Re: [PATCH 4/5] clk: sunxi-ng: a64: Add constraints on PLL-VIDEO0's n/m ratio

[Frank Oltmanns <frank@oltmanns.dev>]

On 2023-12-19 at 17:54:19 +0100, Jernej Škrabec <jernej.skrabec@gmail.com> wrote:
> Dne ponedeljek, 18. december 2023 ob 14:35:22 CET je Frank Oltmanns napisal(a):
>> The Allwinner A64 manual lists the following constraint for the
>> PLL-VIDEO0 clock: 8 <= N/M <= 25
>>
>> Use this constraint.
>>
>> Signed-off-by: Frank Oltmanns <frank@oltmanns.dev>
>> ---
>>  drivers/clk/sunxi-ng/ccu-sun50i-a64.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/clk/sunxi-ng/ccu-sun50i-a64.c b/drivers/clk/sunxi-ng/ccu-sun50i-a64.c
>> index c034ac027d1c..75d839da446c 100644
>> --- a/drivers/clk/sunxi-ng/ccu-sun50i-a64.c
>> +++ b/drivers/clk/sunxi-ng/ccu-sun50i-a64.c
>> @@ -68,7 +68,8 @@ static SUNXI_CCU_NM_WITH_SDM_GATE_LOCK(pll_audio_base_clk, "pll-audio-base",
>>  				       BIT(28),	/* lock */
>>  				       CLK_SET_RATE_UNGATE);
>>
>> -static SUNXI_CCU_NM_WITH_FRAC_GATE_LOCK_MIN_MAX_CLOSEST(pll_video0_clk, "pll-video0",
>> +static SUNXI_CCU_NM_WITH_FRAC_GATE_LOCK_MIN_MAX_FEAT_NM_RATIO(pll_video0_clk,
>> +						"pll-video0",
>>  						"osc24M", 0x010,
>>  						192000000,	/* Minimum rate */
>>  						1008000000,	/* Maximum rate */

I just realized that adding the whole ratio limits for ccu_nm is
superfluous as you could just as well express them in for of a minimum
and maximum range:
Since 8 <= N/M <= 25 and parent_rate = 24 MHz, therefore
  192 MHz <= rate <= 600 MHz.

These absolute limits are also listed in Allwinner's A64 manual.

BUT, here the upper limit was raised to 1008 MHz:
5de39acaf34604bd04834f092479cf4dcc946dd "clk: sunxi-ng: a64: Add max.
rate constraint to video PLL"

With this upper limit the ratio limitation is effectively:
8 <= N/M <= 42

Icenowy Zheng (added to CC) had the reasonable explanation that this was
used in the BSP kernel, so we should probably stick to that and ditch
the two PLL-VIDEO0 related patches. What are your thoughts on that?

>> @@ -80,7 +81,10 @@ static SUNXI_CCU_NM_WITH_FRAC_GATE_LOCK_MIN_MAX_CLOSEST(pll_video0_clk, "pll-vid
>>  						297000000,	/* frac rate 1 */
>>  						BIT(31),	/* gate */
>>  						BIT(28),	/* lock */
>> -						CLK_SET_RATE_UNGATE);
>> +						CLK_SET_RATE_UNGATE,
>> +						CCU_FEATURE_FRACTIONAL |
>> +						CCU_FEATURE_CLOSEST_RATE,
>
> Above flags are unrelated change, put them in new patch if needed.
>
> Best regards,
> Jernej
>
>> +						8, 25);		/* min/max nm ratio */
>>
>>  static SUNXI_CCU_NM_WITH_FRAC_GATE_LOCK(pll_ve_clk, "pll-ve",
>>  					"osc24M", 0x018,
>>
>>