Re: [PATCH] sound/soc/adi/axi-spdif.c: Support programmable master clock

From: Lars-Peter Clausen
Date: Thu Dec 04 2014 - 07:45:52 EST

On 12/04/2014 07:52 AM, Mike Looijmans wrote:
If the master clock supports programmable rates, program it to generate
the desired frequency. Only apply constraints when the clock is fixed.
This allows proper clock generation for both 44100 and 48000 Hz based
sampling rates if the platform supports it.

The clock frequency must be set before enabling it. Enabling the clock
was done in "startup", but that occurs before "hw_params" where the rate
is known. Move the clock start to the hw_params routine, and keep track
of whether the clock has been started, because shutdown may be called
without having called hw_params first.

Usually that shouldn't be a problem. If your clock chip requires it to be disabled in order to be reprogrammed than the CLK_SET_RATE_GATE flag should be set. This will tell the core to disable the clock before changing it.

static const struct snd_soc_dai_ops axi_spdif_dai_ops = {
@@ -216,14 +227,17 @@ static int axi_spdif_probe(struct platform_device *pdev)
spdif->dma_data.addr_width = 4;
spdif->dma_data.maxburst = 1;

- spdif->ratnum.num = clk_get_rate(spdif->clk_ref) / 128;
- spdif->ratnum.den_step = 1;
- spdif->ratnum.den_min = 1;
- spdif->ratnum.den_max = 64;
- spdif->rate_constraints.rats = &spdif->ratnum;
- spdif->rate_constraints.nrats = 1;
+ /* Determine if the clock rate is fixed. If it cannot change frequency,
+ * it returns an error here. */
+ if (clk_round_rate(spdif->clk_ref, 128 * 44100) < 0) {

I don't think this works. For a fixed clock clk_round_rate() will return the fixed rate rather than an error. I tried the patch and even though I have a fixed clock the constraints are no longer set.

There is unfortunately no good way to enumerate which frequencies are supported by a clock other than just calling round_rate for all possible rates.

I think the best way to implement this for now is to try e.g. 32000 * 128, 44100 * 128, 48000 * 128 and then check if clk_round_rate returns the expected rate and if it does set up a rate constraint for that rate.
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