Re: [PATCH v5 3/3] clk: meson: add sub MMC clock controller driver

[Jianxin Pan <jianxin.pan@amlogic.com>]

Hi Jerome,

Thanks for the review, we really appreciate your time.

I'm very sorry maybe I don't catch all your meaning very well. 

Please see my comments below.

On 2018/10/29 3:16, Jerome Brunet wrote:
> On Thu, 2018-10-25 at 22:58 +0200, Martin Blumenstingl wrote:
>> Hi Jerome,
>>
>> On Thu, Oct 25, 2018 at 2:54 PM Jerome Brunet <jbrunet@baylibre.com> wrote:
>> [snip]
>>>>>> +static void clk_regmap_div_init(struct clk_hw *hw)
>>>>>> +{
>>>>>> + struct clk_regmap *clk = to_clk_regmap(hw);
>>>>>> + struct clk_regmap_div_data *div = clk_get_regmap_div_data(clk);
>>>>>> + unsigned int val;
>>>>>> + int ret;
>>>>>> +
>>>>>> + ret = regmap_read(clk->map, div->offset, &val);
>>>>>> + if (ret)
>>>>>> +         return;
>>>>>>
>>>>>> + val &= (clk_div_mask(div->width) << div->shift);
>>>>>> + if (!val)
>>>>>> +         regmap_update_bits(clk->map, div->offset,
>>>>>> +                            clk_div_mask(div->width) << div->shift,
>>>>>> +                            clk_div_mask(div->width));
>>>>>
>>>>> This is wrong for several reasons:
>>>>> * You should hard code the initial value in the driver.
>>>>> * If shift is not 0, I doubt this will give the expected result.
>>>>
>>>> The value 0x00 of divider means nand clock off then read/write nand register is forbidden.
>>>
>>> That is not entirely true, you can access the clock register or you'd be in a
>>> chicken and egg situation.
>>>
>>>> Should we set the initial value in nand driver, or in sub emmc clk driver?
>>>
>>> In the nand driver, which is the consumer of the clock. see my previous comments
>>> about it.
>>
>> an old version of this series had the code still in the NAND driver
>> (by writing to the registers directly instead of using the clk API).
>> this looks pretty much like a "sclk-div" to me (as I commented in v3
>> of this series: [0]):
>> - value 0 means disabled
>> - positive divider values
>> - (probably no duty control, but that's optional as far as I
>> understand sclk-div)
>> - uses max divider value when enabling the clock
>>
>> if switching to sclk-div works then we can get rid of some duplicate code
> 
> It is possible:
> There is a couple of things to note though:
> 
> * sclk does not 'uses max divider value when enabling the clock': Since this
> divider can gate, it needs to save the divider value when disabling, since the
> divider value is no longer stored in the register,
> On init, this cached value is  saved as it is. If the divider is initially
> disabled, we have to set the cached value to something that makes sense, in case
> the clock is enabled without a prior call to clk_set_rate().
>> So in sclk, the clock setting is not changed nor hard coded in init, and this is
> a very important difference.
>
I think It's ok for the latest sub mmc clock and nand driver now:
1. in mmc_clkc_register_clk_with_parent("div", ...) from mmc_clkc_probe():
   cached_div is set to div_max durning clk register，but is not set to div hw register.

2. In meson nand driver v6: 	https://lore.kernel.org/lkml/1541090542-19618-3-git-send-email-jianxin.pan@amlogic.com
1) In meson_nfc_clk_init() from probe:   get clock handle, then prepare_enable and set default rate.
   nfc->device_clk = devm_clk_get(nfc->dev, "device");
   ret = clk_prepare_enable(nfc->device_clk);			//Here div hw register changed from 0 -> cached_div.
   default_clk_rate = clk_round_rate(nfc->device_clk, 24000000);
   ret = clk_set_rate(nfc->device_clk, default_clk_rate);	//Then register and cached_div are both updated to the default 24M.
2) In meson_nfc_select_chip(), set the actual frequency
  ret = clk_set_rate(nfc->device_clk, meson_chip->clk_rate);	//Here register and cached_div are changed again.
3) if clk_disable() is called, set div hw register to zero, and cached_div  keep unchagned.
   if clk_disable() is called again,  cached_div is restored to div hw register.

When enabling the clock, divider register does not need to be div_max.  
Any value is OK except ZERO, the cached_div from init or set_rate is ok
>  
> * Even if sclk zero value means gated, it is still a zero based divider, while
> eMMC/Nand divider is one based. It this controller was to sclk, then something
> needs to be done for this.
Could I add another patch to this patchset for sclk to support CLK_DIVIDER_ONE_BASED ?
> 
> * Since sclk caches a value in its data, and there can multiple instance of eMMC
> /NAND clock controller, some care must be taken when registering the data.
OK, I will fix it and alloc mmc_clkc_div_data danymicly durning probe.
Thank you. 
> 
> Both the generic divider and sclk could work here ... it's up to you Jianxin.
> 
== Why use meson_sclk_div_ops instead of clk_regmap_divider_ops?
The default divider hw register vaule is 0 when system power on.
Then there is a WARNING in divider_recalc_rate() durning clk_hw_register():
[    0.918238] ffe05000.clock-controller#div: Zero divisor and CLK_DIVIDER_ALLOW_ZERO not set
[    0.925581] WARNING: CPU: 3 PID: 1 at drivers/clk/clk-divider.c:127 divider_recalc_rate+0x88/0x90
Then I still need to hard code the initual value to nand driver without CLK_DIVIDER_ALLOW_ZERO flags.
>>
>>
>> Regards
>> Martin
>>
>>
>> [0] https://patchwork.kernel.org/patch/10607157/#22238243
> 
> 
> .
>