linux-kernel - Re: [PATCH v2] ARM: vfp: Always save VFP state in vfp_pm

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Date:	Wed, 16 Feb 2011 11:36:45 -0800
From:	Colin Cross <ccross@...roid.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ARM: vfp: Always save VFP state in vfp_pm_suspend

On Tue, Feb 15, 2011 at 9:03 AM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Mon, Feb 14, 2011 at 02:55:47PM -0800, Colin Cross wrote:
>> diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
>> index 66bf8d1..7231d18 100644
>> --- a/arch/arm/vfp/vfpmodule.c
>> +++ b/arch/arm/vfp/vfpmodule.c
>> @@ -415,13 +415,13 @@ static int vfp_pm_suspend(struct sys_device *dev, pm_message_t state)
>>       struct thread_info *ti = current_thread_info();
>>       u32 fpexc = fmrx(FPEXC);
>>
>> -     /* if vfp is on, then save state for resumption */
>> -     if (fpexc & FPEXC_EN) {
>> +     /* save state for resume */
>> +     if (last_VFP_context[ti->cpu]) {
>
> I'm not entirely happy with this.
>
> It is true that last_VFP_context[] when non-NULL indicates who owns the
> hardware VFP state, so saving it would seem logical.  However, this new
> code now saves the state with the saved fpexc indicating that it's disabled.
>
> This will cause a VFP exception to misbehave by reloading the state, and
> then disabling the VFP unit.  That will cause another VFP exception which
> will find the VFP unit disabled, and re-enable it.  All in all, this is
> rather wasteful.
>
> So...
>        /* If lazy disable, re-enable the VFP ready for it to be saved */
>        if (last_VFP_context[ti->cpu] != &ti->vfpstate) {
>                fpexc |= FPEXC_EN;
>                fmxr(FPEXC, fpexc);
>        }
>        /* If VFP is on, then save state for resumption */
>        if (fpexc & FPEXC_EN) {
>                ...

I think v2 of the patch handles this case correctly:
	/* save state for resume */
	if (last_VFP_context[ti->cpu]) {
		printk(KERN_DEBUG "%s: saving vfp state\n", __func__);
		fmxr(FPEXC, fpexc | FPEXC_EN);
		vfp_save_state(last_VFP_context[ti->cpu], fpexc);
		last_VFP_context[ti->cpu] = NULL;
		fmxr(FPEXC, fpexc & ~FPEXC_EN);
	}

This version enables the VFP if it was not enabled, but saves the
original fpexc value.
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