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

[Jianxin Pan]

On 2018/11/5 17:46, jbrunet@xxxxxxxxxxxx wrote:
> On Sun, 2018-11-04 at 02:01 +0800, Jianxin Pan wrote:
>> 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@xxxxxxxxxxxx>
>>>> 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 registerïbut 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@xxxxxxxxxxx
>> 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.
> 
> You don't need to do all this in your NAND driver: enable - round - set_rate -
> disable is a waste of time. 
> 
> Directly calling set_rate(24000000), with the clock still off, will have the
> same result. Then if your HW needs this clock to be ON to access registers
> (like you told us) you should probably turn it on.
I'm sorry I didn't describe it very clearly in last mail. 
The steps in nand v6 probe are: enable -> round(24M) -> set_rate(24M)ï then this clock is always on.
And it's disabled only in the nand remove() callback. 
I will remove round(24M) in next version .
Thank you.
> 
>>
>> 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 ?
> 
> Yes, you should otherwise the calculation are just wrong for your clock.
OK. Thank you.
> 
>>> * 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
>>>
>>> .
>>>
>>
>>
> 
> 
> .
>