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

Lukas Wunner
2020-02-04T04:38:31Z

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