Date:   Mon, 24 Oct 2022 17:29:09 +0100
From:   Aidan MacDonald <aidanmacdonald.0x0@...il.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     mturquette@...libre.com, sboyd@...nel.org, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, zhouyu@...yeetech.com,
        linux-mips@...r.kernel.org, linux-clk@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 5/5] clk: ingenic: Add X1000 audio clocks


Paul Cercueil <paul@...pouillou.net> writes:

> Hi Aidan,
>
> Le dim. 23 oct. 2022 à 15:56:53 +0100, Aidan MacDonald
> <aidanmacdonald.0x0@...il.com> a écrit :
>> The X1000's CGU supplies the I2S system clock to the AIC module
>> and ultimately the audio codec, represented by the "i2s" clock.
>> It is a simple mux which can either pass through EXCLK or a PLL
>> multiplied by a fractional divider (the "i2s_pll" clock).
>> The AIC contains a separate 1/N divider controlled by the I2S
>> driver, which generates the bit clock from the system clock.
>> The frame clock is always fixed to 1/64th of the bit clock.
>> Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@...il.com>
>> ---
>>  drivers/clk/ingenic/x1000-cgu.c | 69 +++++++++++++++++++++++++++++++++
>>  1 file changed, 69 insertions(+)
>> diff --git a/drivers/clk/ingenic/x1000-cgu.c
>> b/drivers/clk/ingenic/x1000-cgu.c
>> index b2ce3fb83f54..341276e5e1ef 100644
>> --- a/drivers/clk/ingenic/x1000-cgu.c
>> +++ b/drivers/clk/ingenic/x1000-cgu.c
>> @@ -8,6 +8,7 @@
>>  #include <linux/delay.h>
>>  #include <linux/io.h>
>>  #include <linux/of.h>
>> +#include <linux/rational.h>
>>  #include <dt-bindings/clock/ingenic,x1000-cgu.h>
>> @@ -168,6 +169,37 @@ static const struct clk_ops x1000_otg_phy_ops = {
>>  	.is_enabled	= x1000_usb_phy_is_enabled,
>>  };
>> +static void
>> +x1000_i2spll_calc_m_n_od(const struct ingenic_cgu_pll_info *pll_info,
>> +			 unsigned long rate, unsigned long parent_rate,
>> +			 unsigned int *pm, unsigned int *pn, unsigned int *pod)
>> +{
>> +	const unsigned long m_max = GENMASK(pll_info->m_bits - 1, 0);
>> +	const unsigned long n_max = GENMASK(pll_info->n_bits - 1, 0);
>> +	unsigned long m, n;
>> +
>> +	rational_best_approximation(rate, parent_rate, m_max, n_max, &m, &n);
>> +
>> +	/* n should not be less than 2*m */
>> +	if (n < 2 * m)
>> +		n = 2 * m;
>> +
>> +	*pm = m;
>> +	*pn = n;
>> +	*pod = 1;
>> +}
>> +
>> +static void
>> +x1000_i2spll_set_rate_hook(const struct ingenic_cgu_pll_info *pll_info,
>> +			   unsigned long rate, unsigned long parent_rate)
>> +{
>> +	/*
>> +	 * For some reason, the I2S divider doesn't work properly after
>> +	 * updating I2SCDR unless I2SCDR1 is read & written back.
>> +	 */
>> +	writel(readl(cgu->base + CGU_REG_I2SCDR1), cgu->base + CGU_REG_I2SCDR1);
>
> Not fond of the nesting here, just use a variable.
>
> Besides... According to the documentation, bits 31 and 30 of this register are
> misconnected: writing to bit 31 will be reflected in bit 30, and vice-versa. So
> this would work only if the bits 30 and 31 have the same value.

>From my tests it seems that reads are swapped w.r.t. the documented
bit positions, but not writes. I'm assuming this is the case because
when I write to the register with bit 30 (I2S_DEN) set, I can change
I2SDIV_D freely, but if bit 30 is 0, then I2SDIV_D is replaced by
I2SDIV_N/2 like the manual suggests.

> And worse than that, where do you actually set the register's value? Because
> bits 30/31, if cleared, will automatically compute the M/N values to the I2SCDR
> fields, overriding what the driver's .set_rate() callback is doing.

I don't initialize the register, but I2S_NEN (bit 31 of I2SCDR1) doesn't
cause I2SCDR to be automatically updated like I see with I2SCDR1 when
I2S_DEN = 0. It seems setting I2S_NEN to 1 has some other effect on the
clock output, but I don't have an oscilloscope so I'm not sure exactly
what it is. I'm limited to testing by ear.

