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

From: Sascha Hauer <s.hauer@pengutronix.de>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Liu Ying <Ying.Liu@freescale.com>,
	dri-devel@lists.freedesktop.org, stefan.wahren@i2se.com,
	devicetree@vger.kernel.org, kernel@pengutronix.de,
	sboyd@codeaurora.org, linux-kernel@vger.kernel.org,
	a.hajda@samsung.com, andy.yan@rock-chips.com,
	mturquette@linaro.org, linux-arm-kernel@lists.infradead.org,
	Tomi Valkeinen <tomi.valkeinen@ti.com>
Subject: Re: [PATCH RFC v9 01/20] clk: divider: Correct parent clk round rate if no bestdiv is normally found
Date: Thu, 12 Feb 2015 14:41:31 +0100	[thread overview]
Message-ID: <20150212134131.GX12209@pengutronix.de> (raw)
In-Reply-To: <20150212125646.GT8656@n2100.arm.linux.org.uk>

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 <tomi.valkeinen@ti.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 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?

Sascha

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