start_threshold compared in bytes but populated in frames (auto-start path)

[victorzappi]

session_obj.c compares stream_config.start_threshold against a byte counter, but the tinyalsa plugin populates that field from ALSA sw_params, where start_threshold is defined in frames. No unit conversion occurs between the two.

Call chain

1. The plugin's sw_params op receives the raw ALSA structure — include/tinyalsa/plugin.h:

int (*sw_params) (struct pcm_plugin *plugin, struct snd_pcm_sw_params *params);

2. In the ALSA ABI the field is frames, per its own comment — sound/asound.h:

snd_pcm_uframes_t start_threshold;  /* min hw_avail frames for automatic start */

(snd_pcm_uframes_t is unsigned long.)

3. plugins/tinyalsa/src/agm_pcm_plugin.c:518 copies it with a plain cast — no unit conversion, plus a 64-to-32-bit narrowing:

session_config->start_threshold = (uint32_t)sparams->start_threshold;

4. service/inc/public/agm/agm_api.h:402 documents the destination field as bytes:

uint32_t start_threshold;  /**< start_threshold: number of buffers * buffer size */

5. Nothing converts it downstream — service/src/session_obj.c:2297 and :2845 are plain struct assignments:

sess_obj->stream_config = *stream_config;

6. service/src/session_obj.c:2533-2536 compares it to an accumulated byte count:

if (sess_obj->stream_config.start_threshold > 0 &&
    sess_obj->state == SESSION_PREPARED) {
    sess_obj->bytes_written += *count;
    if (sess_obj->bytes_written >= sess_obj->stream_config.start_threshold) {

7. count is bytes, per the public API contract — agm_api.h:1000:

 * \param[in] count: actual number of bytes in the buffer.

Consequence

A client setting start_threshold through tinyalsa sw_params gets a session that auto-starts after start_threshold / bytes_per_frame frames instead of start_threshold frames. At stereo 16-bit (4 bytes per frame) the effective prefill is one quarter of what was requested, shrinking the ring-buffer cushion and making underruns more likely under graph load.

Either agm_pcm_plugin.c:518 needs a frames-to-bytes conversion from the session media format, or session_obj.c should accumulate frames rather than bytes — depending on which unit the field is meant to carry.

[quic-plai]

Thanks for catching this bug. Based on agm_api.h definition, I think agm_pcm_plugin.c should be doing the conversion. Will raise PR to address this bug