linux-kernel - Re: [PATCH] clk: clk-gpio: Add dt option to propagate rate change to parent

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Message-ID: <f3992013a3811ec2292e3da403841cb6e5cb95b5.camel@analog.com>
Date:   Thu, 7 Nov 2019 13:25:38 +0000
From:   "Ardelean, Alexandru" <alexandru.Ardelean@...log.com>
To:     "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "sboyd@...nel.org" <sboyd@...nel.org>
CC:     "Hennerich, Michael" <Michael.Hennerich@...log.com>,
        "jsarha@...com" <jsarha@...com>, "ce3a@....de" <ce3a@....de>,
        "mturquette@...libre.com" <mturquette@...libre.com>
Subject: Re: [PATCH] clk: clk-gpio: Add dt option to propagate rate change to
 parent

On Wed, 2019-11-06 at 14:43 -0800, Stephen Boyd wrote:
> Quoting Alexandru Ardelean (2019-11-06 03:35:51)
> > From: Michael Hennerich <michael.hennerich@...log.com>
> > 
> > For certain setups/boards it's useful to propagate the rate change of
> > the
> > clock up one level to the parent clock.
> > 
> > This change implements this by defining a `clk-set-rate-parent` device-
> > tree
> > property which sets the `CLK_SET_RATE_PARENT` flag to the clock (when
> > set).
> > 
> > Signed-off-by: Michael Hennerich <michael.hennerich@...log.com>
> > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@...log.com>
> > ---
> >  drivers/clk/clk-gpio.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/clk/clk-gpio.c b/drivers/clk/clk-gpio.c
> > index 9d930edd6516..6dfbc4b952fe 100644
> > --- a/drivers/clk/clk-gpio.c
> > +++ b/drivers/clk/clk-gpio.c
> > @@ -241,6 +241,7 @@ static int gpio_clk_driver_probe(struct
> > platform_device *pdev)
> >         struct device_node *node = pdev->dev.of_node;
> >         const char **parent_names, *gpio_name;
> >         unsigned int num_parents;
> > +       unsigned long clk_flags;
> >         struct gpio_desc *gpiod;
> >         struct clk *clk;
> >         bool is_mux;
> > @@ -274,13 +275,16 @@ static int gpio_clk_driver_probe(struct
> > platform_device *pdev)
> >                 return ret;
> >         }
> >  
> > +       clk_flags = of_property_read_bool(node, "clk-set-rate-parent")
> > ?
> > +                       CLK_SET_RATE_PARENT : 0;
> 
> Is there a DT binding update somewhere? It looks like a linux-ism from

Good point. I did not think about the DT, and I guess I didn't search it
thoroughly enough. Found DT files now.

> the DT perspective. I wonder if we can somehow figure out that it's OK
> to call clk_set_rate() on the parent here? Or is it safe to assume that
> we can just always call set rate on the parent? I think for a gate it's
> good and we can just do so, but for a mux maybe not. Care to describe
> your scenario a little more so we can understand why you want to set
> this flag? Is it for a mux or a gate type gpio?
> 

For our case we are using it here [with a slight name variation in the prop
name]:
https://github.com/analogdevicesinc/linux/blob/master/arch/arm/boot/dts/zynq-adrv9361-z7035.dtsi#L43

And on this board:
https://wiki.analog.com/resources/eval/user-guides/adrv936x_rfsom/user-guide/introduction

It's for a gate-type GPIO.
The clock defined in that DT (ad9361_clkin) is attached to a clock that has
a fixed rate (xo_40mhz_fixed_clk), that can become a variable rate [from an
external source, when it is provided].

Our understanding, is that a GPIO gate clock should propagate the rate
change to the parent clock. The same goes for the GPIO MUX clock.
So, the default mode would be to always set CLK_SET_RATE_PARENT.

But, given that there are users of this driver, such a behavior change
could break other users, so we are using this DT prop.

There seems to be only one user of gpio-mux-clock in
arch/arm64/boot/dts/renesas/ulcb-kf.dtsi

Whereas for gpio-gate-clock, there are multiple users. I can't say whether
this change would break anything for them, or it would be fine.