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

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Date:   Mon, 7 Dec 2020 17:53:44 +0100
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Adam Ford <aford173@...il.com>
Cc:     linux-clk <linux-clk@...r.kernel.org>,
        Adam Ford-BE <aford@...conembedded.com>,
        charles.steves@...icpd.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>,
        arm-soc <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] clk: imx: Fix reparenting of UARTs not associated with
 sdout

Hi Adam,

On Mon, Dec 07, 2020 at 09:51:33AM -0600, Adam Ford wrote:
> On Sun, Dec 6, 2020 at 11:24 PM Sascha Hauer <s.hauer@...gutronix.de> wrote:
> >
> > Hi Adam,
> >
> > On Fri, Dec 04, 2020 at 12:31:54PM -0600, Adam Ford wrote:
> > > The default clock source on i.MX8M Mini and Nano boards use a 24MHz clock,
> > > but users who need to re-parent the clock source run into issues because
> > > all the UART clocks are enabled whether or not they're needed by sdout.
> > >
> > > Any attempt to change the parent results in an busy error because the
> > > clocks have been enabled already.
> > >
> > >   clk: failed to reparent uart1 to sys_pll1_80m: -16
> > >
> > > Instead of pre-initializing all UARTS, scan the device tree to see if UART
> > > clock is used as stdout before initializing it.  Only enable the UART clock
> > > if it's needed in order to delay the clock initialization until after the
> > > re-parenting of the clocks.
> > >
> > > Fixes: 9461f7b33d11c ("clk: fix CLK_SET_RATE_GATE with clock rate protection")
> > > Suggested-by: Aisheng Dong <aisheng.dong@....com>
> > > Signed-off-by: Adam Ford <aford173@...il.com>
> > >
> > > diff --git a/drivers/clk/imx/clk.c b/drivers/clk/imx/clk.c
> > > index 47882c51cb85..6dcc5fbd8f3f 100644
> > > --- a/drivers/clk/imx/clk.c
> > > +++ b/drivers/clk/imx/clk.c
> > > @@ -163,12 +163,18 @@ __setup_param("earlyprintk", imx_keep_uart_earlyprintk,
> > >
> > >  void imx_register_uart_clocks(struct clk ** const clks[])
> > >  {
> > > +     struct clk *uart_clk;
> > >       if (imx_keep_uart_clocks) {
> > >               int i;
> > >
> > >               imx_uart_clocks = clks;
> > > -             for (i = 0; imx_uart_clocks[i]; i++)
> > > -                     clk_prepare_enable(*imx_uart_clocks[i]);
> > > +             for (i = 0; imx_uart_clocks[i]; i++) {
> > > +                     uart_clk = of_clk_get(of_stdout, i);
> >
> > This looks wrong. imx_uart_clocks is an array containing all clocks that
> > could possibly be used for an UART. With of_clk_get(of_stdout, i) you
> > get the nth clock for one specific UART.
> > What you have to do here is: For each of imx_uart_clocks[] you have to
> > iterate over all clocks of the of_stdout node.
> 
> Sascha,
> 
> Thanks for the review.
> 
> I agree that I'm using the wrong index when checking of_stdout, but I
> am not sure we need to loop through  imx_uart_clocks at all.
> I looked at the NXP repo, and they just focus on looping through
> of_stdout and don't reference the imx_uart_clocks. Can't we just loop
> through the of_stdout and enable any clocks found in that?

This sounds good. That way we could get rid of the imx_uart_clocks array
entirely.

Note that right below imx_register_uart_clocks() there is imx_clk_disable_uart()
which disables the previously enabled clocks again later when the real
driver has taken over. You'll have to change that as well accordingly.

Sascha

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