[+cc Alex, beginning of thread: https://lore.kernel.org/r/20210903034029.306816-1-nathan@xxxxxxxxxxxxxxx] On Mon, Sep 06, 2021 at 04:01:20PM +1000, Nathan Rossi wrote: > On Fri, 3 Sept 2021 at 16:18, Lukas Wunner <lukas@xxxxxxxxx> wrote: > > > > On Fri, Sep 03, 2021 at 03:40:29AM +0000, Nathan Rossi wrote: > > > The Pericom PI7C9X2G404 PCIe switch has an errata for ACS P2P Request > > > Redirect behaviour when used in the cut-through forwarding mode. The > > > recommended work around for this issue is to use the switch in store and > > > forward mode. Is there a URL for this erratum? What is the issue? Does the switch fail to redirect P2P requests upstream? How would someone recognize that they are affected by it? > > > This change adds a fixup specific to this switch that when enabling the > > > downstream port it checks if it has enabled ACS P2P Request Redirect, > > > and if so changes the device (via the upstream port) to use the store > > > and forward operating mode. > > > > From a quick look at the datasheet, this switch seems to support > > hot-plug on its Downstream Ports: > > > > https://www.diodes.com/assets/Datasheets/PI7C9X2G404SL.pdf > > > > I think your quirk isn't executed if a device is hotplugged to an > > initially-empty Downstream Port. > > The device I am testing against has the ports wired directly to > devices (though can be disconnected) without hotplug so I will see if > I can find a development board with this switch to test the hotplug > behaviour. However it should be noted that the downstream ports are > probed with the switch, and are enabled with the ACS P2P Request > Redirect configured regardless of the presence of a device connected > to the downstream port. > > > Also, if a device which triggered the quirk is hot-removed and none > > of its siblings uses ACS P2P Request Redirect, cut-through forwarding > > isn't reinstated. > > The quirk is enabled on the downstream port of the switch, using the > state of the downstream port and not the device attached to it. My > understanding is that the only path that enables/disables the ACS P2P > Request Redirect on the downstream port is the initial pci_enable_acs. pci_enable_acs() is called during enumeration of each device (including the switch, of course): pci_init_capabilities pci_acs_init pci_enable_acs and your quirk is DECLARE_PCI_FIXUP_ENABLE(), so it runs later, during pci_enable_device(). I don't think we ever turn on ACS P2P Request Redirect after enumeration. But we do also call pci_enable_acs() from pci_restore_state(), so this happens during resume. I assume your quirk would also need to run then? Is there a pci_enable_device() somewhere in the resume path that will do this? > This means that devices attached to the downstream port either > initially or with hotplugging should not change the ACS configuration > of the switches downstream port. > > Which means nothing can cause the switch to need to be reinstated with > cut-through forwarding except the switch itself being hotplugged, > which would cause reset of the switch and the enable fixup to be > called again. Seems right to me, since we enable ACS P2P Request Redirect regardless of whether any downstream device exists. > > Perhaps we need additional pci_fixup ELF sections which are used on > > hot-add and hot-remove? > > > > Thanks, > > > > Lukas
