From: Greg KH <greg@kroah.com>
To: Andi Kleen <ak@linux.intel.com>
Cc: kan.liang@linux.intel.com, peterz@infradead.org,
mingo@redhat.com, linux-kernel@vger.kernel.org,
eranian@google.com, namhyung@kernel.org, acme@kernel.org,
jolsa@redhat.com
Subject: Re: [PATCH 2/7] perf: Create a symlink for a PMU
Date: Thu, 24 Jun 2021 17:31:32 +0200 [thread overview]
Message-ID: <YNSlVPcjHInk4un6@kroah.com> (raw)
In-Reply-To: <1e536604-cf93-0f09-401e-2073924c5582@linux.intel.com>
On Thu, Jun 24, 2021 at 08:24:29AM -0700, Andi Kleen wrote:
>
> 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
But this is NOT how busses work in the driver model.
PCI classes are great, but we do NOT suddenly add a symlink in sysfs if
a driver goes from being handled by "generic_pci_type_foo" to
"vendor_foo". Userspace can handle the change and life goes on.
> 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.
So a device name will move from "generic" to "specific", right?
Why does a bus have to do with any of this?
> > > 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.
Those two sentences do not describe a sysfs path to me.
Where are the Documentation/ABI/ entries for all of this?
> > 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.
But a driver does not caer. And if perf does not care, who cares?
still totally confused,
greg k-h
next prev parent reply other threads:[~2021-06-24 15:31 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-24 1:22 [PATCH 0/7] perf: Add Sapphire Rapids server uncore support kan.liang
2021-06-24 1:22 ` [PATCH 1/7] driver core: Add a way to get to bus devices kset kan.liang
2021-06-24 5:41 ` Greg KH
2021-06-24 1:22 ` [PATCH 2/7] perf: Create a symlink for a PMU kan.liang
2021-06-24 5:48 ` Greg KH
2021-06-24 14:24 ` Andi Kleen
2021-06-24 14:29 ` Greg KH
2021-06-24 15:24 ` Andi Kleen
2021-06-24 15:31 ` Greg KH [this message]
2021-06-24 17:07 ` Liang, Kan
2021-06-24 17:35 ` Greg KH
2021-06-24 17:49 ` Andi Kleen
2021-06-25 5:18 ` Greg KH
2021-06-25 5:17 ` Greg KH
2021-06-24 17:28 ` Andi Kleen
2021-06-25 5:19 ` Greg KH
2021-06-25 14:22 ` Andi Kleen
2021-06-25 14:38 ` Greg KH
2021-06-25 14:49 ` Andi Kleen
2021-06-25 15:03 ` Liang, Kan
2021-06-25 15:44 ` Andi Kleen
2021-06-25 15:57 ` Liang, Kan
2021-06-25 16:18 ` Liang, Kan
2021-06-27 11:02 ` Greg KH
2021-06-27 16:30 ` Andi Kleen
2021-06-28 6:55 ` Greg KH
2021-06-28 15:00 ` Andi Kleen
2021-06-24 1:22 ` [PATCH 3/7] perf/x86/intel/uncore: Create a symlink for an uncore PMU kan.liang
2021-06-24 5:44 ` Greg KH
2021-06-24 1:22 ` [PATCH 4/7] perf/x86/intel/uncore: Add Sapphire Rapids server support kan.liang
2021-06-24 1:22 ` [PATCH 5/7] perf/x86/intel/uncore: Factor out snr_uncore_mmio_map() kan.liang
2021-06-24 1:22 ` [PATCH 6/7] perf/x86/intel/uncore: Support free-running counters on Sapphire Rapids server kan.liang
2021-06-24 1:22 ` [PATCH 7/7] perf/x86/intel/uncore: Fix invalid unit check kan.liang
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