[PATCH 3/3] ASoC: DaVinci: More accurate continuous serial clock for McBSP (I2S)

Raffaele Recalcati
2010-07-06T08:39:10Z

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>
---
arch/arm/mach-davinci/include/mach/asp.h | 33 ++++++++++++++++++++++++++++++
sound/soc/davinci/davinci-i2s.c | 24 +++++++++++++++++----
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 0847d21..b12c69e 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 ba5644b..b251bc9 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)) {
--
1.7.0.4