Re: [PATCH] iommu/arm-smmu: Don't disable next-page prefetcher on devices it works on

[Pankaj Patil <quic_pankpati@quicinc.com>]

On 9/4/2024 1:59 PM, Pankaj Patil wrote:
> On 5/17/2024 10:49 PM, Doug Anderson wrote:
>> Hi,
>>
>> On Fri, May 17, 2024 at 9:37 AM Will Deacon <will@kernel.org> wrote:
>>>
>>> Hi Doug,
>>>
>>> On Mon, May 13, 2024 at 04:13:47PM -0700, Douglas Anderson wrote:
>>>> On sc7180 trogdor devices we get a scary warning at bootup:
>>>>   arm-smmu 15000000.iommu:
>>>>   Failed to disable prefetcher [errata #841119 and #826419], check ACR.CACHE_LOCK
>>>>
>>>> We spent some time trying to figure out how we were going to fix these
>>>> errata and whether we needed to do a firmware update. Upon closer
>>>> inspection, however, we realized that the errata don't apply to us.
>>>> Specifically, the errata document says that for these errata:
>>>> * Found in: r0p0
>>>> * Fixed in: r2p2
>>>>
>>>> ...and trogdor devices appear to be running r2p4. That means that they
>>>> are unaffected despite the scary warning.
>>>>
>>>> The issue is that the kernel unconditionally tries to disable the
>>>> prefetcher even on unaffected devices and then warns when it's unable
>>>> to.
>>>>
>>>> Let's change the kernel to only disable the prefetcher on affected
>>>> devices, which will get rid of the scary warning on devices that are
>>>> unaffected. As per the comment the prefetcher is
>>>> "not-particularly-beneficial" but it shouldn't hurt to leave it on for
>>>> devices where it doesn't cause problems.
>>>>
>>>> Fixes: f87f6e5b4539 ("iommu/arm-smmu: Warn once when the perfetcher errata patch fails to apply")
>>>> Signed-off-by: Douglas Anderson <dianders@chromium.org>
>>>> ---
>>>>
>>>>  drivers/iommu/arm/arm-smmu/arm-smmu-impl.c | 21 +++++++++++++--------
>>>>  1 file changed, 13 insertions(+), 8 deletions(-)
>>>
>>>
>>> Just curious, but did you see any performance impact (good or bad) as a
>>> result of this patch? The next-page prefetcher has always looked a little
>>> naive to me and, with a tendency for tiny TLBs in some implementations,
>>> there's a possibility it could do more harm than good.
>>
>> This patch actually makes no difference on trogdor today other than
>> getting rid of the scary warning. Specifically on trogdor the
>> ACR.CACHE_LOCK bit seems to be set so the kernel is unable to change
>> the setting anyway and has never been able to. We are working on
>> figuring out how to fix the firmware and then we have to get a
>> firmware spin before we can really see any changes. I'll keep an eye
>> out to see if performance numbers change when the firmware uprevs.
>>
>> BTW: any idea how big of a deal these errata are? We're _just_
>> finishing a firmware uprev process and there is always pushback
>> against kicking off a new one unless the issue is important. Given
>> that we've been living with this issue since devices shipped I'm going
>> to assume we don't need to rush a firmware update, but if this is
>> really scary and needs to be addressed sooner we can figure that out.
>>
>> -Doug
> 
> Receiving the warning on pre-silicon platforms as well, despite being unaffected. If merged, it will help in reducing log clutter.
> The patch applies cleanly on the tip of linux-next, tag: next-20240904.
> 
Following up on the patch. Please let me know if any additional 
changes are required.