linux-kernel - Re: Issues with "PCI/LINK: Report degraded links via link bandwidth notification"

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Date:   Tue, 4 Feb 2020 05:38:25 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Alex Deucher <alexdeucher@...il.com>
Cc:     Dave Airlie <airlied@...il.com>,
        Lucas Stach <l.stach@...gutronix.de>,
        Ben Skeggs <skeggsb@...il.com>,
        Karol Herbst <karolherbst@...il.com>,
        "Alex G." <mr.nuke.me@...il.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Alexandru Gagniuc <alex_gagniuc@...lteam.com>,
        Keith Busch <keith.busch@...el.com>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jan Vesely <jano.vesely@...il.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Austin Bolen <austin_bolen@...l.com>,
        Shyam Iyer <Shyam_Iyer@...l.com>,
        Sinan Kaya <okaya@...nel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Issues with "PCI/LINK: Report degraded links via link bandwidth
 notification"

On Mon, Feb 03, 2020 at 04:16:36PM -0500, Alex Deucher wrote:
> AMD has had a micro-controller on the GPU handling pcie link speeds
> and widths dynamically (in addition to GPU clocks and voltages) for
> about 12 years or so at this point to save power when the GPU is idle
> and improve performance when it's required.  The micro-controller
> changes the link parameters dynamically based on load independent of
> the driver.  The driver can tweak the heuristics, or even disable the
> dynamic changes, but by default it's enabled when the driver loads.
> The ucode for this micro-controller is loaded by the driver so you'll
> see fixed clocks and widths prior to the driver loading.  We'd need
> some sort of opt out I suppose for periods when the driver has enabled
> dynamic pcie power management in the micro-controller.

Note that there are *two* bits in the Link Status Register:

* Link Autonomous Bandwidth Status
  "This bit is Set by hardware to indicate that hardware has
  autonomously changed Link speed or width, without the Port
  transitioning through DL_Down status, for reasons other than to
  attempt to correct unreliable Link operation.  This bit must be set if
  the Physical Layer reports a speed or width change was initiated by
  the Downstream component that was indicated as an autonomous change."

* Link Bandwidth Management Status
  "This bit is Set by hardware to indicate that either of the
  following has occurred without the Port transitioning through
  DL_Down status. [...] Hardware has changed Link speed or width to
  attempt to correct unreliable Link operation, either through an
  LTSSM timeout or a higher level process."

See PCIe Base Spec 4.0 sec 7.8.8, 7.8.7, 4.2.6.3.3.1.

The two bits generate *separate* interrupts.  We only enable the
interrupt for the latter.

If AMD GPUs generate a Link Bandwidth Management Interrupt upon
autonomously changing bandwidth for power management reasons
(instead of to correct unreliability issues), that would be a
spec violation.

So the question is, do your GPUs violate the spec in this regard?

Thanks,

Lukas