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

From: [email-masked] (Liu Ying)
Date: Thu, 12 Feb 2015 22:06:27 +0800

On Thu, Feb 12, 2015 at 02:41:31PM +0100, Sascha Hauer wrote:
> On Thu, Feb 12, 2015 at 12:56:46PM +0000, Russell King - ARM Linux wrote:
> > 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 <[email-masked]>
> > > | 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 your patch only fixes the behaviour in your case by accident,
> > > it's not a correct fix for this issue.
> > 
> > Well, it's defined that:
> > 
> > 	new_rate = clk_round_rate(clk, rate);
> > 
> > returns the rate which you would get if you did:
> > 
> > 	clk_set_rate(clk, rate);
> > 	new_rate = clk_get_rate(clk);
> > 
> > The reasoning here is that clk_round_rate() gives you a way to query what
> > rate you would get if you were to ask for the rate to be set, without
> > effecting a change in the hardware.
> > 
> > The idea that you should call clk_round_rate() first before clk_set_rate()
> > and pass the returned rounded rate into clk_set_rate() is really idiotic
> > given that.  Please don't do it, and please remove code which does it, and
> > in review comment on it.  Thanks.
> 
> Tomis patch is based on the assumption that clk_set_rate(clk_round_rate(rate))
> is equal to clk_round_rate(rate). So when this assumption is wrong then
> it should simply be reverted.
> So Liu, could you test if reverting Tomis patch fixes your problem?

Yes, I'll test tomorrow when I have access to my board.

Regards,
Liu Ying

> 
> Sascha
> 
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