linux-kernel - Re: [PATCH] HPET: Workaround for a BIOS workaround on AMD SB700 platform

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Date:	Fri, 15 Aug 2008 10:09:26 +0800
From:	crane cai <crane.cai@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	vojtech@...e.cz, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] HPET: Workaround for a BIOS workaround on AMD SB700
	platform

Hi Thomas,

Your modification has been checked OK on our SB700 platform. Thank you.
Now could you help us to merge your patch to kernel source? Or do I need
to resubmit a patch on it?

Best regards,
- Crane

On Thu, 2008-08-14 at 18:11 +0800, Thomas Gleixner wrote:

> Crane,
> 
> thanks for debugging this. I think we don't need a quirk for this
> workaround. Checking the config register value for 0xffffffff is safe
> on all machines. I simplified your patch to the one below and added a
> printk in case the check times out.
> 
> One thing I'm a bit wary about is the readout of the HPET_PERIOD
> register. Is the value correct even _before_ the SMM machinery is
> started ?

> Thanks,
> 
>         tglx
> -----
> diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
> index ad2b15a..59fd3b6 100644
> --- a/arch/x86/kernel/hpet.c
> +++ b/arch/x86/kernel/hpet.c
> @@ -359,6 +359,7 @@ static int hpet_clocksource_register(void)
>  int __init hpet_enable(void)
>  {
>         unsigned long id;
> +       int i;
> 
>         if (!is_hpet_capable())
>                 return 0;
> @@ -369,6 +370,29 @@ int __init hpet_enable(void)
>          * Read the period and check for a sane value:
>          */
>         hpet_period = hpet_readl(HPET_PERIOD);
> +
> +       /*
> +        * AMD SB700 based systems with spread spectrum enabled use a
> +        * SMM based HPET emulation to provide proper frequency
> +        * setting. The SMM code is initialized with the first HPET
> +        * register access and takes some time to complete. During
> +        * this time the config register reads 0xffffffff. We check
> +        * for max. 1000 loops whether the config register reads a non
> +        * 0xffffffff value to make sure that HPET is up and running
> +        * before we go further. A counting loop is safe, as the HPET
> +        * access takes thousands of CPU cycles. On non SB700 based
> +        * machines this check is only done once and has no side
> +        * effects.
> +        */
> +       for (i = 0; hpet_readl(HPET_CFG) == 0xFFFFFFFF; i++) {
> +               if (i == 1000) {
> +                       printk(KERN_WARNING
> +                              "HPET config register value = 0xFFFFFFFF. "
> +                              "Disabling HPET\n");
> +                       goto out_nohpet;
> +               }
> +       }
> +
>         if (hpet_period < HPET_MIN_PERIOD || hpet_period > HPET_MAX_PERIOD)
>                 goto out_nohpet;
> 


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