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

[Pankaj Patil]

On 5/17/2024 10:49 PM, Doug Anderson wrote:
> Hi,
> 
> On Fri, May 17, 2024 at 9:37 AM Will Deacon <will@xxxxxxxxxx> 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@xxxxxxxxxxxx>
>>> ---
>>>
>>>  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.