linux-kernel - Re: [PATCH v2 22/65] clk: davinci: da8xx-cfgchip: Add a determine

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Date:   Fri, 4 Nov 2022 11:46:02 -0500
From:   David Lechner <david@...hnology.com>
To:     Maxime Ripard <maxime@...no.tech>, Stephen Boyd <sboyd@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Chen-Yu Tsai <wens@...e.org>, Daniel Vetter <daniel@...ll.ch>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Shawn Guo <shawnguo@...nel.org>,
        Fabio Estevam <festevam@...il.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        Michael Turquette <mturquette@...libre.com>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Paul Cercueil <paul@...pouillou.net>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Manivannan Sadhasivam <mani@...nel.org>,
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Subject: Re: [PATCH v2 22/65] clk: davinci: da8xx-cfgchip: Add a
 determine_rate hook

On 11/4/22 8:17 AM, Maxime Ripard wrote:
> The Davinci DA8xxx cfgchip "clk48" clock implements a mux with a
> set_parent hook, but doesn't provide a determine_rate implementation.
> 
> This is a bit odd, since set_parent() is there to, as its name implies,
> change the parent of a clock. However, the most likely candidate to
> trigger that parent change is a call to clk_set_rate(), with
> determine_rate() figuring out which parent is the best suited for a
> given rate.
> 
> The other trigger would be a call to clk_set_parent(), but it's far less
> used, and it doesn't look like there's any obvious user for that clock.
> 
> So, the set_parent hook is effectively unused, possibly because of an
> oversight. However, it could also be an explicit decision by the
> original author to avoid any reparenting but through an explicit call to
> clk_set_parent().
> 
> The latter case would be equivalent to setting the flag
> CLK_SET_RATE_NO_REPARENT, together with setting our determine_rate hook
> to __clk_mux_determine_rate(). Indeed, if no determine_rate
> implementation is provided, clk_round_rate() (through
> clk_core_round_rate_nolock()) will call itself on the parent if
> CLK_SET_RATE_PARENT is set, and will not change the clock rate
> otherwise. __clk_mux_determine_rate() has the exact same behavior when
> CLK_SET_RATE_NO_REPARENT is set.
> 
> And if it was an oversight, then we are at least explicit about our
> behavior now and it can be further refined down the line.

The parent is defined in the device tree and is not expected to change
at runtime, so if I am understanding the patch correctly, setting the
CLK_SET_RATE_NO_REPARENT flag seems correct.

> 
> Signed-off-by: Maxime Ripard <maxime@...no.tech>
> ---
>   drivers/clk/davinci/da8xx-cfgchip.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/clk/davinci/da8xx-cfgchip.c b/drivers/clk/davinci/da8xx-cfgchip.c
> index c04276bc4051..4c1cc59bba53 100644
> --- a/drivers/clk/davinci/da8xx-cfgchip.c
> +++ b/drivers/clk/davinci/da8xx-cfgchip.c
> @@ -565,6 +565,7 @@ static u8 da8xx_usb1_clk48_get_parent(struct clk_hw *hw)
>   }
>   
>   static const struct clk_ops da8xx_usb1_clk48_ops = {
> +	.determine_rate	= __clk_mux_determine_rate,
>   	.set_parent	= da8xx_usb1_clk48_set_parent,
>   	.get_parent	= da8xx_usb1_clk48_get_parent,
>   };
> @@ -589,6 +590,7 @@ da8xx_cfgchip_register_usb1_clk48(struct device *dev,
>   
>   	init.name = "usb1_clk48";
>   	init.ops = &da8xx_usb1_clk48_ops;
> +	init.flags = CLK_SET_RATE_NO_REPARENT;
>   	init.parent_names = parent_names;
>   	init.num_parents = 2;
>   
>