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

From: Abel Vesa <abel.vesa@nxp.com>
To: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Adam Ford <aford173@gmail.com>,
	arm-soc <linux-arm-kernel@lists.infradead.org>,
	Adam Ford-BE <aford@beaconembedded.com>,
	Aisheng Dong <aisheng.dong@nxp.com>,
	Michael Turquette <mturquette@baylibre.com>,
	Stephen Boyd <sboyd@kernel.org>, Shawn Guo <shawnguo@kernel.org>,
	Pengutronix Kernel Team <kernel@pengutronix.de>,
	Fabio Estevam <festevam@gmail.com>,
	NXP Linux Team <linux-imx@nxp.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Jerome Brunet <jbrunet@baylibre.com>,
	linux-clk <linux-clk@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH V3] clk: imx: Fix reparenting of UARTs not associated with sdout
Date: Wed, 20 Jan 2021 18:14:21 +0200	[thread overview]
Message-ID: <20210120161421.h3yng57m3fetwwih@fsr-ub1664-175> (raw)
In-Reply-To: <20210120155001.GD19063@pengutronix.de>

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@nxp.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.