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Diffstat (limited to 'Documentation/sound/alsa/soc/clocking.txt')
| -rw-r--r-- | Documentation/sound/alsa/soc/clocking.txt | 273 |
1 files changed, 5 insertions, 268 deletions
diff --git a/Documentation/sound/alsa/soc/clocking.txt b/Documentation/sound/alsa/soc/clocking.txt index 1f55fd8cb117..e93960d53a1e 100644 --- a/Documentation/sound/alsa/soc/clocking.txt +++ b/Documentation/sound/alsa/soc/clocking.txt @@ -40,275 +40,12 @@ BCLK = LRC * x BCLK = LRC * Channels * Word Size -This relationship depends on the codec or SoC CPU in particular. ASoC can quite -easily match BCLK generated by division (SND_SOC_DAI_BFS_DIV) with BCLK by -multiplication (SND_SOC_DAI_BFS_RATE) or BCLK generated by -Rate * Channels * Word size (RCW or SND_SOC_DAI_BFS_RCW). +This relationship depends on the codec or SoC CPU in particular. In general +it's best to configure BCLK to the lowest possible speed (depending on your +rate, number of channels and wordsize) to save on power. +It's also desireable to use the codec (if possible) to drive (or master) the +audio clocks as it's usually gives more accurate sample rates than the CPU. -ASoC Clocking -------------- -The ASoC core determines the clocking for each particular configuration at -runtime. This is to allow for dynamic audio clocking wereby the audio clock is -variable and depends on the system state or device usage scenario. i.e. a voice -call requires slower clocks (and hence less power) than MP3 playback. -ASoC will call the config_sysclock() function for the target machine during the -audio parameters configuration. The function is responsible for then clocking -the machine audio subsytem and returning the audio clock speed to the core. -This function should also call the codec and cpu DAI clock_config() functions -to configure their respective internal clocking if required. - - -ASoC Clocking Control Flow --------------------------- - -The ASoC core will call the machine drivers config_sysclock() when most of the -DAI capabilities are known. The machine driver is then responsible for calling -the codec and/or CPU DAI drivers with the selected capabilities and the current -MCLK. Note that the machine driver is also resonsible for setting the MCLK (and -enabling it). - - (1) Match Codec and CPU DAI capabilities. At this point we have - matched the majority of the DAI fields and now need to make sure this - mode is currently clockable. - - (2) machine->config_sysclk() is now called with the matched DAI FS, sample - rate and BCLK master. This function then gets/sets the current audio - clock (depening on usage) and calls the codec and CPUI DAI drivers with - the FS, rate, BCLK master and MCLK. - - (3) Codec/CPU DAI config_sysclock(). This function checks that the FS, rate, - BCLK master and MCLK are acceptable for the codec or CPU DAI. It also - sets the DAI internal state to work with said clocks. - -The config_sysclk() functions for CPU, codec and machine should return the MCLK -on success and 0 on failure. - - -Examples (b = BCLK, l = LRC) -============================ - -Example 1 ---------- - -Simple codec that only runs at 48k @ 256FS in master mode. - -CPU only runs as slave DAI, however it generates a variable MCLK. - - -------- --------- - | | <----mclk--- | | - | Codec |b -----------> | CPU | - | |l -----------> | | - | | | | - -------- --------- - -The codec driver has the following config_sysclock() - - static unsigned int config_sysclk(struct snd_soc_codec_dai *dai, - struct snd_soc_clock_info *info, unsigned int clk) - { - /* make sure clock is 256 * rate */ - if(info->rate << 8 == clk) { - dai->mclk = clk; - return clk; - } - - return 0; - } - -The CPU I2S DAI driver has the following config_sysclk() - - static unsigned int config_sysclk(struct snd_soc_codec_dai *dai, - struct snd_soc_clock_info *info, unsigned int clk) - { - /* can we support this clk */ - if(set_audio_clk(clk) < 0) - return -EINVAL; - - dai->mclk = clk; - return dai->clk; - } - -The machine driver config_sysclk() in this example is as follows:- - - unsigned int machine_config_sysclk(struct snd_soc_pcm_runtime *rtd, - struct snd_soc_clock_info *info) - { - int clk = info->rate * info->fs; - - /* check that CPU can deliver clock */ - if(rtd->cpu_dai->config_sysclk(rtd->cpu_dai, info, clk) < 0) - return -EINVAL; - - /* can codec work with this clock */ - return rtd->codec_dai->config_sysclk(rtd->codec_dai, info, clk); - } - - -Example 2 ---------- - -Codec that can master at 8k and 48k at various FS (and hence supports a fixed -set of input MCLK's) and can also be slave at various FS . - -The CPU can master at 8k and 48k @256 FS and can be slave at any FS. - -MCLK is a 12.288MHz crystal on this machine. - - -------- --------- - | | <---xtal---> | | - | Codec |b <----------> | CPU | - | |l <----------> | | - | | | | - -------- --------- - - -The codec driver has the following config_sysclock() - - /* supported input clocks */ - const static int hifi_clks[] = {11289600, 12000000, 12288000, - 16934400, 18432000}; - - static unsigned int config_hsysclk(struct snd_soc_codec_dai *dai, - struct snd_soc_clock_info *info, unsigned int clk) - { - int i; - - /* is clk supported */ - for(i = 0; i < ARRAY_SIZE(hifi_clks); i++) { - if(clk == hifi_clks[i]) { - dai->mclk = clk; - return clk; - } - } - - /* this clk is not supported */ - return 0; - } - -The CPU I2S DAI driver has the following config_sysclk() - - static unsigned int config_sysclk(struct snd_soc_codec_dai *dai, - struct snd_soc_clock_info *info, unsigned int clk) - { - /* are we master or slave */ - if (info->bclk_master & - (SND_SOC_DAIFMT_CBM_CFM | SND_SOC_DAIFMT_CBM_CFS)) { - - /* we can only master @ 256FS */ - if(info->rate << 8 == clk) { - dai->mclk = clk; - return dai->mclk; - } - } else { - /* slave we can run at any FS */ - dai->mclk = clk; - return dai->mclk; - } - - /* not supported */ - return dai->clk; - } - -The machine driver config_sysclk() in this example is as follows:- - - unsigned int machine_config_sysclk(struct snd_soc_pcm_runtime *rtd, - struct snd_soc_clock_info *info) - { - int clk = 12288000; /* 12.288MHz */ - - /* who's driving the link */ - if (info->bclk_master & - (SND_SOC_DAIFMT_CBM_CFM | SND_SOC_DAIFMT_CBM_CFS)) { - /* codec master */ - - /* check that CPU can work with clock */ - if(rtd->cpu_dai->config_sysclk(rtd->cpu_dai, info, clk) < 0) - return -EINVAL; - - /* can codec work with this clock */ - return rtd->codec_dai->config_sysclk(rtd->codec_dai, info, clk); - } else { - /* cpu master */ - - /* check that codec can work with clock */ - if(rtd->codec_dai->config_sysclk(rtd->codec_dai, info, clk) < 0) - return -EINVAL; - - /* can CPU work with this clock */ - return rtd->cpu_dai->config_sysclk(rtd->cpu_dai, info, clk); - } - } - - - -Example 3 ---------- - -Codec that masters at 8k ... 48k @256 FS. Codec can also be slave and -doesn't care about FS. The codec has an internal PLL and dividers to generate -the necessary internal clocks (for 256FS). - -CPU can only be slave and doesn't care about FS. - -MCLK is a non controllable 13MHz clock from the CPU. - - - -------- --------- - | | <----mclk--- | | - | Codec |b <----------> | CPU | - | |l <----------> | | - | | | | - -------- --------- - -The codec driver has the following config_sysclock() - - /* valid PCM clock dividers * 2 */ - static int pcm_divs[] = {2, 6, 11, 4, 8, 12, 16}; - - static unsigned int config_vsysclk(struct snd_soc_codec_dai *dai, - struct snd_soc_clock_info *info, unsigned int clk) - { - int i, j, best_clk = info->fs * info->rate; - - /* can we run at this clk without the PLL ? */ - for (i = 0; i < ARRAY_SIZE(pcm_divs); i++) { - if ((best_clk >> 1) * pcm_divs[i] == clk) { - dai->pll_in = 0; - dai->clk_div = pcm_divs[i]; - dai->mclk = best_clk; - return dai->mclk; - } - } - - /* now check for PLL support */ - for (i = 0; i < ARRAY_SIZE(pll_div); i++) { - if (pll_div[i].pll_in == clk) { - for (j = 0; j < ARRAY_SIZE(pcm_divs); j++) { - if (pll_div[i].pll_out == pcm_divs[j] * (best_clk >> 1)) { - dai->pll_in = clk; - dai->pll_out = pll_div[i].pll_out; - dai->clk_div = pcm_divs[j]; - dai->mclk = best_clk; - return dai->mclk; - } - } - } - } - - /* this clk is not supported */ - return 0; - } - - -The CPU I2S DAI driver has the does not need a config_sysclk() as it can slave -at any FS. - - unsigned int config_sysclk(struct snd_soc_pcm_runtime *rtd, - struct snd_soc_clock_info *info) - { - /* codec has pll that generates mclk from 13MHz xtal */ - return rtd->codec_dai->config_sysclk(rtd->codec_dai, info, 13000000); - } |