Messages in this thread | | | Subject | Re: [PATCH 2/7] perf: Create a symlink for a PMU | From | Andi Kleen <> | Date | Thu, 24 Jun 2021 08:24:29 -0700 |
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On 6/24/2021 7:29 AM, Greg KH wrote: > On Thu, Jun 24, 2021 at 07:24:31AM -0700, Andi Kleen wrote: >>> But first off, why is this symlink suddenly needed? What is so special >>> about this new hardware that it breaks the existing model? >> The driver can be in two modes: >> >> - Driver fully knows the hardware and puts in the correct Linux names >> >> - Driver doesn't know the hardware but is in a fallback mode where it only >> looks at a discovery table. There we don't have the correct names, just an >> numeric identifier for the different hardware sub components. > Why does this matter? Why would the driver not "know" the hardware? If > it doesn't know it, why would it bind to it?
It's a similar concept as a PCI class. How to have a driver that can handle future hardware, but with some restrictions
The perf CPU PMU has had a similar concept for a long time. The driver can be either in architectural mode (with a subset of features), or be fully enabled. This allows users who are on an older kernel to still use at least a subset of the functionality.
It will bind as long as the discovery table is there.
> >> In the later mode the numeric identifier is used in sysfs, in the former >> case the full Linux name. But we want to keep some degree of Linux user >> space compatibility between the two, that is why the full mode creates a >> symlink from the "numeric" name. This way the (ugly) identifiers needed for >> the fallback mode work everywhere. > So what _exactly_ does the symlink do here? What is it from->to?
It's from numeric identifier to full perf name
In fallback mode there is no symlink, only the numeric identifier.
> > And where is it being documented? What userspace tool needs to be fixed > up so that the symlink can be removed?
The names are visible in the perf command lines. Perf supports either name without changes. So it's not about fixing a specific tool, but about using the drivers in both modes, with limited compatibility between the two.
Yes probably it needs better documentation.
-Andi
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