For example, with f_in = 1008 MHz, M = 7, N = 2500, the output should
nominally be 2.8224 MHz (= 64 * 44.1 KHz). Now that works just fine
for playing 44.1 KHz audio if I set I2S_NEN to 0, but setting it to 1,
I need to increase M to 16 to get things to sound right. I've tested
a couple other frequencies but there isn't an obvious pattern to what
increase in M is needed (it always seems to need an increase).

I don't really care to play around with it because everything works
fine if I keep I2S_NEN = I2S_DEN = 0. I've tested that configuration
from 8 KHz to 192 KHz at typical audio frequencies and randomly chosen
non-standard frequencies, and haven't experienced any issues.

> Either we want that, and in that case the I2S clock should be a custom clock
> (since it wouldn't need to compute or write M/N), or we don't, and in this case
> bits 30/31 of this register should be set.

My experience suggests we should set I2S_NEN and I2S_DEN to 0 and
calculate output frequency as F_out = F_in * I2SDIV_M / I2SDIV_N.

Reading I2SCDR1 and writing the same value back is something I took
from the Ingenic BSP kernels. (The manual also suggests doing this.)
It worked fine for me so I didn't bother to invesigate it further.

The important part actually seems to be refreshing I2SDIV_D to a
"reasonable" value, which happens automatically when I2S_DEN = 0.
But the automatic calculation only happens when writing the register.

Writing 0 to I2SCDR1 solves both problems: the register is properly
initialized and I2SDIV_D gets updated automatically. That seems to
be the best solution.

>> +}
>> +
>>  static const s8 pll_od_encoding[8] = {itt
>>  	0x0, 0x1, -1, 0x2, -1, -1, -1, 0x3,
>>  };
>> @@ -319,6 +351,37 @@ static const struct ingenic_cgu_clk_info
>> x1000_cgu_clocks[] = {
>>  		.gate = { CGU_REG_CLKGR, 25 },
>>  	},
>> +	[X1000_CLK_I2SPLLMUX] = {
>> +		"i2s_pll_mux", CGU_CLK_MUX,
>> +		.parents = { X1000_CLK_SCLKA, X1000_CLK_MPLL, -1, -1 },
>
> If you have only 1 bit you can only have two parents, so you can remove the
> -1s.
>

Thanks, will do.

Regards,
Aidan

>> +		.mux = { CGU_REG_I2SCDR, 31, 1 },
>> +	},
>> +
>> +	[X1000_CLK_I2SPLL] = {
>> +		"i2s_pll", CGU_CLK_PLL,
>> +		.parents = { X1000_CLK_I2SPLLMUX, -1, -1, -1 },
>
> .parents = { X1000_CLK_I2SPLLMUX, },
>
>> +		.pll = {
>> +			.reg = CGU_REG_I2SCDR,
>> +			.rate_multiplier = 1,
>> +			.m_shift = 13,
>> +			.m_bits = 9,
>> +			.n_shift = 0,
>> +			.n_bits = 13,
>> +			.calc_m_n_od = x1000_i2spll_calc_m_n_od,
>> +			.set_rate_hook = x1000_i2spll_set_rate_hook,
>> +		},
>> +	},
>> +
>> +	[X1000_CLK_I2S] = {
>> +		"i2s", CGU_CLK_MUX,
>> +		.parents = { X1000_CLK_EXCLK, -1, -1, X1000_CLK_I2SPLL },
>> +		/*
>> +		 * NOTE: the mux is at bit 30; bit 29 enables the M/N divider.
>> +		 * Therefore, the divider is disabled when EXCLK is selected.
>> +		 */
>> +		.mux = { CGU_REG_I2SCDR, 29, 2 },
>> +	},
>> +
>>  	[X1000_CLK_LCD] = {
>>  		"lcd", CGU_CLK_MUX | CGU_CLK_DIV | CGU_CLK_GATE,
>>  		.parents = { X1000_CLK_SCLKA, X1000_CLK_MPLL },
>> @@ -426,6 +489,12 @@ static const struct ingenic_cgu_clk_info
>> x1000_cgu_clocks[] = {
>>  		.gate = { CGU_REG_CLKGR, 9 },
>>  	},
>> +	[X1000_CLK_AIC] = {
>> +		"aic", CGU_CLK_GATE,
>> +		.parents = { X1000_CLK_EXCLK, -1, -1, -1 },
>
> .parents = { X1000_CLK_EXCLK, },
>
> Cheers,
> -Paul
>
>> +		.gate = { CGU_REG_CLKGR, 11 },
>> +	},
>> +
>>  	[X1000_CLK_UART0] = {
>>  		"uart0", CGU_CLK_GATE,
>>  		.parents = { X1000_CLK_EXCLK, -1, -1, -1 },
>> --
>> 2.38.1
>>

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