ASoC: DaVinci: More accurate continuous serial clock for McBSP (I2S)

    i2s_accurate_sck switch can be used to have a better approximate
    sampling frequency.
    The clock is an externally visible bit clock and it is named
    i2s continuous serial clock (I2S_SCK).
    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@bticino.it>
Signed-off-by: Davide Bonfanti <davide.bonfanti@bticino.it>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Acked-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

2 files changed, 52 insertions, 5 deletions

diff --git a/arch/arm/mach-davinci/include/mach/asp.h b/arch/arm/mach-davinci/include/mach/asp.h
index 0847d2144f1f..b12c69edeea5 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 clock (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 ba5644b5fbbf..b251bc9a9812 100644
--- a/sound/soc/davinci/davinci-i2s.c
+++ b/sound/soc/davinci/davinci-i2s.c
@@ -155,6 +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,
@@ -447,11 +448,23 @@ 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 +675,7 @@ 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)) {