linux-kernel - Re: [PATCH 2/7] perf: Create a symlink for a PMU

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Date:   Thu, 24 Jun 2021 07:24:31 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Greg KH <greg@...ah.com>, kan.liang@...ux.intel.com
Cc:     peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, eranian@...gle.com,
        namhyung@...nel.org, acme@...nel.org, jolsa@...hat.com
Subject: Re: [PATCH 2/7] perf: Create a symlink for a PMU

> 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.

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.

-Andi