Re: [PATCH] PCI/portdrv: Disable bwctrl service if port is fixed at 2.5 GT/s

[Lukas Wunner <lukas@wunner.de>]

On Thu, Dec 12, 2024 at 10:17:21AM +0100, Niklas Schnelle wrote:
> On Thu, 2024-12-12 at 10:08 +0100, Lukas Wunner wrote:
> > After re-reading the spec I'm convinced now
> > that we're doing this wrong and that we should honor the Max Link Speed
> > instead of blindly deeming all set bits in the Link Capabilities 2
> > Register as supported speeds:
> > 
> > https://lore.kernel.org/r/e3386d62a766be6d0ef7138a001dabfe563cdff8.1733991971.git.lukas@wunner.de/
> > 
> > @Niklas, could you test if this is sufficient to avoid the issue?
> > Or do we still need to stop instantiating the bandwidth controller
> > if more than one speed is supported?
> 
> Yes, I will test this but will only get to do so tonight (UTC +2).

Hey, no worries.  We're not on the run!

> If it's not sufficient I think we could use the modified
> pcie_get_supported_speeds() to check if only one link speed is
> supported, right?

pcie_get_supported_speeds() is used to fill in the supported_speeds
field in struct pci_dev.

And that field is used in a number of places (exposure of the max link
speed in sysfs, delay handling in pci_bridge_wait_for_secondary_bus(),
link tuning in radeon/amdgpu drivers, etc).

So we can't use pcie_get_supported_speeds() to (exclusively) influence
the behavior of the bandwidth controller.  Instead, the solution is your
patch for get_port_device_capability(), but future-proofed such that
bwctrl is only instantiated if more than one link speed is supported.

Thanks!

Lukas