Re: Problem with PCIe enumeration of Google/Coral TPU Edge module on Linux

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Apr 09, 2020 at 04:25:40PM +0100, Luís Mendes wrote:
> Hi Bjorn,
> 
> I've good news. I've found the culprit and it is a pretty simple
> issue, however the good solution is not obvious to me.
> Can you help in finding the best way to patch this issue?
> 
> So first detailing the problem in file setup_bus.c there is this *if
> condition* to ignore resources from classless devices and so
> it is that this Google/Coral Edge TPU is a classless device with class 0xff:
> 
> static void __dev_sort_resources(struct pci_dev *dev, struct list_head *head)
> {
>     u16 class = dev->class >> 8;
> 
>        pci_info(dev, "%s\n", __func__);
>     /* Don't touch classless devices or host bridges or IOAPICs */
>     if (class == PCI_CLASS_NOT_DEFINED || class == PCI_CLASS_BRIDGE_HOST)
>         return;
>    ....
> 
> So the one possible trivial, non generic, attempt that works is to do:
> static void __dev_sort_resources(struct pci_dev *dev, struct list_head *head)
> {
>     u16 class = dev->class >> 8;
> 
>        pci_info(dev, "%s\n", __func__);
>     /* Don't touch classless devices or host bridges or IOAPICs */
>     if ((class == PCI_CLASS_NOT_DEFINED &&  !(dev->vendor == 0x1ac1 &&
> dev->device==0x089a)) || class == PCI_CLASS_BRIDGE_HOST)
>         return;
>    ....
> 
> What is your suggestion to make the solution generic? Create a
> whitelist? Remove this verification? I have no idea... nothing sounds
> good to me...

Good detective work, thanks for chasing this down!

I should have seen that check when adding the debug.  Guess I thought
"sort", hmmm, that just re-orders things without actually changing the
content.  But pdev_sort_resources() in fact *adds* resources to a
list, and if resources aren't on the list, we apparently don't assign
space for them.

In any event, I would first check to see if there's an Edge TPU
firmware update that might set the class code.

If not, we should probably add a quirk to override the class code,
similar to quirk_eisa_bridge(), fixup_rev1_53c810(),
fixup_ti816x_class(), quirk_tw686x_class().

Bjorn