linux-kernel - Re: [PATCH V3] clk: imx: Fix reparenting of UARTs not associated with sdout

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Message-ID: <20210120161421.h3yng57m3fetwwih@fsr-ub1664-175>
Date:   Wed, 20 Jan 2021 18:14:21 +0200
From:   Abel Vesa <abel.vesa@....com>
To:     Sascha Hauer <s.hauer@...gutronix.de>
Cc:     Adam Ford <aford173@...il.com>,
        arm-soc <linux-arm-kernel@...ts.infradead.org>,
        Adam Ford-BE <aford@...conembedded.com>,
        Aisheng Dong <aisheng.dong@....com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Jerome Brunet <jbrunet@...libre.com>,
        linux-clk <linux-clk@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3] clk: imx: Fix reparenting of UARTs not associated
 with sdout

On 21-01-20 16:50:01, Sascha Hauer wrote:
> On Wed, Jan 20, 2021 at 05:28:13PM +0200, Abel Vesa wrote:
> > On 21-01-20 16:13:05, Sascha Hauer wrote:
> > > Hi Abel,
> > > 
> > > On Wed, Jan 20, 2021 at 04:44:54PM +0200, Abel Vesa wrote:
> > > > On 21-01-18 08:00:43, Adam Ford wrote:
> > > > > On Mon, Jan 18, 2021 at 6:52 AM Abel Vesa <abel.vesa@....com> wrote:
> > 
> > ...
> > 
> > > > > 
> > > > > >
> > > > > > TBH, I'm against the idea of having to call consumer API from a clock provider driver.
> > > > > > I'm still investigating a way of moving the uart clock control calls in drivers/serial/imx,
> > > > > > where they belong.
> > > > > 
> > > > > That makes sense.
> > > > > 
> > > > 
> > > > Just a thought. The uart clock used for console remains on from u-boot,
> > > > so maybe it's enough to just add the CLK_IGNORE_UNUSED flag to all the
> > > > uart root clocks and remove the prepare/enable calls for uart clocks 
> > > > for good. I don't really have a way to test it right now, but maybe
> > > > you could give it a try.
> > > 
> > > That would mean that UART clocks will never be disabled, regardless of
> > > whether they are used for console or not. That doesn't sound very
> > > appealing.
> > 
> > AFAIK, the only uart clock that is enabled by u-boot is the one used for
> > the console. Later on, when the serial driver probes, it will enable it itself.
> > 
> > Unless I'm missing something, this is exactly what we need.
> 
> It might enable it, but with CLK_IGNORE_UNUSED the clock won't be
> disabled again when a port is closed after usage

OK, tell me what I'm getting wrong in the following scenario:

U-boot leaves the console uart clock enabled. All the other ones are disabled.

Kernel i.MX clk driver registers the uart clocks with flag CLK_IGNORE_UNUSED.

Serial driver probes, then enables/disables the clocks when the port opens/closes.

The clk_disable_unused will ignore the uart clocks entirely due to the flag.