linux-kernel - Re: [PATCH v4 08/20] clk: tegra: pll: Add logic for handling SDM data

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Date:	Wed, 6 May 2015 12:16:33 -0400
From:	Rhyland Klein <rklein@...dia.com>
To:	Thierry Reding <thierry.reding@...il.com>
CC:	Benson Leung <bleung@...omium.org>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	Mike Turquette <mturquette@...aro.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Alexandre Courbot <gnurou@...il.com>,
	<linux-clk@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 08/20] clk: tegra: pll: Add logic for handling SDM
 data

On 5/6/2015 9:57 AM, Thierry Reding wrote:
> * PGP Signed by an unknown key
> 
> On Tue, May 05, 2015 at 03:16:08PM -0400, Rhyland Klein wrote:
>> On 5/4/2015 7:01 PM, Benson Leung wrote:
>>> On Mon, May 4, 2015 at 9:37 AM, Rhyland Klein <rklein@...dia.com> wrote:
>>>> @@ -495,6 +505,28 @@ static int _calc_rate(struct clk_hw *hw, struct tegra_clk_pll_freq_table *cfg,
>>>>         return 0;
>>>>  }
>>>>
>>>> +static void clk_pll_set_sdm_data(struct clk_hw *hw,
>>>> +                                struct tegra_clk_pll_freq_table *cfg)
>>>> +{
>>>> +       struct tegra_clk_pll *pll = to_clk_pll(hw);
>>>> +       u32 val;
>>>> +
>>>> +       if (!pll->params->sdm_din_reg)
>>>> +               return;
>>>> +
>>>> +       if (cfg->sdm_data) {
>>>> +               val = pll_readl_sdm_din(pll) & (~sdm_din_mask(pll));
>>>> +               val |= sdin_data_to_din(cfg->sdm_data) & sdm_din_mask(pll);
>>>> +               pll_writel_sdm_din(val, pll);
>>>> +       }
>>>> +
>>>> +       val = pll_readl_sdm_ctrl(pll);
>>>> +       if (!cfg->sdm_data != !(val & pll->params->sdm_ctrl_en_mask)) {
>>>
>>> You can use sdm_en_mask(pll) here.
>>>
>>> I'm not super clear about what you're trying to accomplish here with
>>> !cfg->sdm_data != !(val & mask).
>>> Are you just checking if the masked value is different from sdm_data,
>>> but accounting for the integer widths being different (u16 vs u32)?
>>
>> So I got clarification from the downstream author to be sure, and this
>> is the answer to what this is checking:
>>
>> (<Configuration has non zero  SDM_DATA>  AND <sdm control is disabled>)
>> OR
>> (<Configuration has zero  SDM_DATA>  AND <sdm control is enabled>)
>>
>> So the check is correct, just a complicated way of expressing it.
> 
> Can it be rewritten to be less complicated? I hate it when I have to
> look at code for several seconds and still not understand what it's
> doing. Why not something that is closer to the pseudo code you gave:
> 
> 	bool enabled = (val & pll->params->sdm_ctrl_en_mask) != 0;
> 
> 	if ((enabled && cfg->sdm_data == 0) || (!enabled && cfg->sdm_data != 0))
> 
> ? Also I think this could use some could comments explaining what's
> going on. Perhaps this could be simplified even further:
> 
> 	if (cfg->sdm_data == 0 && enabled)
> 		val &= ~pll->params->sdm_ctrl_en_mask;
> 
> 	if (cfg->sdm_data != 0 && !enabled)
> 		val |= pll->params->sdm_ctrl_en_mask;
> 
> 	pll_writel_sdm_ctrl(val, pll);
> 
> Now that I have a /much/ easier time reading and understanding. That may
> not even require comments because it's pretty plain what's going on. But
> there may be some advantage in adding comments about SDM in general. The
> comment could be something like what you have in the commit message, so
> that people don't have to go find the commit that added the code to find
> out what this is doing.
> 
> Thierry

I agree that the original statement was confusing. I'll replace it with
a simpler version, and comments never hurt :)

-rhyland

> 
> * Unknown Key
> * 0x7F3EB3A1
> 


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