Re: [PATCH] PCI: Add ACS errata for Pericom PI7C9X2G404 switch

[Lukas Wunner <lukas@wunner.de>]

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.
> 
> 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.

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.

Perhaps we need additional pci_fixup ELF sections which are used on
hot-add and hot-remove?

Thanks,

Lukas