linux-kernel - Re: [PATCH v2] clk: Add fractional scale clock support

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Message-ID: <CAFHUOYw9rJe7+1kvz9gwxSZO=FucztfhdO_6vOz8kA7QsS9EyA@mail.gmail.com>
Date:	Wed, 20 Jul 2016 11:36:03 -0700
From:	Hoan Tran <hotran@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Michael Turquette <mturquette@...libre.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-clk <linux-clk@...r.kernel.org>, Duc Dang <dhdang@....com>,
	Loc Ho <lho@....com>
Subject: Re: [PATCH v2] clk: Add fractional scale clock support

On Fri, Jul 1, 2016 at 5:09 PM, Hoan Tran <hotran@....com> wrote:
>
> Hi Stephen,
>
> On Fri, Jul 1, 2016 at 11:38 AM, Stephen Boyd <sboyd@...eaurora.org> wrote:
> > Sorry I replied offlist before. Pressed the wrong key.
> >
> > On 06/30, Hoan Tran wrote:
> >> On Thu, Jun 30, 2016 at 1:23 PM, Stephen Boyd <sboyd@...eaurora.org> wrote:
> >> >
> >> > How is this different from clk-fractional-divider.c?
> >> >
> >>
> >> This is a driver which clock output is multiplied with a fixed fractional
> >> scale (denominator).
> >> A field inside a register is used to configure the multiplier.
> >>
> >> Example: With fractional scale is 1/8.
> >> Freq_out = Freq_parent * multiplier * (1/8)
> >>
> >> For fractional-divider, there are 2 fields of a register are used which
> >>  - A field for numerator
> >>  - A field for denominator
> >> Freq_out = Freq_parent * numerator / denominator
> >>
> >
> > Ok so the difference is that the denominator is a fixed value?
>
> The major difference is a fixed denominator.
> Another difference is:
> In case CLK_FRACTIONAL_SCALE_INVERTED=1, the freq_out is calculated as below
>
> Freq_out = Freq_parent * (fixed_denominator - multiplier) / fixed_denominator.
>
> > Perhaps that can be modeled as a clk-multiplier that is used as
> > the only parent of a fixed divider?
>
> Because of CLK_FRACTIONAL_SCALE_INVERTED flag, I don't know how to
> model as a clk-multiplier. And how to pass the fixed denominator into
> a clk-multiplier.
>
> > Or we can add a flag to the
> > clk-fractional-divider code to handle this minor difference.
>
> This driver is used 2 register fields for numerator, denominator and
> calculates both of them.
> Do you think I can integrate this fractional scale into it ?
>
> Thanks
> Hoan
>

Hi Stephen,

Do you have any comments ?

Thanks
Hoan


>
> >
> > --
> > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> > a Linux Foundation Collaborative Project