linux-kernel - [PATCH 7/8] ASoC: cs42l42: Assume 24-bit samples are in 32-bit slots

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Message-ID: <20210805161111.10410-7-rf@opensource.cirrus.com>
Date:   Thu, 5 Aug 2021 17:11:10 +0100
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     <broonie@...nel.org>
CC:     <alsa-devel@...a-project.org>, <patches@...nsource.cirrus.com>,
        <linux-kernel@...r.kernel.org>,
        Richard Fitzgerald <rf@...nsource.cirrus.com>
Subject: [PATCH 7/8] ASoC: cs42l42: Assume 24-bit samples are in 32-bit slots

If the machine driver doesn't call snd_soc_dai_set_sysclk() the
SCLK is assumed to be sample_rate * sample_bits * 2 (that is, the
rate necessary for a standard I2S frame).

But 24-bit samples can be sent in either a 24-bit slot or a 32-bit
slot. If the PLL is configured for a 24-bit slot, but a 32-bit slot is
used, cs42l42 will be overclocked.

Ultimately it is the machine driver's responsibilty to call
snd_soc_dai_set_sysclk() if SLK will be different from the standard
I2S rate. However, it is convenient to assume 32-bit slots to allow
this common case without needing special machine driver support. The
machine driver then only has to set SCLK if the slots are 24-bit, but
if it fails to do this cs42l42 won't be overclocked.

Signed-off-by: Richard Fitzgerald <rf@...nsource.cirrus.com>
---
 sound/soc/codecs/cs42l42.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/sound/soc/codecs/cs42l42.c b/sound/soc/codecs/cs42l42.c
index b2ee51443a22..3677ed4670d0 100644
--- a/sound/soc/codecs/cs42l42.c
+++ b/sound/soc/codecs/cs42l42.c
@@ -844,6 +844,13 @@ static int cs42l42_pcm_hw_params(struct snd_pcm_substream *substream,
 	if (channels == 1)
 		cs42l42->bclk *= 2;

+	/*
+	 * Assume 24-bit samples are in 32-bit slots, to prevent SCLK being
+	 * more than assumed (which would result in overclocking).
+	 */
+	if (params_width(params) == 24)
+		cs42l42->bclk = (cs42l42->bclk / 3) * 4;
+
 	switch(substream->stream) {
 	case SNDRV_PCM_STREAM_CAPTURE:
 		if (channels == 2) {
-- 
2.11.0