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

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Message-ID: <20110220125745.GC14495@n2100.arm.linux.org.uk>
Date:	Sun, 20 Feb 2011 12:57:45 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Colin Cross <ccross@...roid.com>
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 Wed, Feb 16, 2011 at 11:36:45AM -0800, Colin Cross wrote:
> 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);

This saves fpexc with the enable flag possibly clear.

> 		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.

Which is wrong as I said above.
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