel platforms (TGL, ARL, PTL) using HDA / ACE DMA controllers, the host-to-DSP transfer is executed autonomously by hardware DMA stream descriptors from host DRAM without requiring per-transfer register doorbells or mailbox IPCs (identical to SOF PCM where ops->ack is unused / NULL).
   • In contrast, mailbox-driven DSPs (e.g. legacy Atom SST or DSPs with non-coherent SRAM) require an explicit doorbell interrupt per commit.
   • The callback is implemented as a no-op in SOF to fulfill struct snd_compress_ops, preserve subsystem symmetry with doorbell-driven platforms, and provide a hook for future platform tracing.

Summary of Changes

1. ALSA Compress Core & UAPI (sound/core/compress_offload.c, include/uapi/sound/compress_offload.h):

   • Implement snd_compr_mmap():
     • Dispatches to stream->ops->mmap(stream, vma) when supported by the driver.
     • Falls back to snd_dma_buffer_mmap(runtime->dma_buffer_p, vma) if available.
   • Introduce SNDRV_COMPRESS_ACK ioctl (_IOW('C', 0x24, __u64)):
     • Allows user space operating in mmap mode to inform the driver of committed/consumed bytes.
     • Validates byte count against available buffer space.
     • Calls optional stream->ops->ack(stream, bytes).
     • Updates runtime->total_bytes_available.
     • Transitions state from SNDRV_PCM_STATE_SETUP to SNDRV_PCM_STATE_PREPARED on initial commit.
     • Wakes up sleeping poll/wait threads (&runtime->sleep).

2. ASoC Compress Layer (include/sound/soc-component.h, sound/soc/soc-component.c, sound/soc/soc-compress.c):

   • Add snd_soc_component_compr_mmap() to iterate over runtime components and call component->driver->compress_ops->mmap().
   • Wire .mmap = soc_compr_mmap in both soc_compr_ops and soc_compr_dyn_ops.

3. SOF IPC4 Compressed Driver (sound/soc/sof/ipc4-compress.c):

   • Implement sof_ipc4_compr_mmap() to map the runtime DMA buffer directly using snd_dma_buffer_mmap().
   • Implement sof_ipc4_compr_ack() callback (no-op for autonomous HDA/ACE DMA).
   • Align SOF_IPC4_COMPR_MIN_FRAGMENT_SIZE to 128 bytes to match Intel HDA / ACE DMA burst hardware constraints.
   • Register .mmap and .ack in sof_ipc4_compressed_ops.

Testing & Verification

• Verified clean kernel build on x86_64 (sound/core/compress_offload.o, sound/soc/soc-component.o, sound/soc/soc-compress.o, sound/soc/sof/ipc4-compress.o).
• Tested with Steam Audio adjacent MMAP and compressed stream pipelines on physical Arrow Lake hardware (Dragon Fly DUT) running kernel 7.3.0-rc1-sof-dev+.
• All unit tests and audio loopback tests passing with zero underruns/overruns.

Signed-off-by: Liam Girdwood liam.r.girdwood@linux.intel.com