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

Vineet Gupta
2017-06-01T21:16:55Z

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@mellanox.com>
>>
>> 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@mellanox.com>
>> ---
>> 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.