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

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Date:   Tue, 30 Oct 2018 21:41:31 +0800
From:   Jianxin Pan <jianxin.pan@...ogic.com>
To:     Jerome Brunet <jbrunet@...libre.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
CC:     Neil Armstrong <narmstrong@...libre.com>, <yixun.lan@...ogic.com>,
        <khilman@...libre.com>, <carlo@...one.org>,
        <mturquette@...libre.com>, <sboyd@...nel.org>, <robh@...nel.org>,
        <miquel.raynal@...tlin.com>, <boris.brezillon@...tlin.com>,
        <liang.yang@...ogic.com>, <jian.hu@...ogic.com>,
        <qiufang.dai@...ogic.com>, <hanjie.lin@...ogic.com>,
        <victor.wan@...ogic.com>, <linux-clk@...r.kernel.org>,
        <linux-amlogic@...ts.infradead.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3/3] clk: meson: add sub MMC clock controller driver

Hi Jerome,

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@...libre.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.
>  
> * 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.
> 
> * 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.
> 
> Both the generic divider and sclk could work here ... it's up to you Jianxin.
Thank you for the detailed explanation.
I will use sclk here.

With generic divider, there is a WARNING in divider_recalc_rate() durning clk_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, or add CLK_DIVIDER_ALLOW_ZERO flags.
> 
>>
>>
>> Regards
>> Martin
>>
>>
>> [0] https://patchwork.kernel.org/patch/10607157/#22238243
> 
> 
> .
>