linux-kernel - Re: [PATCH] soc/tegra: pmc: Fix "scheduling while atomic"

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Date:	Thu, 5 May 2016 14:17:18 +0100
From:	Jon Hunter <jonathanh@...dia.com>
To:	Dmitry Osipenko <digetx@...il.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	"Alexandre Courbot" <gnurou@...il.com>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	Prashant Gaikwad <pgaikwad@...dia.com>
CC:	<linux-clk@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] soc/tegra: pmc: Fix "scheduling while atomic"


On 17/04/16 14:34, Dmitry Osipenko wrote:
> clk_get_rate() takes a mutex, hence cannot be used while IRQ's been
> disabled. Replace it with a locked version.
> 
> [    3.430853] [<c0850fcc>] (dump_stack) from [<c00411f8>] (__schedule_bug+0x50/0x64)
> [    3.431079] [<c00411f8>] (__schedule_bug) from [<c08553a8>] (__schedule+0x5c8/0x688)
> [    3.431453] [<c08553a8>] (__schedule) from [<c08558f4>] (schedule_preempt_disabled+0x24/0x34)
> [    3.431835] [<c08558f4>] (schedule_preempt_disabled) from [<c0856f24>] (__mutex_lock_slowpath+0xbc/0x170)
> [    3.432204] [<c0856f24>] (__mutex_lock_slowpath) from [<c0857024>] (mutex_lock+0x4c/0x50)
> [    3.432427] [<c0857024>] (mutex_lock) from [<c0610368>] (clk_prepare_lock+0x88/0xfc)
> [    3.432800] [<c0610368>] (clk_prepare_lock) from [<c0611034>] (clk_get_rate+0xc/0x60)
> [    3.433177] [<c0611034>] (clk_get_rate) from [<c034f10c>] (tegra_pmc_enter_suspend_mode+0x188/0x20c)
> [    3.433580] [<c034f10c>] (tegra_pmc_enter_suspend_mode) from [<c0020d48>] (tegra_idle_lp2_last+0xc/0x40)
> [    3.433795] [<c0020d48>] (tegra_idle_lp2_last) from [<c0021e1c>] (tegra20_idle_lp2_coupled+0x118/0x1fc)
> [    3.434171] [<c0021e1c>] (tegra20_idle_lp2_coupled) from [<c055ec24>] (cpuidle_enter_state+0x3c/0x160)
> [    3.434551] [<c055ec24>] (cpuidle_enter_state) from [<c0560ce8>] (cpuidle_enter_state_coupled+0x3dc/0x3f4)
> [    3.434959] [<c0560ce8>] (cpuidle_enter_state_coupled) from [<c0055f1c>] (cpu_startup_entry+0x240/0x288)
> [    3.435340] [<c0055f1c>] (cpu_startup_entry) from [<c0b29c84>] (start_kernel+0x3b4/0x3c0)
> [    3.435557] [<c0b29c84>] (start_kernel) from [<00008074>] (0x8074)
> 
> Signed-off-by: Dmitry Osipenko <digetx@...il.com>

Thanks for the report. I have been unable to reproduce this, but then I
don't see my tegra20 entering LP2 during cpuidle. I did force my tegra20
into LP2 during suspend which will exercise the same code but I did not
trigger this either. However, from looking at the code it does appear
that we could hit this.

> ---
>  drivers/clk/tegra/clk-tegra-pmc.c | 9 +++++++++
>  drivers/soc/tegra/pmc.c           | 2 +-
>  include/linux/clk/tegra.h         | 2 ++
>  3 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/clk/tegra/clk-tegra-pmc.c b/drivers/clk/tegra/clk-tegra-pmc.c
> index 91377ab..1ccf414 100644
> --- a/drivers/clk/tegra/clk-tegra-pmc.c
> +++ b/drivers/clk/tegra/clk-tegra-pmc.c
> @@ -78,6 +78,8 @@ static struct pmc_clk_init_data pmc_clks[] = {
>  	PMC_CLK(3, 22, 18),
>  };
>  
> +static struct clk_hw *pclk_hw;
> +
>  void __init tegra_pmc_clk_init(void __iomem *pmc_base,
>  				struct tegra_clk *tegra_clks)
>  {
> @@ -112,6 +114,9 @@ void __init tegra_pmc_clk_init(void __iomem *pmc_base,
>  		clk_register_clkdev(clk, data->dev_name, data->gate_name);
>  	}
>  
> +	dt_clk = tegra_lookup_dt_id(tegra_clk_pclk, tegra_clks);
> +	pclk_hw = __clk_get_hw(*dt_clk);
> +
>  	/* blink */
>  	writel_relaxed(0, pmc_base + PMC_BLINK_TIMER);
>  	clk = clk_register_gate(NULL, "blink_override", "clk_32k", 0,
> @@ -129,3 +134,7 @@ void __init tegra_pmc_clk_init(void __iomem *pmc_base,
>  	*dt_clk = clk;
>  }
>  
> +unsigned long tegra_pmc_get_pclk_rate(void)
> +{
> +	return clk_hw_get_rate(pclk_hw);
> +}

Ideally, it would be great if we did not need to add another custom API
for this, but I did not find anything in the CCF that would allow us to
avoid but that was only a quick look. However, we could ask the CCF folks.

What I plan to do next is to understand if the pclk is likely to change.
I know that it comes from one of the plls but I am not sure if we ever
change the rate. If not we may be able to move this to probe time and
avoid this.

Cheers
Jon