linux-kernel - Re: [PATCH RFC v9 01/20] clk: divider: Correct parent clk round rate if no bestdiv is normally found

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Date:	Sat, 21 Feb 2015 09:56:20 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Sascha Hauer <s.hauer@...gutronix.de>
Cc:	Liu Ying <Ying.Liu@...escale.com>, stefan.wahren@...e.com,
	devicetree@...r.kernel.org, linux@....linux.org.uk,
	mturquette@...aro.org, sboyd@...eaurora.org,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	a.hajda@...sung.com, Tomi Valkeinen <tomi.valkeinen@...com>,
	kernel@...gutronix.de, andy.yan@...k-chips.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH RFC v9 01/20] clk: divider: Correct parent clk round rate
 if no bestdiv is normally found

Hello,

On Thu, Feb 12, 2015 at 01:24:05PM +0100, Sascha Hauer wrote:
> On Thu, Feb 12, 2015 at 06:39:45PM +0800, Liu Ying wrote:
> > On Thu, Feb 12, 2015 at 10:33:56AM +0100, Sascha Hauer wrote:
> > > On Thu, Feb 12, 2015 at 02:01:24PM +0800, Liu Ying wrote:
> > > > If no best divider is normally found, we will try to use the maximum divider.
> > > > We should not set the parent clock rate to be 1Hz by force for being rounded.
> > > > Instead, we should take the maximum divider as a base and calculate a correct
> > > > parent clock rate for being rounded.
> > > 
> > > Please add an explanation why you think the current code is wrong and
> > > what this actually fixes, maybe an example?
> > 
> > The MIPI DSI panel's pixel clock rate is 26.4MHz and it's derived from PLL5 on
> > the MX6DL SabreSD board.
> > 
> > These are the clock tree summaries with or without the patch applied:
> > 1) With the patch applied:
> > pll5_bypass_src                       1            1    24000000          0 0
> >    pll5                               1            1   844800048          0 0
> >       pll5_bypass                     1            1   844800048          0 0
> >          pll5_video                   1            1   844800048          0 0
> >             pll5_post_div             1            1   211200012          0 0
> >                pll5_video_div           1            1   211200012        0 0
> >                   ipu1_di0_pre_sel           1            1   211200012   0 0
> >                      ipu1_di0_pre           1            1    26400002    0 0
> >                         ipu1_di0_sel           1            1    26400002 0 0
> >                            ipu1_di0           1            1    26400002  0 0
> > 
> > 2) Without the patch applied:
> > pll5_bypass_src                       1            1    24000000          0 0
> >    pll5                               1            1   648000000          0 0
> >       pll5_bypass                     1            1   648000000          0 0
> >          pll5_video                   1            1   648000000          0 0
> >             pll5_post_div             1            1   162000000          0 0
> >                pll5_video_div           1            1    40500000        0 0
> >                   ipu1_di0_pre_sel           1            1    40500000   0 0
> >                      ipu1_di0_pre           1            1    20250000    0 0
> >                         ipu1_di0_sel           1            1    20250000 0 0
> >                            ipu1_di0           1            1    20250000  0 0
> 
> This seems to be broken since:
> 
> | commit b11d282dbea27db1788893115dfca8a7856bf205
> | Author: Tomi Valkeinen <tomi.valkeinen@...com>
> | Date:   Thu Feb 13 12:03:59 2014 +0200
> | 
> |     clk: divider: fix rate calculation for fractional rates
> 
> This patch fixed a case when clk_set_rate(clk_round_rate(rate)) resulted
> in a lower frequency than clk_round_rate(rate) returned.
> 
> Since then the MULT_ROUND_UP in clk_divider_bestdiv() is inconsistent to
> the rest of the divider. Maybe this should be a simple rate * i now, but
> I'm unsure what side effects this has.
I think this is the right fix that I found, too, while fixing all
problems that your test code with the three dividers shows (at least the
subset of all problem I was able to identify).

> I think your patch only fixes the behaviour in your case by accident,
> it's not a correct fix for this issue.
Right.

I will follow up with a patch (and two more that fix different things).

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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