From: Liu Ying <victor.liu@nxp.com>
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Heiko Stuebner <heiko@sntech.de>,
Elaine Zhang <zhangqing@rock-chips.com>,
Stephen Boyd <sboyd@kernel.org>,
linux-kernel@vger.kernel.org, linux-clk@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
linux-rockchip@lists.infradead.org,
Michael Turquette <mturquette@baylibre.com>,
NXP Linux Team <linux-imx@nxp.com>, Jacky Bai <ping.bai@nxp.com>
Subject: Re: [PATCH v1 2/3] clk: fractional-divider: Introduce NO_PRESCALER flag
Date: Mon, 19 Jul 2021 11:16:07 +0800 [thread overview]
Message-ID: <bfa0c16e88c0d445137290b2bef104e5fa74d78a.camel@nxp.com> (raw)
In-Reply-To: <YPGHbvaCv/x/JlgH@smile.fi.intel.com>
On Fri, 2021-07-16 at 16:19 +0300, Andy Shevchenko wrote:
> On Fri, Jul 16, 2021 at 10:43:57AM +0800, Liu Ying wrote:
> > On Thu, 2021-07-15 at 15:07 +0300, Andy Shevchenko wrote:
> > > The newly introduced flag, when set, makes the flow to skip
> > > the assumption that the caller will use an additional 2^scale
> > > prescaler to get the desired clock rate.
> >
> > Now, I start to be aware of the reason why the "left shifting" is
> > needed but still not 100% sure that details are all right. IIUC, you
> > are considering a potential HW prescaler here, while I thought the HW
> > model is just a fractional divider(M/N) and the driver is fully
> > agnostic to the potential HW prescaler.
>
> It's not AFAICS. Otherwise we will get saturated values which is much worse
> then shifted left frequency. Anyway, this driver appeared first for the hardware
> that has it for all users, so currently the assumption stays.
>
> ...
>
> > > scale = fls_long(*parent_rate / rate - 1);
> > > - if (scale > fd->nwidth)
> > > + if (scale > fd->nwidth && !(fd->flags & CLK_FRAC_DIVIDER_NO_PRESCALER))
> > > rate <<= scale - fd->nwidth;
> >
> > First of all, check the CLK_FRAC_DIVIDER_NO_PRESCALER flag for the
> > entire above snippet of code?
>
> OK.
>
> > Second and more important, it seems that it would be good to decouple
> > the prescaler knowledge from this fractional divider clk driver so as
> > to make it simple(Output rate = (m / n) * parent_rate). This way, the
> > CLK_FRAC_DIVIDER_NO_PRESCALER flag is not even needed at the first
> > place, which means rational_best_approximation() just _directly_
> > offer best_{numerator,denominator} for all cases.
>
> Feel free to submit a patch, just give a good test to avoid breakage of almost
> all users of this driver.
Maybe someone may do that. I just shared my thought that it sounds
like a good idea to decouple the prescaler knowledge from this
fractional divider clk driver.
>
> > Further more, is it
> > possilbe for rational_best_approximation() to make sure there is no
> > risk of overflow for best_{numerator,denominator}, since
> > max_{numerator,denominator} are already handed over to
> > rational_best_approximation()?
>
> How? It can not be satisfied for all possible inputs.
Just have rational_best_approximation() make sure
best_{numerator,denominator} are in the range of
[1, max_{numerator,denominator}] for all given_{numerator,denominator}.
At the same time, best_numerator/best_denominator should be as close
to given_numerator/given_denominator as possible. For this particular
fractional divider clk use case, clk_round_rate() can be called
multiple times until users find rounded rate is ok.
>
> > Overflowed/unreasonable
> > best_{numerator,denominator} don't sound like the "best" offered value.
>
> I don't follow here. If you got saturated values it means that your input is
> not convergent. In practice it means that we will supply quite a bad value to
> the caller.
Just like I mentioned above, if given_{numerator,denominator} are not
convergent, best_numerator/best_denominator should be as close
to given_numerator/given_denominator as possible and at the same time
best_{numerator,denominator} are in the range of
[1, max_{numerator,denominator}]. This way, caller may have chance to
propose convergent inputs.
Regards,
Liu Ying
>
> > If that's impossible, then audit best_{numerator,denominator} after
> > calling rational_best_approximation()?
>
> And? I do not understand what you will do if you get the values of m and n
> as m = 1, n = 2^nlim - 1.
>
> > Make sense?
>
> Not really. I probably miss your point, sorry.
>
> So, I will submit v2 with addressed first comment and LKP noticed compiler
> error.
>
next prev parent reply other threads:[~2021-07-19 3:18 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-15 12:07 [PATCH v1 1/3] clk: fractional-divider: Export approximation algo to the CCF users Andy Shevchenko
2021-07-15 12:07 ` [PATCH v1 2/3] clk: fractional-divider: Introduce NO_PRESCALER flag Andy Shevchenko
2021-07-16 2:43 ` Liu Ying
2021-07-16 13:19 ` Andy Shevchenko
2021-07-19 3:16 ` Liu Ying [this message]
2021-07-19 12:09 ` Andy Shevchenko
2021-07-22 6:02 ` Liu Ying
2021-07-22 7:24 ` Andy Shevchenko
2021-07-22 9:08 ` Liu Ying
2021-07-22 9:34 ` Andy Shevchenko
2021-07-22 9:59 ` Liu Ying
2021-07-15 12:07 ` [PATCH v1 3/3] clk: fractional-divider: Document the arithmetics used behind the code Andy Shevchenko
2021-07-15 15:38 ` [PATCH v1 1/3] clk: fractional-divider: Export approximation algo to the CCF users kernel test robot
2021-07-15 16:51 ` Andy Shevchenko
2021-07-15 17:58 ` Robin Murphy
2021-07-16 13:09 ` Andy Shevchenko
2021-07-15 22:41 ` kernel test robot
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=bfa0c16e88c0d445137290b2bef104e5fa74d78a.camel@nxp.com \
--to=victor.liu@nxp.com \
--cc=andriy.shevchenko@linux.intel.com \
--cc=heiko@sntech.de \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-clk@vger.kernel.org \
--cc=linux-imx@nxp.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-rockchip@lists.infradead.org \
--cc=mturquette@baylibre.com \
--cc=ping.bai@nxp.com \
--cc=sboyd@kernel.org \
--cc=zhangqing@rock-chips.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).