ASoC: SOF: Intel: hda-pcm: Place the constraint on period time instead of buffer time

Instead of constraining the buffer time to be double of the host buffer
size it is better to set it for the priod time.
This will implicitly constrain the buffer time to a safe value
(num_periods is at least 2) and prohibits applications to set smaller
period size than what will be covered by the initial DMA burst.

Fixes: 02ea2a0 ("ASoC: SOF: Intel: hda-pcm: Use dsp_max_burst_size_in_ms to place constraint")
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>

Author: ujfalusi

sound/soc/sof/intel/hda-pcm.c

@@ -291,19 +291,25 @@ int hda_dsp_pcm_open(struct snd_sof_dev *sdev,
 	 * On playback start the DMA will transfer dsp_max_burst_size_in_ms
 	 * amount of data in one initial burst to fill up the host DMA buffer.
 	 * Consequent DMA burst sizes are shorter and their length can vary.
-	 * To make sure that userspace allocate large enough ALSA buffer we need
-	 * to place a constraint on the buffer time.
+	 * To avoid immediate xrun by the initial burst we need to place
+	 * constraint on the period size (via PERIOD_TIME) to cover the size of
+	 * the host buffer.
+	 * We need to add headroom of max 10ms as the firmware needs time to
+	 * settle to the 1ms pacing and initially it can run faster for few
+	 * internal periods.
 	 *
 	 * On capture the DMA will transfer 1ms chunks.
-	 *
-	 * Exact dsp_max_burst_size_in_ms constraint is racy, so set the
-	 * constraint to a minimum of 2x dsp_max_burst_size_in_ms.
 	 */
-	if (spcm->stream[direction].dsp_max_burst_size_in_ms)
+	if (spcm->stream[direction].dsp_max_burst_size_in_ms) {
+		unsigned int period_time = spcm->stream[direction].dsp_max_burst_size_in_ms;
+
+		period_time += min(period_time, 10);
+
 		snd_pcm_hw_constraint_minmax(substream->runtime,
-			SNDRV_PCM_HW_PARAM_BUFFER_TIME,
-			spcm->stream[direction].dsp_max_burst_size_in_ms * USEC_PER_MSEC * 2,
+			SNDRV_PCM_HW_PARAM_PERIOD_TIME,
+			period_time * USEC_PER_MSEC,
 			UINT_MAX);
+	}
 
 	/* binding pcm substream to hda stream */
 	substream->runtime->private_data = &dsp_stream->hstream;