[PATCH] ASoC: DaVinci: More accurate calculation for clock divider for McBSP (I2S)

[Raffaele Recalcati]

From: Raffaele Recalcati <raffaele.recalcati at bticino.it>

    i2s_accurate_sck switch can be used to have a better approximate
    sampling frequency.
    The trade off is between more accurate clock (fast clock)
    and less accurate clock (slow clock).
    The waveform will be not symmetric.
    Probably it is possible to get a better algorithm for calculating
    the divider, trying to keep a slower clock as possible.

    This patch has been developed against the
        http://git.kernel.org/pub/scm/linux/kernel/git/khilman/linux-davinci.git
    git tree and has been tested on bmx board (similar to dm365 evm, but using
    uda1345 as external audio codec).

Signed-off-by: Raffaele Recalcati <raffaele.recalcati at bticino.it>
Signed-off-by: Davide Bonfanti <davide.bonfanti at bticino.it>
---
 arch/arm/mach-davinci/include/mach/asp.h |   33 ++++++++++++++++++++++++++++++
 sound/soc/davinci/davinci-i2s.c          |   28 ++++++++++++++++++++-----
 2 files changed, 55 insertions(+), 6 deletions(-)

diff --git a/arch/arm/mach-davinci/include/mach/asp.h b/arch/arm/mach-davinci/include/mach/asp.h
index 0847d21..5149abe 100644
--- a/arch/arm/mach-davinci/include/mach/asp.h
+++ b/arch/arm/mach-davinci/include/mach/asp.h
@@ -73,6 +73,39 @@ struct snd_platform_data {
 	 */
 	int clk_input_pin;
 
+	/*
+	 * This flag works when both clock and FS are outputs for the cpu
+	 * and makes clock more accurate (FS is not symmetrical and the
+	 * clock is very fast.
+	 * The clock becoming faster is named
+	 * i2s continuous serial clocl (I2S_SCK) and it is an externally
+	 * visible bit clock.
+	 *
+	 * first line : WordSelect
+	 * second line : ContinuousSerialClock
+	 * third line: SerialData
+	 *
+	 * SYMMETRICAL APPROACH:
+	 *   _______________________          LEFT
+	 * _|         RIGHT         |______________________|
+	 *     _   _         _   _   _   _         _   _
+	 *   _| |_| |_ x16 _| |_| |_| |_| |_ x16 _| |_| |_
+	 *     _   _         _   _   _   _         _   _
+	 *   _/ \_/ \_ ... _/ \_/ \_/ \_/ \_ ... _/ \_/ \_
+	 *    \_/ \_/       \_/ \_/ \_/ \_/       \_/ \_/
+	 *
+	 * ACCURATE CLOCK APPROACH:
+	 *   ______________          LEFT
+	 * _|     RIGHT    |_______________________________|
+	 *     _         _   _         _   _   _   _   _   _
+	 *   _| |_ x16 _| |_| |_ x16 _| |_| |_| |_| |_| |_| |
+	 *     _         _   _          _      dummy cycles
+	 *   _/ \_ ... _/ \_/ \_  ... _/ \__________________
+	 *    \_/       \_/ \_/        \_/
+	 *
+	 */
+	bool i2s_accurate_sck;
+
 	/* McASP specific fields */
 	int tdm_slots;
 	u8 op_mode;
diff --git a/sound/soc/davinci/davinci-i2s.c b/sound/soc/davinci/davinci-i2s.c
index 17f594f..e6dcd81 100644
--- a/sound/soc/davinci/davinci-i2s.c
+++ b/sound/soc/davinci/davinci-i2s.c
@@ -122,6 +122,7 @@ static const unsigned char double_fmt[SNDRV_PCM_FORMAT_S32_LE + 1] = {
 };
 
 struct davinci_mcbsp_dev {
+	struct device *dev;
 	struct davinci_pcm_dma_params	dma_params[2];
 	void __iomem			*base;
 #define MOD_DSP_A	0
@@ -154,6 +155,7 @@ struct davinci_mcbsp_dev {
 	unsigned int fmt;
 	int clk_div;
 	int clk_input_pin;
+	bool i2s_accurate_sck;
 };
 
 static inline void davinci_mcbsp_write_reg(struct davinci_mcbsp_dev *dev,
@@ -296,7 +298,7 @@ static int davinci_i2s_set_dai_fmt(struct snd_soc_dai *cpu_dai,
 			pcr |= DAVINCI_MCBSP_PCR_SCLKME;
 			break;
 		default:
-			dev_err(&pdev->dev, "bad clk_input_pin\n");
+			dev_err(dev->dev, "bad clk_input_pin\n");
 			return -EINVAL;
 		}
 
@@ -447,11 +449,23 @@ static int davinci_i2s_hw_params(struct snd_pcm_substream *substream,
 		       DAVINCI_MCBSP_SRGR_CLKSM;
 		srgr |= DAVINCI_MCBSP_SRGR_FWID(mcbsp_word_length *
 						8 - 1);
-		/* symmetric waveforms */
-		clk_div = freq / (mcbsp_word_length * 16) /
-			  params->rate_num * params->rate_den;
-		srgr |= DAVINCI_MCBSP_SRGR_FPER(mcbsp_word_length *
-						16 - 1);
+		if (dev->i2s_accurate_sck) {
+			clk_div = 256;
+			do {
+				framesize = (freq / (--clk_div)) /
+				params->rate_num *
+					params->rate_den;
+			} while (((framesize < 33) || (framesize > 4095)) &&
+				 (clk_div));
+			clk_div--;
+			srgr |= DAVINCI_MCBSP_SRGR_FPER(framesize - 1);
+		} else {
+			/* symmetric waveforms */
+			clk_div = freq / (mcbsp_word_length * 16) /
+				  params->rate_num * params->rate_den;
+			srgr |= DAVINCI_MCBSP_SRGR_FPER(mcbsp_word_length *
+							16 - 1);
+		}
 		clk_div &= 0xFF;
 		srgr |= clk_div;
 		break;
@@ -662,6 +676,7 @@ static int davinci_i2s_probe(struct platform_device *pdev)
 		dev->dma_params[SNDRV_PCM_STREAM_CAPTURE].sram_size =
 			pdata->sram_size_capture;
 		dev->clk_input_pin = pdata->clk_input_pin;
+		dev->i2s_accurate_sck = pdata->i2s_accurate_sck;
 	}
 	dev->clk = clk_get(&pdev->dev, NULL);
 	if (IS_ERR(dev->clk)) {
@@ -694,6 +709,7 @@ static int davinci_i2s_probe(struct platform_device *pdev)
 		goto err_free_mem;
 	}
 	dev->dma_params[SNDRV_PCM_STREAM_CAPTURE].channel = res->start;
+	dev->dev = &pdev->dev;
 
 	davinci_i2s_dai.private_data = dev;
 	davinci_i2s_dai.capture.dma_data = dev->dma_params;
-- 
1.7.0.4