ALSA: firewire-lib: tune the maximum available size of PCM period

Linux driver for 1394 OHCI controller voluntarily flushes isoc context
when total size of accumulated context header reached PAGE_SIZE. This
kicks tasklet for the isoc context. This is inconvenient to process
runtime of PCM substream.

This commit adds a restriction of the maximum size of PCM period to
avoid this behaviour.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20191017155424.885-12-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/firewire/amdtp-stream.c b/sound/firewire/amdtp-stream.c
index fd7c3e4d5374e8d7c0fab6e56c6da0e9bcaf40b2..8a7818130382a78764e4f0b8a78ae13ffd97cddc 100644 (file)
--- a/sound/firewire/amdtp-stream.c
+++ b/sound/firewire/amdtp-stream.c
@@ -176,6 +176,8 @@ int amdtp_stream_add_pcm_hw_constraints(struct amdtp_stream *s,
                                        struct snd_pcm_runtime *runtime)
 {
        struct snd_pcm_hardware *hw = &runtime->hw;
+       unsigned int ctx_header_size;
+       unsigned int maximum_usec_per_period;
        int err;
 
        hw->info = SNDRV_PCM_INFO_BATCH |
@@ -196,19 +198,24 @@ int amdtp_stream_add_pcm_hw_constraints(struct amdtp_stream *s,
        hw->period_bytes_max = hw->period_bytes_min * 2048;
        hw->buffer_bytes_max = hw->period_bytes_max * hw->periods_min;
 
-       /*
-        * Currently firewire-lib processes 16 packets in one software
-        * interrupt callback. This equals to 2msec but actually the
-        * interval of the interrupts has a jitter.
-        * Additionally, even if adding a constraint to fit period size to
-        * 2msec, actual calculated frames per period doesn't equal to 2msec,
-        * depending on sampling rate.
-        * Anyway, the interval to call snd_pcm_period_elapsed() cannot 2msec.
-        * Here let us use 5msec for safe period interrupt.
-        */
+       // Linux driver for 1394 OHCI controller voluntarily flushes isoc
+       // context when total size of accumulated context header reaches
+       // PAGE_SIZE. This kicks tasklet for the isoc context and brings
+       // callback in the middle of scheduled interrupts.
+       // Although AMDTP streams in the same domain use the same events per
+       // IRQ, use the largest size of context header between IT/IR contexts.
+       // Here, use the value of context header in IR context is for both
+       // contexts.
+       if (!(s->flags & CIP_NO_HEADER))
+               ctx_header_size = IR_CTX_HEADER_SIZE_CIP;
+       else
+               ctx_header_size = IR_CTX_HEADER_SIZE_NO_CIP;
+       maximum_usec_per_period = USEC_PER_SEC * PAGE_SIZE /
+                                 CYCLES_PER_SECOND / ctx_header_size;
+
        err = snd_pcm_hw_constraint_minmax(runtime,
                                           SNDRV_PCM_HW_PARAM_PERIOD_TIME,
-                                          5000, UINT_MAX);
+                                          5000, maximum_usec_per_period);
        if (err < 0)
                goto end;