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Message-ID: <20150212134131.GX12209@pengutronix.de> Date: Thu, 12 Feb 2015 14:41:31 +0100 From: Sascha Hauer <s.hauer@...gutronix.de> To: Russell King - ARM Linux <linux@....linux.org.uk> Cc: Liu Ying <Ying.Liu@...escale.com>, dri-devel@...ts.freedesktop.org, stefan.wahren@...e.com, devicetree@...r.kernel.org, kernel@...gutronix.de, sboyd@...eaurora.org, linux-kernel@...r.kernel.org, a.hajda@...sung.com, andy.yan@...k-chips.com, mturquette@...aro.org, linux-arm-kernel@...ts.infradead.org, Tomi Valkeinen <tomi.valkeinen@...com> Subject: Re: [PATCH RFC v9 01/20] clk: divider: Correct parent clk round rate if no bestdiv is normally found 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@...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 -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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