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Date:   Fri, 01 Mar 2019 09:31:23 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Chen-Yu Tsai <wens@...e.org>,
        Jassi Brar <jassisinghbrar@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Samuel Holland <samuel@...lland.org>
Cc:     devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-sunxi@...glegroups.com, Samuel Holland <samuel@...lland.org>
Subject: Re: [PATCH v2 03/10] clk: sunxi-ng: sun50i: Mark the msgbox clock as critical

Quoting Samuel Holland (2019-02-28 21:29:40)
> The msgbox clock is critical because the hardware is shared between
> Linux and system firmware. The message box may be used by the EL3 secure
> monitor's PSCI implementation. On 64-bit sunxi SoCs, this is provided by
> ARM TF-A; 32-bit SoCs use a different implementation. The secure monitor
> uses the message box to forward requests to power management firmware
> running on a separate CPU.
> 
> It is not enough for the secure monitor to enable the clock each time
> Linux performs a SMC into EL3, as both the firmware and Linux can run
> concurrently on separate CPUs. So it is never safe for Linux to turn
> this clock off, and it should be marked as critical.
> 
> At this time, such power management firmware only exists for the A64.
> However, it makes sense to take care of all CCU drivers now for
> consistency, and to ease the transition in the future, once firmware is
> ported to the other SoCs.
> 
> Signed-off-by: Samuel Holland <samuel@...lland.org>
> ---
>  drivers/clk/sunxi-ng/ccu-sun50i-a64.c | 2 +-
>  drivers/clk/sunxi-ng/ccu-sun50i-h6.c  | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/clk/sunxi-ng/ccu-sun50i-a64.c b/drivers/clk/sunxi-ng/ccu-sun50i-a64.c
> index 932836d26e2b..7780e855c56f 100644
> --- a/drivers/clk/sunxi-ng/ccu-sun50i-a64.c
> +++ b/drivers/clk/sunxi-ng/ccu-sun50i-a64.c
> @@ -350,7 +350,7 @@ static SUNXI_CCU_GATE(bus_de_clk,   "bus-de",       "ahb1",
>  static SUNXI_CCU_GATE(bus_gpu_clk,     "bus-gpu",      "ahb1",
>                       0x064, BIT(20), 0);
>  static SUNXI_CCU_GATE(bus_msgbox_clk,  "bus-msgbox",   "ahb1",
> -                     0x064, BIT(21), 0);
> +                     0x064, BIT(21), CLK_IS_CRITICAL);

Please add a comment explaining why CLK_IS_CRITICAL is used. It helps us
remember years from now without having to dig in the git history.

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