linux-kernel - Re: [PATCH] kernel: x86: early-quirks: Replace mdelay with usleep_range in apple_airport

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Date:   Wed, 24 Jan 2018 06:33:01 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        paulo.r.zanoni@...el.com, kstewart@...uxfoundation.org,
        gregkh@...uxfoundation.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel: x86: early-quirks: Replace mdelay with
 usleep_range in apple_airport_reset

On Wed, Jan 24, 2018 at 10:38:22AM +0800, Jia-Ju Bai wrote:
> The function apple_airport_reset is not called in atomic context.
> Thus mdelay can be replaced with usleep_range, to avoid busy wait.
> 
> This is reported by a static analysis tool named DCNS written by myself.

No, usleep_range() is built on hrtimers and at this point in the
boot sequence we haven't called hrtimers_init() yet.

Look at init/main.c:start_kernel() and note that setup_arch()
(which calls early_quirks()) is called way before hrtimers_init().

Please amend your static checker to consider every call to mdelay()
that occurs before hrtimers_init() a false positive, or manually
verify each patch for this constraint before submission.

I'll test your patch later today on my MacBook Pro but I suspect
it'll cause a boot crash or hang.  In any case the choice of mdelay()
here was deliberate.

Thanks,

Lukas

> 
> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
> ---
>  arch/x86/kernel/early-quirks.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/early-quirks.c b/arch/x86/kernel/early-quirks.c
> index 1e82f78..559e81a 100644
> --- a/arch/x86/kernel/early-quirks.c
> +++ b/arch/x86/kernel/early-quirks.c
> @@ -604,7 +604,7 @@ static void __init apple_airport_reset(int bus, int slot, int func)
>  	if ((pmcsr & PCI_PM_CTRL_STATE_MASK) != PCI_D0) {
>  		pmcsr &= ~PCI_PM_CTRL_STATE_MASK;
>  		write_pci_config_16(bus, slot, func, BCM4331_PM_CAP + PCI_PM_CTRL, pmcsr);
> -		mdelay(10);
> +		usleep_range(10000, 11000);
>  
>  		pmcsr = read_pci_config_16(bus, slot, func, BCM4331_PM_CAP + PCI_PM_CTRL);
>  		if ((pmcsr & PCI_PM_CTRL_STATE_MASK) != PCI_D0) {
> -- 
> 1.7.9.5
>