Re: [PATCH 06/10] ARC: [plat-eznps] Fix TLB Errata

[Vineet Gupta]

On 05/25/2017 04:00 AM, Alexey Brodkin wrote:
> Hi Noam,
>
> On Thu, 2017-05-25 at 05:34 +0300, Noam Camus wrote:
> > From: Noam Camus <noamca@xxxxxxxxxxxx>
> >
> > Due to a HW bug in NPS400 we get from time to time false TLB miss.
> > Workaround this by validating each miss.
> >
> > Signed-off-by: Noam Camus <noamca@xxxxxxxxxxxx>
> > ---
> >   arch/arc/mm/tlbex.S |   10 ++++++++++
> >   1 files changed, 10 insertions(+), 0 deletions(-)
> >
> > diff --git a/arch/arc/mm/tlbex.S b/arch/arc/mm/tlbex.S
> > index b30e4e3..1d48723 100644
> > --- a/arch/arc/mm/tlbex.S
> > +++ b/arch/arc/mm/tlbex.S
> > @@ -274,6 +274,13 @@ ex_saved_reg1:
> >   .macro COMMIT_ENTRY_TO_MMU
> >   #if (CONFIG_ARC_MMU_VER < 4)
> >   
> > +#ifdef CONFIG_EZNPS_MTM_EXT
> > +	/* verify if entry for this vaddr+ASID already exists */
> > +	sr    TLBProbe, [ARC_REG_TLBCOMMAND]
> > +	lr    r0, [ARC_REG_TLBINDEX]
> > +	bbit0 r0, 31, 88f
> > +#endif
>
> That's funny. I think we used to have something like that in the past.

Not here as this is fast path TLB refill handler and landign here implies entry 
was *not* present, unless there's a hardware bug, hence this patch.

Perhaps you are remembering the slow path TLB update code (tlb.c) which has always 
had this - as mm code can call update_mmu_cache() in various cases and in soem of 
those, the entry can be already present so for ARC700 cores we need to ensure that 
dups are not inserted !

> >   	/* Get free TLB slot: Set = computed from vaddr, way = random */
> >   	sr  TLBGetIndex, [ARC_REG_TLBCOMMAND]
> >   
> > @@ -287,6 +294,9 @@ ex_saved_reg1:
> >   #else
> >   	sr TLBInsertEntry, [ARC_REG_TLBCOMMAND]
> >   #endif
> > +#ifdef CONFIG_EZNPS_MTM_EXT
> > +88:
> > +#endif
>
> Not sure if label itself required wrapping in ifdefs. It just makes code bulkier
> and harder to read.

I agree !

FWIW, after this patch, COMMIT_ENTRY_TO_MMU is totally unreadable - perhaps one of 
us needs to break it up into MMU ver specific implementations. But at any rate 
that can be after this patch.