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

[Martin Blumenstingl <martin.blumenstingl@googlemail.com>]

Hi Jerome,

many thanks for the whole explanation!

On Sun, Oct 28, 2018 at 8:16 PM Jerome Brunet <jbrunet@baylibre.com> 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.
>
> * 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.
to give even more options:
the generic divider will (probably) get a CLK_DIVIDER_ZERO_GATE flag
in the next development cycle, see [0] (this may require a small
change to clk-regmap on top)


Regards
Martin


[0] https://patchwork.kernel.org/patch/10650797/