linux-kernel - Re: [PATCH v8 2/9] clk: tegra: Fix refcounting of gate clocks

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Date:   Wed, 14 Jul 2021 18:58:58 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        Prashant Gaikwad <pgaikwad@...dia.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>
CC:     Rob Herring <robh+dt@...nel.org>, <linux-tegra@...r.kernel.org>,
        <linux-clk@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH v8 2/9] clk: tegra: Fix refcounting of gate clocks


On 14/07/2021 12:59, Dmitry Osipenko wrote:
> 14.07.2021 14:48, Jon Hunter пишет:
>>
>> On 16/05/2021 17:30, Dmitry Osipenko wrote:
>>> The refcounting of the gate clocks has a bug causing the enable_refcnt
>>> to underflow when unused clocks are disabled. This happens because clk
>>> provider erroneously bumps the refcount if clock is enabled at a boot
>>> time, which it shouldn't be doing, and it does this only for the gate
>>> clocks, while peripheral clocks are using the same gate ops and the
>>> peripheral clocks are missing the initial bump. Hence the refcount of
>>> the peripheral clocks is 0 when unused clocks are disabled and then the
>>> counter is decremented further by the gate ops, causing the integer
>>> underflow.
>>>
>>> Fix this problem by removing the erroneous bump and by implementing the
>>> disable_unused() callback, which disables the unused gates properly.
>>>
>>> The visible effect of the bug is such that the unused clocks are never
>>> gated if a loaded kernel module grabs the unused clocks and starts to use
>>> them. In practice this shouldn't cause any real problems for the drivers
>>> and boards supported by the kernel today.
>>>
>>> Acked-by: Thierry Reding <treding@...dia.com>
>>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>

...

> Seems you'll need to implement the disable_unused() callback for the
> clk_sdmmc_mux to fix it. It's good that this problem has been caught.
> 
> diff --git a/drivers/clk/tegra/clk-sdmmc-mux.c
> b/drivers/clk/tegra/clk-sdmmc-mux.c
> index 316912d3b1a4..4f2c3309eea4 100644
> --- a/drivers/clk/tegra/clk-sdmmc-mux.c
> +++ b/drivers/clk/tegra/clk-sdmmc-mux.c
> @@ -194,6 +194,15 @@ static void clk_sdmmc_mux_disable(struct clk_hw *hw)
>  	gate_ops->disable(gate_hw);
>  }
> 
> +static void clk_sdmmc_mux_disable_unused(struct clk_hw *hw)
> +{
> +	struct tegra_sdmmc_mux *sdmmc_mux = to_clk_sdmmc_mux(hw);
> +	const struct clk_ops *gate_ops = sdmmc_mux->gate_ops;
> +	struct clk_hw *gate_hw = &sdmmc_mux->gate.hw;
> +
> +	gate_ops->disable_unused(gate_hw);
> +}
> +
>  static void clk_sdmmc_mux_restore_context(struct clk_hw *hw)
>  {
>  	struct clk_hw *parent = clk_hw_get_parent(hw);
> @@ -218,6 +227,7 @@ static const struct clk_ops tegra_clk_sdmmc_mux_ops = {
>  	.is_enabled = clk_sdmmc_mux_is_enabled,
>  	.enable = clk_sdmmc_mux_enable,
>  	.disable = clk_sdmmc_mux_disable,
> +	.disable_unused = clk_sdmmc_mux_disable_unused,
>  	.restore_context = clk_sdmmc_mux_restore_context,
>  };
> 


Thanks, that fixes it! Please feel free to add my test-by and acked-by
when you send out the patch.

Acked-by: Jon Hunter <jonathanh@...dia.com>
Tested-by: Jon Hunter <jonathanh@...dia.com>

Cheers!
Jon

-- 
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