STRUCT VIRTIO_PCM_SUBSTREAM(9)

💖 struct virtio_pcm_substream(9) 💖

okay so real quick before we dive in: this is a page from the Kernel API Manual and it's all about struct virtio_pcm_substream — yes bestie, the VirtIO PCM substream itself, the main character of the sound subsystem. buckle up 🎀

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✨ NAME ✨

struct virtio_pcm_substream — VirtIO PCM substream. that's it, that's the vibe. 💅

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🌸 SYNOPSIS 🌸

here's the whole struct, copied exactly because we do not paraphrase code in this house 🫶

struct virtio_pcm_substream {
    struct virtio_snd *snd;
    u32 nid;
    u32 sid;
    u32 direction;
    u32 features;
    struct snd_pcm_substream *substream;
    struct snd_pcm_indirect pcm_indirect;
    struct snd_pcm_hardware hw;
    struct work_struct elapsed_period;
    spinlock_t lock;
    size_t buffer_bytes;
    size_t hw_ptr;
    bool xfer_enabled;
    bool xfer_xrun;
    bool stopped;
    bool suspended;
    struct virtio_pcm_msg **msgs;
    unsigned int nmsgs;
    unsigned int msg_count;
    wait_queue_head_t msg_empty;
 };

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💅 Members 💅

every single field, no skips, no cap. let's go field by field 🔥

• snd — the VirtIO sound device. the whole reason we're here tbh 👑
• nid — function group node identifier.
• sid — stream identifier.
• direction — which way the stream data flows, (SNDRV_PCM_STREAM_XXX). pick a lane bestie 🦋
• features — the stream's VirtIO feature bit map, written as (1 << VIRTIO_SND_PCM_F_XXX).
• substream — the kernel ALSA substream. iconic struct adjacent ✨
• pcm_indirect — the kernel indirect pcm structure.
• hw — the kernel ALSA substream hardware descriptor.
• elapsed_period — kernel work that handles the elapsed period state. lowkey busy little worker bee 🐝
• lock — spinlock that protects fields shared by interrupt handlers and substream operators. very much the bouncer at the club 🚪💀
• buffer_bytes — current buffer size in bytes.
• hw_ptr — substream hardware pointer value in bytes, and its range is literally [0 ... buffer_bytes) — note that the top end is exclusive, so don't come for me 🌈
• xfer_enabled — data transfer state (0 - off, 1 - on).
• xfer_xrun — data underflow/overflow state (0 - no xrun, 1 - xrun). if this is 1 we're having a bad time 😭
• stopped — True if the substream is stopped and must be released on the device side.
• suspended — True if the substream is suspended and must be reconfigured on the device side at resume.
• msgs — allocated I/O messages.
• nmsgs — number of allocated I/O messages.
• msg_count — number of pending I/O messages in the virtqueue.
• msg_empty — the thing that notifies when msg_count is zero. the "we're empty, come thru" signal 🫶

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⭐ SEE ALSO ⭐

go read these next, iykyk:

• Kernel file ./sound/virtio/virtio_pcm.h
• struct virtio_pcm_stream(9)
• struct virtio_pcm(9)

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ok that's the whole page, every field accounted for, nothing dropped 💖 now go make some sound drivers slay 💅

and finally, mwah, this little writeup lives in ./sound/virtio and it's dated October 2026 — signing off, struct virtio_pcm_substream(9), no notes, periodt 🎀✨