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

[Thierry Reding]

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@xxxxxxxxxx> 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

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