linux-kernel - Re: [PATCH] arc: Flush and invalidate caches on start

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Date:   Thu, 31 Aug 2017 16:34:29 +0000
From:   Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
Subject: Re: [PATCH] arc: Flush and invalidate caches on start

Hi Vineet,

On Thu, 2017-08-31 at 09:31 -0700, Vineet Gupta wrote:
> On 08/31/2017 07:22 AM, Alexey Brodkin wrote:
> > 
> > This is useful to make sure no stale data exists in caches after bootloaders.
> > The worst thing could be some lines of cache were locked in a bootloader
> > for example during DDR recalibration and never unlocked. This may lead
> > to really unpredictable issues later down the line.
> > 
> > Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
> > ---
> >   arch/arc/kernel/head.S | 16 ++++++++++++++++
> >   1 file changed, 16 insertions(+)
> > 
> > diff --git a/arch/arc/kernel/head.S b/arch/arc/kernel/head.S
> > index 8b90d25a15cc..04e28b664183 100644
> > --- a/arch/arc/kernel/head.S
> > +++ b/arch/arc/kernel/head.S
> > @@ -34,6 +34,10 @@
> >   #endif
> >   	sr	r5, [ARC_REG_IC_CTRL]
> >   
> > +	; Invalidate entire I$
> > +	mov	r5, 1
> > +	sr	r5, [ARC_REG_IC_IVIC]
> > +
> >   1:
> >   	lr	r5, [ARC_REG_DC_BCR]
> >   	breq    r5, 0, 1f		; D$ doesn't exist
> > @@ -46,6 +50,18 @@
> >   #endif
> >   	sr	r5, [ARC_REG_DC_CTRL]
> >   
> > +	; Flush entire D$
> > +	mov	r5, 1
> > +	sr	r5, [ARC_REG_DC_FLSH]
> > +	; Wait for flush operation to complete
> > +1:
> > +	lr	r5, [ARC_REG_DC_CTRL]
> > +	bbit1	r5, DC_CTRL_FLUSH_STATUS, 1b
> > +
> > +	; Invalidate entire D$
> > +	mov	r5, 1
> > +	sr	r5, [ARC_REG_DC_IVDC]
> > +
> 
> AFAIK uboot already flushes the caches before handing control over to kernel - so 
> why do we need it here.
> If uboot is locking lines, it needs to fix that and not penalize the general case 
> with or w/o uboot !

U-Boot indeed flushes caches.. but doesn't invalidate them!
And only invalidation unlocks locked lines.

That indeed should be added in U-Boot but I'd say above stuff doesn't
influence a lot code size or execution time while makes system more fool-proof.

-Alexey