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

From: Alex Deucher <alexdeucher@gmail.com>
To: Lukas Wunner <lukas@wunner.de>
Cc: Dave Airlie <airlied@gmail.com>,
	Lucas Stach <l.stach@pengutronix.de>,
	Ben Skeggs <skeggsb@gmail.com>,
	Karol Herbst <karolherbst@gmail.com>,
	"Alex G." <mr.nuke.me@gmail.com>,
	Bjorn Helgaas <helgaas@kernel.org>,
	Alexandru Gagniuc <alex_gagniuc@dellteam.com>,
	Keith Busch <keith.busch@intel.com>, Jens Axboe <axboe@fb.com>,
	Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
	David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
	Jan Vesely <jano.vesely@gmail.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	Austin Bolen <austin_bolen@dell.com>,
	Shyam Iyer <Shyam_Iyer@dell.com>, Sinan Kaya <okaya@kernel.org>,
	Linux PCI <linux-pci@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: Issues with "PCI/LINK: Report degraded links via link bandwidth notification"
Date: Tue, 4 Feb 2020 09:47:50 -0500	[thread overview]
Message-ID: <CADnq5_PFuZZnZXzhJ0PkuPdCPNRt=ogXFYgrp4k5XSC9XTZGeg@mail.gmail.com> (raw)
In-Reply-To: <20200204043825.thpbqpz3ao7zqvlh@wunner.de>

On Mon, Feb 3, 2020 at 11:38 PM Lukas Wunner <lukas@wunner.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