Re: [RFC 1/6] video: hdmi: add helper function for N and CTS

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Tue, Jan 19, 2016 at 02:40:31PM +0100, Arnaud Pouliquen wrote:
> From: Moise Gergaud <moise.gergaud@st.com>
> 
> Add helper function to compute HDMI CTS and N parameters
> Implementation is based on HDMI 1.4b specification.

You should note that these CTS/N parameters are for where the audio and
video clocks are coherent: iow, they are derived from the same source.
If they can drift (because they're generated from different sources)
then a hardware implementation which automatically calculates CTS is
needed.

However, some of the values given in the HDMI specs for CTS are actually
two values, because a single value does not specify the audio clock even
with the N values given.  For these, the values returned will not give
an accurate clock - this should also be noted.

Next, does "pixel clock" make sense when you have pixel repetition?
Would calling this "tdms_clock" be more accurate to the specification
and less mis-leading to users?

> Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
> ---
>  drivers/video/hdmi.c | 146 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/hdmi.h |  11 ++++
>  2 files changed, 157 insertions(+)
> 
> diff --git a/drivers/video/hdmi.c b/drivers/video/hdmi.c
> index 1626892..8af33c1 100644
> --- a/drivers/video/hdmi.c
> +++ b/drivers/video/hdmi.c
> @@ -1242,3 +1242,149 @@ int hdmi_infoframe_unpack(union hdmi_infoframe *frame, void *buffer)
>  	return ret;
>  }
>  EXPORT_SYMBOL(hdmi_infoframe_unpack);
> +
> +/**
> + * audio clock regeneration (acr) parameters
> + * N and CTS computation are based on HDMI specification 1.4b
> + */
> +enum audio_rate {
> +	HDMI_AUDIO_N_CTS_32KHZ,
> +	HDMI_AUDIO_N_CTS_44_1KHZ,
> +	HDMI_AUDIO_N_CTS_48KHZ,
> +};
> +
> +struct hdmi_audio_acr {
> +	unsigned long pixel_clk;
> +	struct hdmi_audio_n_cts n_cts;
> +};
> +
> +static const struct hdmi_audio_acr hdmi_audio_standard_acr[3][12] = {
> +	{ /*32 kHz*/
> +		{  25175, {  4576,  28125 } }, /* 25,20/1.001  MHz */
> +		{  25200, {  4096,  25200 } }, /* 25.20        MHz */
> +		{  27000, {  4096,  27000 } }, /* 27.00        MHz */
> +		{  27027, {  4096,  27027 } }, /* 27.00*1.001  MHz */
> +		{  54000, {  4096,  54000 } }, /* 54.00        MHz */
> +		{  54054, {  4096,  54054 } }, /* 54.00*1.001  MHz */
> +		{  74176, { 11648, 310938 } }, /* 74.25/1.001  MHz */
> +		{  74250, {  4096,  74250 } }, /* 74.25        MHz */
> +		{ 148352, { 11648, 421875 } }, /* 148.50/1.001 MHz */
> +		{ 148500, {  4096, 148500 } }, /* 148.50       MHz */
> +		{ 296703, {  5824, 421875 } }, /* 297/1.001    MHz */
> +		{ 297000, {  3072, 222750 } }, /* 297          MHz */
> +	},
> +	{ /*44.1 kHz, 88.2 kHz  176.4 kHz*/
> +		{  25175, {  7007,  31250 } }, /* 25,20/1.001  MHz */
> +		{  25200, {  6272,  28000 } }, /* 25.20        MHz */
> +		{  27000, {  6272,  30000 } }, /* 27.00        MHz */
> +		{  27027, {  6272,  30030 } }, /* 27.00*1.001  MHz */
> +		{  54000, {  6272,  60000 } }, /* 54.00        MHz */
> +		{  54054, {  6272,  60060 } }, /* 54.00*1.001  MHz */
> +		{  74176, { 17836, 234375 } }, /* 74.25/1.001  MHz */
> +		{  74250, {  6272,  82500 } }, /* 74.25        MHz */
> +		{ 148352, {  8918, 234375 } }, /* 148.50/1.001 MHz */
> +		{ 148500, {  6272, 165000 } }, /* 148.50       MHz */
> +		{ 296703, {  4459, 234375 } }, /* 297/1.001    MHz */
> +		{ 297000, {  4704, 247500 } }, /* 297          MHz */
> +	},
> +	{ /*48 kHz, 96 kHz  192 kHz*/
> +		{  25175, {  6864,  28125 } }, /* 25,20/1.001  MHz */
> +		{  25200, {  6144,  25200 } }, /* 25.20        MHz */
> +		{  27000, {  6144,  27000 } }, /* 27.00        MHz */
> +		{  27027, {  6144,  27027 } }, /* 27.00*1.001  MHz */
> +		{  54000, {  6144,  54000 } }, /* 54.00        MHz */
> +		{  54054, {  6144,  54054 } }, /* 54.00*1.001  MHz */
> +		{  74176, { 11648, 140625 } }, /* 74.25/1.001  MHz */
> +		{  74250, {  6144,  74250 } }, /* 74.25        MHz */
> +		{ 148352, {  5824, 140625 } }, /* 148.50/1.001 MHz */
> +		{ 148500, {  6144, 148500 } }, /* 148.50       MHz */
> +		{ 296703, {  5824, 281250 } }, /* 297/1.001    MHz */
> +		{ 297000, {  5120, 247500 } }, /* 297          MHz */
> +	}
> +};
> +
> +/**
> + * hdmi_compute_n_cts() - compute N and CTS parameters
> + * @audio_fs: audio frame clock frequency in kHz

