linux-kernel - Re: [RFC] clk: imx: Allow re-parenting by default on set rate

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Message-ID: <20190313104149.blhwqgn53lsgow6z@pengutronix.de>
Date:   Wed, 13 Mar 2019 11:41:49 +0100
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Abel Vesa <abel.vesa@....com>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Sascha Hauer <kernel@...gutronix.de>,
        Lucas Stach <l.stach@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>,
        Aisheng Dong <aisheng.dong@....com>,
        Jacky Bai <ping.bai@....com>,
        Anson Huang <anson.huang@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>,
        Fabio Estevam <fabio.estevam@....com>,
        "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC] clk: imx: Allow re-parenting by default on set rate

On Mon, Mar 11, 2019 at 10:41:40AM +0000, Abel Vesa wrote:
> On 19-03-11 11:28:25, Sascha Hauer wrote:
> > Hi Abel,
> > 
> > On Thu, Mar 07, 2019 at 09:20:37AM +0000, Abel Vesa wrote:
> > > By default, the muxes should re-parent on set_rate.
> > > This would allow the drivers to control only the leaf clock node,
> > > leaving the rest to the clock driver, that way simplifying the
> > > clock control.
> > 
> > I am afraid of this change. Besides the rate there might be other
> > reasons to choose one mux input over another, consider for example low
> > power audio playback where we need one specific mux setting because it
> > provides a clock which runs at low power mode.
> > On the IPU on i.MX5/6 there are clocks being used as pixel clocks
> > derived from different muxes. I don't think you want to pick an input
> > clock just because it happens to deliver the best clock rate at that
> > point in time, but really is shared with some other clock that changes
> > its rate in the next moment.
> > 
> 
> > I have no concrete examples for things that break with this change, but
> > I would be more confident if we change the behaviour explicitly only for
> > the muxes that we have reviewed to cope with this change.
> > 
> 
> Fair enough. We could replace all the imx_clk_mux with imx_clk_mux_noreparent
> and after that we can independently switch the clocks that are safe (to switch)
> to imx_clk_mux (which would not have the noreparent flag set).

Ok with me.

> 
> The main idea is to simplify the clock control from drivers point of view.

Which drivers do you have in mind? I hardly ever missed reparenting on
rate changes, so where is this feature useful?

Sascha

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