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

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Date:   Tue, 4 Feb 2020 09:47:50 -0500
From:   Alex Deucher <alexdeucher@...il.com>
To:     Lukas Wunner <lukas@...ner.de>
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 3, 2020 at 11:38 PM Lukas Wunner <lukas@...ner.de> wrote:
>
> 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?

I don't know off hand.  I can ask the firmware team.  That said, I
haven't seen any reports of problems with our GPUs with this code
enabled.

Alex