This is wrong.  You're expecting values such as 32000 here.  32000kHz
would be 32MHz, which is way too high for an audio frame clock.  I
think you mean Hz.

> + * @pixel_clk: pixel clock frequency in kHz

This is correct though.

> + * @n_cts: N and CTS parameter returned to user
> + *
> + * Values computed are based on table described in HDMI specification 1.4b
> + *
> + * Returns 0 on success or a negative error code on failure.
> + */
> +int hdmi_audio_compute_n_cts(unsigned int audio_fs, unsigned long pixel_clk,
> +			     struct hdmi_audio_n_cts *n_cts)
> +{
> +	int audio_freq_id, i;
> +	int ratio = 1;
> +	const struct hdmi_audio_acr  *acr_table;
> +	const struct hdmi_audio_n_cts *predef_n_cts = NULL;
> +
> +	switch (audio_fs) {
> +	case 32000:
> +		audio_freq_id = HDMI_AUDIO_N_CTS_32KHZ;
> +		n_cts->n = 4096;
> +		break;
> +
> +	case 44100:
> +		audio_freq_id = HDMI_AUDIO_N_CTS_44_1KHZ;
> +		n_cts->n = 6272;
> +		break;
> +
> +	case 48000:
> +		audio_freq_id = HDMI_AUDIO_N_CTS_48KHZ;
> +		n_cts->n = 6144;
> +		break;
> +
> +	case 88200:
> +		audio_freq_id = HDMI_AUDIO_N_CTS_44_1KHZ;
> +		ratio = 2;
> +		n_cts->n = 6272 * 2;
> +		break;
> +
> +	case 96000:
> +		audio_freq_id = HDMI_AUDIO_N_CTS_48KHZ;
> +		ratio = 2;
> +		n_cts->n = 6144 * 2;
> +		break;
> +
> +	case 176400:
> +		audio_freq_id = HDMI_AUDIO_N_CTS_44_1KHZ;
> +		ratio = 4;
> +		n_cts->n = 6272 * 4;
> +		break;
> +
> +	case 192000:
> +		audio_freq_id = HDMI_AUDIO_N_CTS_48KHZ;
> +		ratio = 4;
> +		n_cts->n = 6144 * 4;
> +		break;
> +
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	acr_table = hdmi_audio_standard_acr[audio_freq_id];
> +	for (i = 0; i < ARRAY_SIZE(hdmi_audio_standard_acr[0]); i++) {
> +		if (pixel_clk == acr_table[i].pixel_clk) {
> +			predef_n_cts = &acr_table[i].n_cts;
> +			break;
> +		}
> +	}
> +
> +	if (!predef_n_cts) {
> +		/*
> +		 * predefined frequency not found. Compute CTS using formula:
> +		 * CTS = (Ftdms_clk * N) / (128* audio_fs)
> +		 */
> +		n_cts->cts =  pixel_clk * n_cts->n / (128 * audio_fs);

This looks like it could overflow.  192kHz audio clock, 297MHz tdms
clock -> (297000 * 6144 * 4) = 7299072000.  32 bit unsigned math
overflows beyond 4294967295.

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