[alsa-devel] [RFC 1/6] video: hdmi: add helper function for N and CTS
Russell King - ARM Linux
linux at arm.linux.org.uk
Tue Jan 19 17:54:30 CET 2016
On Tue, Jan 19, 2016 at 02:40:31PM +0100, Arnaud Pouliquen wrote:
> From: Moise Gergaud <moise.gergaud at 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 at 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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